1 /* SPDX-License-Identifier: GPL-2.0 */
3 * linux/arch/x86_64/entry.S
5 * Copyright (C) 1991, 1992 Linus Torvalds
6 * Copyright (C) 2000, 2001, 2002 Andi Kleen SuSE Labs
7 * Copyright (C) 2000 Pavel Machek <pavel@suse.cz>
9 * entry.S contains the system-call and fault low-level handling routines.
11 * Some of this is documented in Documentation/x86/entry_64.rst
13 * A note on terminology:
14 * - iret frame: Architecture defined interrupt frame from SS to RIP
15 * at the top of the kernel process stack.
18 * - SYM_FUNC_START/END:Define functions in the symbol table.
19 * - idtentry: Define exception entry points.
21 #include <linux/linkage.h>
22 #include <asm/segment.h>
23 #include <asm/cache.h>
24 #include <asm/errno.h>
25 #include <asm/asm-offsets.h>
27 #include <asm/unistd.h>
28 #include <asm/thread_info.h>
29 #include <asm/hw_irq.h>
30 #include <asm/page_types.h>
31 #include <asm/irqflags.h>
32 #include <asm/paravirt.h>
33 #include <asm/percpu.h>
36 #include <asm/pgtable_types.h>
37 #include <asm/export.h>
38 #include <asm/frame.h>
39 #include <asm/trapnr.h>
40 #include <asm/nospec-branch.h>
41 #include <asm/fsgsbase.h>
42 #include <linux/err.h>
47 .section .entry.text, "ax"
50 * 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
52 * This is the only entry point used for 64-bit system calls. The
53 * hardware interface is reasonably well designed and the register to
54 * argument mapping Linux uses fits well with the registers that are
55 * available when SYSCALL is used.
57 * SYSCALL instructions can be found inlined in libc implementations as
58 * well as some other programs and libraries. There are also a handful
59 * of SYSCALL instructions in the vDSO used, for example, as a
60 * clock_gettimeofday fallback.
62 * 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
63 * then loads new ss, cs, and rip from previously programmed MSRs.
64 * rflags gets masked by a value from another MSR (so CLD and CLAC
65 * are not needed). SYSCALL does not save anything on the stack
66 * and does not change rsp.
69 * rax system call number
71 * r11 saved rflags (note: r11 is callee-clobbered register in C ABI)
75 * r10 arg3 (needs to be moved to rcx to conform to C ABI)
78 * (note: r12-r15, rbp, rbx are callee-preserved in C ABI)
80 * Only called from user space.
82 * When user can change pt_regs->foo always force IRET. That is because
83 * it deals with uncanonical addresses better. SYSRET has trouble
84 * with them due to bugs in both AMD and Intel CPUs.
87 SYM_CODE_START(entry_SYSCALL_64)
92 /* tss.sp2 is scratch space. */
93 movq %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
94 SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
95 movq PER_CPU_VAR(pcpu_hot + X86_top_of_stack), %rsp
97 SYM_INNER_LABEL(entry_SYSCALL_64_safe_stack, SYM_L_GLOBAL)
100 /* Construct struct pt_regs on stack */
101 pushq $__USER_DS /* pt_regs->ss */
102 pushq PER_CPU_VAR(cpu_tss_rw + TSS_sp2) /* pt_regs->sp */
103 pushq %r11 /* pt_regs->flags */
104 pushq $__USER_CS /* pt_regs->cs */
105 pushq %rcx /* pt_regs->ip */
106 SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
107 pushq %rax /* pt_regs->orig_ax */
109 PUSH_AND_CLEAR_REGS rax=$-ENOSYS
113 /* Sign extend the lower 32bit as syscall numbers are treated as int */
116 /* clobbers %rax, make sure it is after saving the syscall nr */
120 call do_syscall_64 /* returns with IRQs disabled */
123 * Try to use SYSRET instead of IRET if we're returning to
124 * a completely clean 64-bit userspace context. If we're not,
125 * go to the slow exit path.
126 * In the Xen PV case we must use iret anyway.
129 ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", \
135 cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
136 jne swapgs_restore_regs_and_return_to_usermode
139 * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
140 * in kernel space. This essentially lets the user take over
141 * the kernel, since userspace controls RSP.
143 * If width of "canonical tail" ever becomes variable, this will need
144 * to be updated to remain correct on both old and new CPUs.
146 * Change top bits to match most significant bit (47th or 56th bit
147 * depending on paging mode) in the address.
149 #ifdef CONFIG_X86_5LEVEL
150 ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
151 "shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
153 shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
154 sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
157 /* If this changed %rcx, it was not canonical */
159 jne swapgs_restore_regs_and_return_to_usermode
161 cmpq $__USER_CS, CS(%rsp) /* CS must match SYSRET */
162 jne swapgs_restore_regs_and_return_to_usermode
165 cmpq %r11, EFLAGS(%rsp) /* R11 == RFLAGS */
166 jne swapgs_restore_regs_and_return_to_usermode
169 * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
170 * restore RF properly. If the slowpath sets it for whatever reason, we
171 * need to restore it correctly.
173 * SYSRET can restore TF, but unlike IRET, restoring TF results in a
174 * trap from userspace immediately after SYSRET. This would cause an
175 * infinite loop whenever #DB happens with register state that satisfies
176 * the opportunistic SYSRET conditions. For example, single-stepping
179 * movq $stuck_here, %rcx
184 * would never get past 'stuck_here'.
186 testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
187 jnz swapgs_restore_regs_and_return_to_usermode
189 /* nothing to check for RSP */
191 cmpq $__USER_DS, SS(%rsp) /* SS must match SYSRET */
192 jne swapgs_restore_regs_and_return_to_usermode
195 * We win! This label is here just for ease of understanding
196 * perf profiles. Nothing jumps here.
198 syscall_return_via_sysret:
203 * Now all regs are restored except RSP and RDI.
204 * Save old stack pointer and switch to trampoline stack.
207 movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
210 pushq RSP-RDI(%rdi) /* RSP */
211 pushq (%rdi) /* RDI */
214 * We are on the trampoline stack. All regs except RDI are live.
215 * We can do future final exit work right here.
217 STACKLEAK_ERASE_NOCLOBBER
219 SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
223 SYM_INNER_LABEL(entry_SYSRETQ_unsafe_stack, SYM_L_GLOBAL)
227 SYM_INNER_LABEL(entry_SYSRETQ_end, SYM_L_GLOBAL)
230 SYM_CODE_END(entry_SYSCALL_64)
236 .pushsection .text, "ax"
237 SYM_FUNC_START(__switch_to_asm)
239 * Save callee-saved registers
240 * This must match the order in inactive_task_frame
250 movq %rsp, TASK_threadsp(%rdi)
251 movq TASK_threadsp(%rsi), %rsp
253 #ifdef CONFIG_STACKPROTECTOR
254 movq TASK_stack_canary(%rsi), %rbx
255 movq %rbx, PER_CPU_VAR(fixed_percpu_data) + FIXED_stack_canary
259 * When switching from a shallower to a deeper call stack
260 * the RSB may either underflow or use entries populated
261 * with userspace addresses. On CPUs where those concerns
262 * exist, overwrite the RSB with entries which capture
263 * speculative execution to prevent attack.
265 FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
267 /* restore callee-saved registers */
276 SYM_FUNC_END(__switch_to_asm)
280 * A newly forked process directly context switches into this address.
282 * rax: prev task we switched from
283 * rbx: kernel thread func (NULL for user thread)
284 * r12: kernel thread arg
286 .pushsection .text, "ax"
288 SYM_CODE_START_NOALIGN(ret_from_fork)
290 ANNOTATE_NOENDBR // copy_thread
293 call schedule_tail /* rdi: 'prev' task parameter */
295 testq %rbx, %rbx /* from kernel_thread? */
296 jnz 1f /* kernel threads are uncommon */
301 call syscall_exit_to_user_mode /* returns with IRQs disabled */
302 jmp swapgs_restore_regs_and_return_to_usermode
310 * A kernel thread is allowed to return here after successfully
311 * calling kernel_execve(). Exit to userspace to complete the execve()
316 SYM_CODE_END(ret_from_fork)
319 .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
320 #ifdef CONFIG_DEBUG_ENTRY
323 testl $X86_EFLAGS_IF, %eax
331 SYM_CODE_START(xen_error_entry)
334 PUSH_AND_CLEAR_REGS save_ret=1
335 ENCODE_FRAME_POINTER 8
336 UNTRAIN_RET_FROM_CALL
338 SYM_CODE_END(xen_error_entry)
341 * idtentry_body - Macro to emit code calling the C function
342 * @cfunc: C function to be called
343 * @has_error_code: Hardware pushed error code on stack
345 .macro idtentry_body cfunc has_error_code:req
348 * Call error_entry() and switch to the task stack if from userspace.
350 * When in XENPV, it is already in the task stack, and it can't fault
351 * for native_iret() nor native_load_gs_index() since XENPV uses its
352 * own pvops for IRET and load_gs_index(). And it doesn't need to
353 * switch the CR3. So it can skip invoking error_entry().
355 ALTERNATIVE "call error_entry; movq %rax, %rsp", \
356 "call xen_error_entry", X86_FEATURE_XENPV
361 movq %rsp, %rdi /* pt_regs pointer into 1st argument*/
363 .if \has_error_code == 1
364 movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
365 movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
370 /* For some configurations \cfunc ends up being a noreturn. */
377 * idtentry - Macro to generate entry stubs for simple IDT entries
378 * @vector: Vector number
379 * @asmsym: ASM symbol for the entry point
380 * @cfunc: C function to be called
381 * @has_error_code: Hardware pushed error code on stack
383 * The macro emits code to set up the kernel context for straight forward
384 * and simple IDT entries. No IST stack, no paranoid entry checks.
386 .macro idtentry vector asmsym cfunc has_error_code:req
387 SYM_CODE_START(\asmsym)
388 UNWIND_HINT_IRET_REGS offset=\has_error_code*8
393 .if \has_error_code == 0
394 pushq $-1 /* ORIG_RAX: no syscall to restart */
397 .if \vector == X86_TRAP_BP
399 * If coming from kernel space, create a 6-word gap to allow the
400 * int3 handler to emulate a call instruction.
402 testb $3, CS-ORIG_RAX(%rsp)
403 jnz .Lfrom_usermode_no_gap_\@
407 UNWIND_HINT_IRET_REGS offset=8
408 .Lfrom_usermode_no_gap_\@:
411 idtentry_body \cfunc \has_error_code
413 _ASM_NOKPROBE(\asmsym)
414 SYM_CODE_END(\asmsym)
418 * Interrupt entry/exit.
420 + The interrupt stubs push (vector) onto the stack, which is the error_code
421 * position of idtentry exceptions, and jump to one of the two idtentry points
424 * common_interrupt is a hotpath, align it to a cache line
426 .macro idtentry_irq vector cfunc
427 .p2align CONFIG_X86_L1_CACHE_SHIFT
428 idtentry \vector asm_\cfunc \cfunc has_error_code=1
432 * System vectors which invoke their handlers directly and are not
433 * going through the regular common device interrupt handling code.
435 .macro idtentry_sysvec vector cfunc
436 idtentry \vector asm_\cfunc \cfunc has_error_code=0
440 * idtentry_mce_db - Macro to generate entry stubs for #MC and #DB
441 * @vector: Vector number
442 * @asmsym: ASM symbol for the entry point
443 * @cfunc: C function to be called
445 * The macro emits code to set up the kernel context for #MC and #DB
447 * If the entry comes from user space it uses the normal entry path
448 * including the return to user space work and preemption checks on
451 * If hits in kernel mode then it needs to go through the paranoid
452 * entry as the exception can hit any random state. No preemption
453 * check on exit to keep the paranoid path simple.
455 .macro idtentry_mce_db vector asmsym cfunc
456 SYM_CODE_START(\asmsym)
457 UNWIND_HINT_IRET_REGS
462 pushq $-1 /* ORIG_RAX: no syscall to restart */
465 * If the entry is from userspace, switch stacks and treat it as
468 testb $3, CS-ORIG_RAX(%rsp)
469 jnz .Lfrom_usermode_switch_stack_\@
471 /* paranoid_entry returns GS information for paranoid_exit in EBX. */
476 movq %rsp, %rdi /* pt_regs pointer */
482 /* Switch to the regular task stack and use the noist entry point */
483 .Lfrom_usermode_switch_stack_\@:
484 idtentry_body noist_\cfunc, has_error_code=0
486 _ASM_NOKPROBE(\asmsym)
487 SYM_CODE_END(\asmsym)
490 #ifdef CONFIG_AMD_MEM_ENCRYPT
492 * idtentry_vc - Macro to generate entry stub for #VC
493 * @vector: Vector number
494 * @asmsym: ASM symbol for the entry point
495 * @cfunc: C function to be called
497 * The macro emits code to set up the kernel context for #VC. The #VC handler
498 * runs on an IST stack and needs to be able to cause nested #VC exceptions.
500 * To make this work the #VC entry code tries its best to pretend it doesn't use
501 * an IST stack by switching to the task stack if coming from user-space (which
502 * includes early SYSCALL entry path) or back to the stack in the IRET frame if
503 * entered from kernel-mode.
505 * If entered from kernel-mode the return stack is validated first, and if it is
506 * not safe to use (e.g. because it points to the entry stack) the #VC handler
507 * will switch to a fall-back stack (VC2) and call a special handler function.
509 * The macro is only used for one vector, but it is planned to be extended in
510 * the future for the #HV exception.
512 .macro idtentry_vc vector asmsym cfunc
513 SYM_CODE_START(\asmsym)
514 UNWIND_HINT_IRET_REGS
520 * If the entry is from userspace, switch stacks and treat it as
523 testb $3, CS-ORIG_RAX(%rsp)
524 jnz .Lfrom_usermode_switch_stack_\@
527 * paranoid_entry returns SWAPGS flag for paranoid_exit in EBX.
528 * EBX == 0 -> SWAPGS, EBX == 1 -> no SWAPGS
535 * Switch off the IST stack to make it free for nested exceptions. The
536 * vc_switch_off_ist() function will switch back to the interrupted
537 * stack if it is safe to do so. If not it switches to the VC fall-back
540 movq %rsp, %rdi /* pt_regs pointer */
541 call vc_switch_off_ist
542 movq %rax, %rsp /* Switch to new stack */
548 movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
549 movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
551 movq %rsp, %rdi /* pt_regs pointer */
556 * No need to switch back to the IST stack. The current stack is either
557 * identical to the stack in the IRET frame or the VC fall-back stack,
558 * so it is definitely mapped even with PTI enabled.
562 /* Switch to the regular task stack */
563 .Lfrom_usermode_switch_stack_\@:
564 idtentry_body user_\cfunc, has_error_code=1
566 _ASM_NOKPROBE(\asmsym)
567 SYM_CODE_END(\asmsym)
572 * Double fault entry. Straight paranoid. No checks from which context
573 * this comes because for the espfix induced #DF this would do the wrong
576 .macro idtentry_df vector asmsym cfunc
577 SYM_CODE_START(\asmsym)
578 UNWIND_HINT_IRET_REGS offset=8
583 /* paranoid_entry returns GS information for paranoid_exit in EBX. */
587 movq %rsp, %rdi /* pt_regs pointer into first argument */
588 movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
589 movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
592 /* For some configurations \cfunc ends up being a noreturn. */
597 _ASM_NOKPROBE(\asmsym)
598 SYM_CODE_END(\asmsym)
602 * Include the defines which emit the idt entries which are shared
603 * shared between 32 and 64 bit and emit the __irqentry_text_* markers
604 * so the stacktrace boundary checks work.
607 .globl __irqentry_text_start
608 __irqentry_text_start:
610 #include <asm/idtentry.h>
613 .globl __irqentry_text_end
617 SYM_CODE_START_LOCAL(common_interrupt_return)
618 SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
620 #ifdef CONFIG_DEBUG_ENTRY
621 /* Assert that pt_regs indicates user mode. */
628 ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
634 * The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
635 * Save old stack pointer and switch to trampoline stack.
638 movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
641 /* Copy the IRET frame to the trampoline stack. */
642 pushq 6*8(%rdi) /* SS */
643 pushq 5*8(%rdi) /* RSP */
644 pushq 4*8(%rdi) /* EFLAGS */
645 pushq 3*8(%rdi) /* CS */
646 pushq 2*8(%rdi) /* RIP */
648 /* Push user RDI on the trampoline stack. */
652 * We are on the trampoline stack. All regs except RDI are live.
653 * We can do future final exit work right here.
655 STACKLEAK_ERASE_NOCLOBBER
657 SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
665 SYM_INNER_LABEL(restore_regs_and_return_to_kernel, SYM_L_GLOBAL)
666 #ifdef CONFIG_DEBUG_ENTRY
667 /* Assert that pt_regs indicates kernel mode. */
674 addq $8, %rsp /* skip regs->orig_ax */
676 * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization
677 * when returning from IPI handler.
680 SYM_INNER_LABEL(early_xen_iret_patch, SYM_L_GLOBAL)
683 .long .Lnative_iret - (. + 4)
687 UNWIND_HINT_IRET_REGS
689 * Are we returning to a stack segment from the LDT? Note: in
690 * 64-bit mode SS:RSP on the exception stack is always valid.
692 #ifdef CONFIG_X86_ESPFIX64
693 testb $4, (SS-RIP)(%rsp)
694 jnz native_irq_return_ldt
697 SYM_INNER_LABEL(native_irq_return_iret, SYM_L_GLOBAL)
698 ANNOTATE_NOENDBR // exc_double_fault
700 * This may fault. Non-paranoid faults on return to userspace are
701 * handled by fixup_bad_iret. These include #SS, #GP, and #NP.
702 * Double-faults due to espfix64 are handled in exc_double_fault.
703 * Other faults here are fatal.
707 #ifdef CONFIG_X86_ESPFIX64
708 native_irq_return_ldt:
710 * We are running with user GSBASE. All GPRs contain their user
711 * values. We have a percpu ESPFIX stack that is eight slots
712 * long (see ESPFIX_STACK_SIZE). espfix_waddr points to the bottom
713 * of the ESPFIX stack.
715 * We clobber RAX and RDI in this code. We stash RDI on the
716 * normal stack and RAX on the ESPFIX stack.
718 * The ESPFIX stack layout we set up looks like this:
720 * --- top of ESPFIX stack ---
725 * RIP <-- RSP points here when we're done
726 * RAX <-- espfix_waddr points here
727 * --- bottom of ESPFIX stack ---
730 pushq %rdi /* Stash user RDI */
731 swapgs /* to kernel GS */
732 SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi /* to kernel CR3 */
734 movq PER_CPU_VAR(espfix_waddr), %rdi
735 movq %rax, (0*8)(%rdi) /* user RAX */
736 movq (1*8)(%rsp), %rax /* user RIP */
737 movq %rax, (1*8)(%rdi)
738 movq (2*8)(%rsp), %rax /* user CS */
739 movq %rax, (2*8)(%rdi)
740 movq (3*8)(%rsp), %rax /* user RFLAGS */
741 movq %rax, (3*8)(%rdi)
742 movq (5*8)(%rsp), %rax /* user SS */
743 movq %rax, (5*8)(%rdi)
744 movq (4*8)(%rsp), %rax /* user RSP */
745 movq %rax, (4*8)(%rdi)
746 /* Now RAX == RSP. */
748 andl $0xffff0000, %eax /* RAX = (RSP & 0xffff0000) */
751 * espfix_stack[31:16] == 0. The page tables are set up such that
752 * (espfix_stack | (X & 0xffff0000)) points to a read-only alias of
753 * espfix_waddr for any X. That is, there are 65536 RO aliases of
754 * the same page. Set up RSP so that RSP[31:16] contains the
755 * respective 16 bits of the /userspace/ RSP and RSP nonetheless
756 * still points to an RO alias of the ESPFIX stack.
758 orq PER_CPU_VAR(espfix_stack), %rax
760 SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
761 swapgs /* to user GS */
762 popq %rdi /* Restore user RDI */
765 UNWIND_HINT_IRET_REGS offset=8
768 * At this point, we cannot write to the stack any more, but we can
771 popq %rax /* Restore user RAX */
774 * RSP now points to an ordinary IRET frame, except that the page
775 * is read-only and RSP[31:16] are preloaded with the userspace
776 * values. We can now IRET back to userspace.
778 jmp native_irq_return_iret
780 SYM_CODE_END(common_interrupt_return)
781 _ASM_NOKPROBE(common_interrupt_return)
784 * Reload gs selector with exception handling
787 * Is in entry.text as it shouldn't be instrumented.
789 SYM_FUNC_START(asm_load_gs_index)
793 ANNOTATE_NOENDBR // error_entry
795 2: ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
800 /* running with kernelgs */
802 swapgs /* switch back to user gs */
804 /* This can't be a string because the preprocessor needs to see it. */
805 movl $__USER_DS, %eax
808 ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
813 _ASM_EXTABLE(.Lgs_change, .Lbad_gs)
815 SYM_FUNC_END(asm_load_gs_index)
816 EXPORT_SYMBOL(asm_load_gs_index)
820 * A note on the "critical region" in our callback handler.
821 * We want to avoid stacking callback handlers due to events occurring
822 * during handling of the last event. To do this, we keep events disabled
823 * until we've done all processing. HOWEVER, we must enable events before
824 * popping the stack frame (can't be done atomically) and so it would still
825 * be possible to get enough handler activations to overflow the stack.
826 * Although unlikely, bugs of that kind are hard to track down, so we'd
827 * like to avoid the possibility.
828 * So, on entry to the handler we detect whether we interrupted an
829 * existing activation in its critical region -- if so, we pop the current
830 * activation and restart the handler using the previous one.
832 * C calling convention: exc_xen_hypervisor_callback(struct *pt_regs)
835 SYM_CODE_START_LOCAL_NOALIGN(exc_xen_hypervisor_callback)
838 * Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
839 * see the correct pointer to the pt_regs
842 movq %rdi, %rsp /* we don't return, adjust the stack frame */
845 call xen_pv_evtchn_do_upcall
848 SYM_CODE_END(exc_xen_hypervisor_callback)
851 * Hypervisor uses this for application faults while it executes.
852 * We get here for two reasons:
853 * 1. Fault while reloading DS, ES, FS or GS
854 * 2. Fault while executing IRET
855 * Category 1 we do not need to fix up as Xen has already reloaded all segment
856 * registers that could be reloaded and zeroed the others.
857 * Category 2 we fix up by killing the current process. We cannot use the
858 * normal Linux return path in this case because if we use the IRET hypercall
859 * to pop the stack frame we end up in an infinite loop of failsafe callbacks.
860 * We distinguish between categories by comparing each saved segment register
861 * with its current contents: any discrepancy means we in category 1.
864 SYM_CODE_START_NOALIGN(xen_failsafe_callback)
879 /* All segments match their saved values => Category 2 (Bad IRET). */
884 UNWIND_HINT_IRET_REGS offset=8
885 jmp asm_exc_general_protection
886 1: /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
890 UNWIND_HINT_IRET_REGS
891 pushq $-1 /* orig_ax = -1 => not a system call */
895 SYM_CODE_END(xen_failsafe_callback)
896 #endif /* CONFIG_XEN_PV */
899 * Save all registers in pt_regs. Return GSBASE related information
900 * in EBX depending on the availability of the FSGSBASE instructions:
903 * N 0 -> SWAPGS on exit
904 * 1 -> no SWAPGS on exit
906 * Y GSBASE value at entry, must be restored in paranoid_exit
909 * R15 - old SPEC_CTRL
911 SYM_CODE_START(paranoid_entry)
914 PUSH_AND_CLEAR_REGS save_ret=1
915 ENCODE_FRAME_POINTER 8
918 * Always stash CR3 in %r14. This value will be restored,
919 * verbatim, at exit. Needed if paranoid_entry interrupted
920 * another entry that already switched to the user CR3 value
921 * but has not yet returned to userspace.
923 * This is also why CS (stashed in the "iret frame" by the
924 * hardware at entry) can not be used: this may be a return
925 * to kernel code, but with a user CR3 value.
927 * Switching CR3 does not depend on kernel GSBASE so it can
928 * be done before switching to the kernel GSBASE. This is
929 * required for FSGSBASE because the kernel GSBASE has to
930 * be retrieved from a kernel internal table.
932 SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
935 * Handling GSBASE depends on the availability of FSGSBASE.
937 * Without FSGSBASE the kernel enforces that negative GSBASE
938 * values indicate kernel GSBASE. With FSGSBASE no assumptions
939 * can be made about the GSBASE value when entering from user
942 ALTERNATIVE "jmp .Lparanoid_entry_checkgs", "", X86_FEATURE_FSGSBASE
945 * Read the current GSBASE and store it in %rbx unconditionally,
946 * retrieve and set the current CPUs kernel GSBASE. The stored value
947 * has to be restored in paranoid_exit unconditionally.
949 * The unconditional write to GS base below ensures that no subsequent
950 * loads based on a mispredicted GS base can happen, therefore no LFENCE
953 SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
954 jmp .Lparanoid_gsbase_done
956 .Lparanoid_entry_checkgs:
957 /* EBX = 1 -> kernel GSBASE active, no restore required */
961 * The kernel-enforced convention is a negative GSBASE indicates
962 * a kernel value. No SWAPGS needed on entry and exit.
964 movl $MSR_GS_BASE, %ecx
967 js .Lparanoid_kernel_gsbase
969 /* EBX = 0 -> SWAPGS required on exit */
972 .Lparanoid_kernel_gsbase:
973 FENCE_SWAPGS_KERNEL_ENTRY
974 .Lparanoid_gsbase_done:
977 * Once we have CR3 and %GS setup save and set SPEC_CTRL. Just like
978 * CR3 above, keep the old value in a callee saved register.
980 IBRS_ENTER save_reg=%r15
981 UNTRAIN_RET_FROM_CALL
984 SYM_CODE_END(paranoid_entry)
987 * "Paranoid" exit path from exception stack. This is invoked
988 * only on return from non-NMI IST interrupts that came
991 * We may be returning to very strange contexts (e.g. very early
992 * in syscall entry), so checking for preemption here would
993 * be complicated. Fortunately, there's no good reason to try
994 * to handle preemption here.
996 * R/EBX contains the GSBASE related information depending on the
997 * availability of the FSGSBASE instructions:
1000 * N 0 -> SWAPGS on exit
1001 * 1 -> no SWAPGS on exit
1003 * Y User space GSBASE, must be restored unconditionally
1006 * R15 - old SPEC_CTRL
1008 SYM_CODE_START_LOCAL(paranoid_exit)
1012 * Must restore IBRS state before both CR3 and %GS since we need access
1013 * to the per-CPU x86_spec_ctrl_shadow variable.
1015 IBRS_EXIT save_reg=%r15
1018 * The order of operations is important. RESTORE_CR3 requires
1021 * NB to anyone to try to optimize this code: this code does
1022 * not execute at all for exceptions from user mode. Those
1023 * exceptions go through error_exit instead.
1025 RESTORE_CR3 scratch_reg=%rax save_reg=%r14
1027 /* Handle the three GSBASE cases */
1028 ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE
1030 /* With FSGSBASE enabled, unconditionally restore GSBASE */
1032 jmp restore_regs_and_return_to_kernel
1034 .Lparanoid_exit_checkgs:
1035 /* On non-FSGSBASE systems, conditionally do SWAPGS */
1037 jnz restore_regs_and_return_to_kernel
1039 /* We are returning to a context with user GSBASE */
1041 jmp restore_regs_and_return_to_kernel
1042 SYM_CODE_END(paranoid_exit)
1045 * Switch GS and CR3 if needed.
1047 SYM_CODE_START(error_entry)
1051 PUSH_AND_CLEAR_REGS save_ret=1
1052 ENCODE_FRAME_POINTER 8
1054 testb $3, CS+8(%rsp)
1055 jz .Lerror_kernelspace
1058 * We entered from user mode or we're pretending to have entered
1059 * from user mode due to an IRET fault.
1062 FENCE_SWAPGS_USER_ENTRY
1063 /* We have user CR3. Change to kernel CR3. */
1064 SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
1066 UNTRAIN_RET_FROM_CALL
1068 leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */
1069 /* Put us onto the real thread stack. */
1073 * There are two places in the kernel that can potentially fault with
1074 * usergs. Handle them here. B stepping K8s sometimes report a
1075 * truncated RIP for IRET exceptions returning to compat mode. Check
1076 * for these here too.
1078 .Lerror_kernelspace:
1079 leaq native_irq_return_iret(%rip), %rcx
1080 cmpq %rcx, RIP+8(%rsp)
1082 movl %ecx, %eax /* zero extend */
1083 cmpq %rax, RIP+8(%rsp)
1085 cmpq $.Lgs_change, RIP+8(%rsp)
1086 jne .Lerror_entry_done_lfence
1089 * hack: .Lgs_change can fail with user gsbase. If this happens, fix up
1090 * gsbase and proceed. We'll fix up the exception and land in
1091 * .Lgs_change's error handler with kernel gsbase.
1096 * Issue an LFENCE to prevent GS speculation, regardless of whether it is a
1097 * kernel or user gsbase.
1099 .Lerror_entry_done_lfence:
1100 FENCE_SWAPGS_KERNEL_ENTRY
1102 leaq 8(%rsp), %rax /* return pt_regs pointer */
1107 /* Fix truncated RIP */
1108 movq %rcx, RIP+8(%rsp)
1113 * We came from an IRET to user mode, so we have user
1114 * gsbase and CR3. Switch to kernel gsbase and CR3:
1117 FENCE_SWAPGS_USER_ENTRY
1118 SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
1120 UNTRAIN_RET_FROM_CALL
1123 * Pretend that the exception came from user mode: set up pt_regs
1124 * as if we faulted immediately after IRET.
1126 leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */
1130 SYM_CODE_END(error_entry)
1132 SYM_CODE_START_LOCAL(error_return)
1134 DEBUG_ENTRY_ASSERT_IRQS_OFF
1136 jz restore_regs_and_return_to_kernel
1137 jmp swapgs_restore_regs_and_return_to_usermode
1138 SYM_CODE_END(error_return)
1141 * Runs on exception stack. Xen PV does not go through this path at all,
1142 * so we can use real assembly here.
1145 * %r14: Used to save/restore the CR3 of the interrupted context
1146 * when PAGE_TABLE_ISOLATION is in use. Do not clobber.
1148 SYM_CODE_START(asm_exc_nmi)
1149 UNWIND_HINT_IRET_REGS
1153 * We allow breakpoints in NMIs. If a breakpoint occurs, then
1154 * the iretq it performs will take us out of NMI context.
1155 * This means that we can have nested NMIs where the next
1156 * NMI is using the top of the stack of the previous NMI. We
1157 * can't let it execute because the nested NMI will corrupt the
1158 * stack of the previous NMI. NMI handlers are not re-entrant
1161 * To handle this case we do the following:
1162 * Check the a special location on the stack that contains
1163 * a variable that is set when NMIs are executing.
1164 * The interrupted task's stack is also checked to see if it
1166 * If the variable is not set and the stack is not the NMI
1168 * o Set the special variable on the stack
1169 * o Copy the interrupt frame into an "outermost" location on the
1171 * o Copy the interrupt frame into an "iret" location on the stack
1172 * o Continue processing the NMI
1173 * If the variable is set or the previous stack is the NMI stack:
1174 * o Modify the "iret" location to jump to the repeat_nmi
1175 * o return back to the first NMI
1177 * Now on exit of the first NMI, we first clear the stack variable
1178 * The NMI stack will tell any nested NMIs at that point that it is
1179 * nested. Then we pop the stack normally with iret, and if there was
1180 * a nested NMI that updated the copy interrupt stack frame, a
1181 * jump will be made to the repeat_nmi code that will handle the second
1184 * However, espfix prevents us from directly returning to userspace
1185 * with a single IRET instruction. Similarly, IRET to user mode
1186 * can fault. We therefore handle NMIs from user space like
1187 * other IST entries.
1193 /* Use %rdx as our temp variable throughout */
1196 testb $3, CS-RIP+8(%rsp)
1197 jz .Lnmi_from_kernel
1200 * NMI from user mode. We need to run on the thread stack, but we
1201 * can't go through the normal entry paths: NMIs are masked, and
1202 * we don't want to enable interrupts, because then we'll end
1203 * up in an awkward situation in which IRQs are on but NMIs
1206 * We also must not push anything to the stack before switching
1207 * stacks lest we corrupt the "NMI executing" variable.
1211 FENCE_SWAPGS_USER_ENTRY
1212 SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
1214 movq PER_CPU_VAR(pcpu_hot + X86_top_of_stack), %rsp
1215 UNWIND_HINT_IRET_REGS base=%rdx offset=8
1216 pushq 5*8(%rdx) /* pt_regs->ss */
1217 pushq 4*8(%rdx) /* pt_regs->rsp */
1218 pushq 3*8(%rdx) /* pt_regs->flags */
1219 pushq 2*8(%rdx) /* pt_regs->cs */
1220 pushq 1*8(%rdx) /* pt_regs->rip */
1221 UNWIND_HINT_IRET_REGS
1222 pushq $-1 /* pt_regs->orig_ax */
1223 PUSH_AND_CLEAR_REGS rdx=(%rdx)
1224 ENCODE_FRAME_POINTER
1230 * At this point we no longer need to worry about stack damage
1231 * due to nesting -- we're on the normal thread stack and we're
1232 * done with the NMI stack.
1240 * Return back to user mode. We must *not* do the normal exit
1241 * work, because we don't want to enable interrupts.
1243 jmp swapgs_restore_regs_and_return_to_usermode
1247 * Here's what our stack frame will look like:
1248 * +---------------------------------------------------------+
1250 * | original Return RSP |
1251 * | original RFLAGS |
1254 * +---------------------------------------------------------+
1255 * | temp storage for rdx |
1256 * +---------------------------------------------------------+
1257 * | "NMI executing" variable |
1258 * +---------------------------------------------------------+
1259 * | iret SS } Copied from "outermost" frame |
1260 * | iret Return RSP } on each loop iteration; overwritten |
1261 * | iret RFLAGS } by a nested NMI to force another |
1262 * | iret CS } iteration if needed. |
1264 * +---------------------------------------------------------+
1265 * | outermost SS } initialized in first_nmi; |
1266 * | outermost Return RSP } will not be changed before |
1267 * | outermost RFLAGS } NMI processing is done. |
1268 * | outermost CS } Copied to "iret" frame on each |
1269 * | outermost RIP } iteration. |
1270 * +---------------------------------------------------------+
1272 * +---------------------------------------------------------+
1274 * The "original" frame is used by hardware. Before re-enabling
1275 * NMIs, we need to be done with it, and we need to leave enough
1276 * space for the asm code here.
1278 * We return by executing IRET while RSP points to the "iret" frame.
1279 * That will either return for real or it will loop back into NMI
1282 * The "outermost" frame is copied to the "iret" frame on each
1283 * iteration of the loop, so each iteration starts with the "iret"
1284 * frame pointing to the final return target.
1288 * Determine whether we're a nested NMI.
1290 * If we interrupted kernel code between repeat_nmi and
1291 * end_repeat_nmi, then we are a nested NMI. We must not
1292 * modify the "iret" frame because it's being written by
1293 * the outer NMI. That's okay; the outer NMI handler is
1294 * about to about to call exc_nmi() anyway, so we can just
1295 * resume the outer NMI.
1298 movq $repeat_nmi, %rdx
1301 movq $end_repeat_nmi, %rdx
1307 * Now check "NMI executing". If it's set, then we're nested.
1308 * This will not detect if we interrupted an outer NMI just
1315 * Now test if the previous stack was an NMI stack. This covers
1316 * the case where we interrupt an outer NMI after it clears
1317 * "NMI executing" but before IRET. We need to be careful, though:
1318 * there is one case in which RSP could point to the NMI stack
1319 * despite there being no NMI active: naughty userspace controls
1320 * RSP at the very beginning of the SYSCALL targets. We can
1321 * pull a fast one on naughty userspace, though: we program
1322 * SYSCALL to mask DF, so userspace cannot cause DF to be set
1323 * if it controls the kernel's RSP. We set DF before we clear
1327 /* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
1328 cmpq %rdx, 4*8(%rsp)
1329 /* If the stack pointer is above the NMI stack, this is a normal NMI */
1332 subq $EXCEPTION_STKSZ, %rdx
1333 cmpq %rdx, 4*8(%rsp)
1334 /* If it is below the NMI stack, it is a normal NMI */
1337 /* Ah, it is within the NMI stack. */
1339 testb $(X86_EFLAGS_DF >> 8), (3*8 + 1)(%rsp)
1340 jz first_nmi /* RSP was user controlled. */
1342 /* This is a nested NMI. */
1346 * Modify the "iret" frame to point to repeat_nmi, forcing another
1347 * iteration of NMI handling.
1350 leaq -10*8(%rsp), %rdx
1357 /* Put stack back */
1363 /* We are returning to kernel mode, so this cannot result in a fault. */
1370 /* Make room for "NMI executing". */
1373 /* Leave room for the "iret" frame */
1376 /* Copy the "original" frame to the "outermost" frame */
1380 UNWIND_HINT_IRET_REGS
1382 /* Everything up to here is safe from nested NMIs */
1384 #ifdef CONFIG_DEBUG_ENTRY
1386 * For ease of testing, unmask NMIs right away. Disabled by
1387 * default because IRET is very expensive.
1390 pushq %rsp /* RSP (minus 8 because of the previous push) */
1391 addq $8, (%rsp) /* Fix up RSP */
1393 pushq $__KERNEL_CS /* CS */
1395 iretq /* continues at repeat_nmi below */
1396 UNWIND_HINT_IRET_REGS
1401 ANNOTATE_NOENDBR // this code
1403 * If there was a nested NMI, the first NMI's iret will return
1404 * here. But NMIs are still enabled and we can take another
1405 * nested NMI. The nested NMI checks the interrupted RIP to see
1406 * if it is between repeat_nmi and end_repeat_nmi, and if so
1407 * it will just return, as we are about to repeat an NMI anyway.
1408 * This makes it safe to copy to the stack frame that a nested
1411 * RSP is pointing to "outermost RIP". gsbase is unknown, but, if
1412 * we're repeating an NMI, gsbase has the same value that it had on
1413 * the first iteration. paranoid_entry will load the kernel
1414 * gsbase if needed before we call exc_nmi(). "NMI executing"
1417 movq $1, 10*8(%rsp) /* Set "NMI executing". */
1420 * Copy the "outermost" frame to the "iret" frame. NMIs that nest
1421 * here must not modify the "iret" frame while we're writing to
1422 * it or it will end up containing garbage.
1430 ANNOTATE_NOENDBR // this code
1433 * Everything below this point can be preempted by a nested NMI.
1434 * If this happens, then the inner NMI will change the "iret"
1435 * frame to point back to repeat_nmi.
1437 pushq $-1 /* ORIG_RAX: no syscall to restart */
1440 * Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
1441 * as we should not be calling schedule in NMI context.
1442 * Even with normal interrupts enabled. An NMI should not be
1443 * setting NEED_RESCHED or anything that normal interrupts and
1444 * exceptions might do.
1453 /* Always restore stashed SPEC_CTRL value (see paranoid_entry) */
1454 IBRS_EXIT save_reg=%r15
1456 /* Always restore stashed CR3 value (see paranoid_entry) */
1457 RESTORE_CR3 scratch_reg=%r15 save_reg=%r14
1460 * The above invocation of paranoid_entry stored the GSBASE
1461 * related information in R/EBX depending on the availability
1464 * If FSGSBASE is enabled, restore the saved GSBASE value
1465 * unconditionally, otherwise take the conditional SWAPGS path.
1467 ALTERNATIVE "jmp nmi_no_fsgsbase", "", X86_FEATURE_FSGSBASE
1473 /* EBX == 0 -> invoke SWAPGS */
1484 * Skip orig_ax and the "outermost" frame to point RSP at the "iret"
1485 * at the "iret" frame.
1490 * Clear "NMI executing". Set DF first so that we can easily
1491 * distinguish the remaining code between here and IRET from
1492 * the SYSCALL entry and exit paths.
1494 * We arguably should just inspect RIP instead, but I (Andy) wrote
1495 * this code when I had the misapprehension that Xen PV supported
1496 * NMIs, and Xen PV would break that approach.
1499 movq $0, 5*8(%rsp) /* clear "NMI executing" */
1502 * iretq reads the "iret" frame and exits the NMI stack in a
1503 * single instruction. We are returning to kernel mode, so this
1504 * cannot result in a fault. Similarly, we don't need to worry
1505 * about espfix64 on the way back to kernel mode.
1508 SYM_CODE_END(asm_exc_nmi)
1510 #ifndef CONFIG_IA32_EMULATION
1512 * This handles SYSCALL from 32-bit code. There is no way to program
1513 * MSRs to fully disable 32-bit SYSCALL.
1515 SYM_CODE_START(ignore_sysret)
1520 SYM_CODE_END(ignore_sysret)
1523 .pushsection .text, "ax"
1525 SYM_CODE_START_NOALIGN(rewind_stack_and_make_dead)
1527 /* Prevent any naive code from trying to unwind to our caller. */
1530 movq PER_CPU_VAR(pcpu_hot + X86_top_of_stack), %rax
1531 leaq -PTREGS_SIZE(%rax), %rsp
1535 SYM_CODE_END(rewind_stack_and_make_dead)