x86/entry/64: Fix an IRQ state error on ptregs-using syscalls

I messed up the IRQ state when jumping off the fast path due to
invocation of a ptregs-using syscall.  This bug shouldn't have
had any impact yet, but it would have caused problems with
subsequent context tracking cleanups.

Reported-and-tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1e423bff959e x86/entry/64: ("Migrate the 64-bit syscall slow path to C")
Link: http://lkml.kernel.org/r/ab92cd365fb7b0a56869e920017790d96610fdca.1454261517.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 567aa522ac0afd25c7ff9bf52e636b07d730f7e3..9f7bb808035efd1ffc0bbdefa264bf4f83279f94 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -191,8 +191,8 @@ entry_SYSCALL_64_fastpath:
 
        /*
         * This call instruction is handled specially in stub_ptregs_64.
-        * It might end up jumping to the slow path.  If it jumps, RAX is
-        * clobbered.
+        * It might end up jumping to the slow path.  If it jumps, RAX
+        * and all argument registers are clobbered.
         */
        call    *sys_call_table(, %rax, 8)
 .Lentry_SYSCALL_64_after_fastpath_call:
@@ -315,17 +315,24 @@ END(entry_SYSCALL_64)
 ENTRY(stub_ptregs_64)
        /*
         * Syscalls marked as needing ptregs land here.
-        * If we are on the fast path, we need to save the extra regs.
-        * If we are on the slow path, the extra regs are already saved.
+        * If we are on the fast path, we need to save the extra regs,
+        * which we achieve by trying again on the slow path.  If we are on
+        * the slow path, the extra regs are already saved.
         *
         * RAX stores a pointer to the C function implementing the syscall.
+        * IRQs are on.
         */
        cmpq    $.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
        jne     1f
 
-       /* Called from fast path -- pop return address and jump to slow path */
+       /*
+        * Called from fast path -- disable IRQs again, pop return address
+        * and jump to slow path
+        */
+       DISABLE_INTERRUPTS(CLBR_NONE)
+       TRACE_IRQS_OFF
        popq    %rax
-       jmp     entry_SYSCALL64_slow_path       /* called from fast path */
+       jmp     entry_SYSCALL64_slow_path
 
 1:
        /* Called from C */