arch/x86/include/asm/nospec-branch.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2
   3 #ifndef _ASM_X86_NOSPEC_BRANCH_H_
   4 #define _ASM_X86_NOSPEC_BRANCH_H_
   5
   6 #include <linux/static_key.h>
   7 #include <linux/objtool.h>
   8 #include <linux/linkage.h>
   9
  10 #include <asm/alternative.h>
  11 #include <asm/cpufeatures.h>
  12 #include <asm/msr-index.h>
  13 #include <asm/unwind_hints.h>
  14 #include <asm/percpu.h>
  15 #include <asm/current.h>
  16
  17 /*
  18  * Call depth tracking for Intel SKL CPUs to address the RSB underflow
  19  * issue in software.
  20  *
  21  * The tracking does not use a counter. It uses uses arithmetic shift
  22  * right on call entry and logical shift left on return.
  23  *
  24  * The depth tracking variable is initialized to 0x8000.... when the call
  25  * depth is zero. The arithmetic shift right sign extends the MSB and
  26  * saturates after the 12th call. The shift count is 5 for both directions
  27  * so the tracking covers 12 nested calls.
  28  *
  29  *  Call
  30  *  0: 0x8000000000000000       0x0000000000000000
  31  *  1: 0xfc00000000000000       0xf000000000000000
  32  * ...
  33  * 11: 0xfffffffffffffff8       0xfffffffffffffc00
  34  * 12: 0xffffffffffffffff       0xffffffffffffffe0
  35  *
  36  * After a return buffer fill the depth is credited 12 calls before the
  37  * next stuffing has to take place.
  38  *
  39  * There is a inaccuracy for situations like this:
  40  *
  41  *  10 calls
  42  *   5 returns
  43  *   3 calls
  44  *   4 returns
  45  *   3 calls
  46  *   ....
  47  *
  48  * The shift count might cause this to be off by one in either direction,
  49  * but there is still a cushion vs. the RSB depth. The algorithm does not
  50  * claim to be perfect and it can be speculated around by the CPU, but it
  51  * is considered that it obfuscates the problem enough to make exploitation
  52  * extremely difficult.
  53  */
  54 #define RET_DEPTH_SHIFT                 5
  55 #define RSB_RET_STUFF_LOOPS             16
  56 #define RET_DEPTH_INIT                  0x8000000000000000ULL
  57 #define RET_DEPTH_INIT_FROM_CALL        0xfc00000000000000ULL
  58 #define RET_DEPTH_CREDIT                0xffffffffffffffffULL
  59
  60 #ifdef CONFIG_CALL_THUNKS_DEBUG
  61 # define CALL_THUNKS_DEBUG_INC_CALLS                            \
  62         incq    %gs:__x86_call_count;
  63 # define CALL_THUNKS_DEBUG_INC_RETS                             \
  64         incq    %gs:__x86_ret_count;
  65 # define CALL_THUNKS_DEBUG_INC_STUFFS                           \
  66         incq    %gs:__x86_stuffs_count;
  67 # define CALL_THUNKS_DEBUG_INC_CTXSW                            \
  68         incq    %gs:__x86_ctxsw_count;
  69 #else
  70 # define CALL_THUNKS_DEBUG_INC_CALLS
  71 # define CALL_THUNKS_DEBUG_INC_RETS
  72 # define CALL_THUNKS_DEBUG_INC_STUFFS
  73 # define CALL_THUNKS_DEBUG_INC_CTXSW
  74 #endif
  75
  76 #if defined(CONFIG_CALL_DEPTH_TRACKING) && !defined(COMPILE_OFFSETS)
  77
  78 #include <asm/asm-offsets.h>
  79
  80 #define CREDIT_CALL_DEPTH                                       \
  81         movq    $-1, PER_CPU_VAR(pcpu_hot + X86_call_depth);
  82
  83 #define ASM_CREDIT_CALL_DEPTH                                   \
  84         movq    $-1, PER_CPU_VAR(pcpu_hot + X86_call_depth);
  85
  86 #define RESET_CALL_DEPTH                                        \
  87         xor     %eax, %eax;                                     \
  88         bts     $63, %rax;                                      \
  89         movq    %rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);
  90
  91 #define RESET_CALL_DEPTH_FROM_CALL                              \
  92         movb    $0xfc, %al;                                     \
  93         shl     $56, %rax;                                      \
  94         movq    %rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);   \
  95         CALL_THUNKS_DEBUG_INC_CALLS
  96
  97 #define INCREMENT_CALL_DEPTH                                    \
  98         sarq    $5, %gs:pcpu_hot + X86_call_depth;              \
  99         CALL_THUNKS_DEBUG_INC_CALLS
 100
 101 #define ASM_INCREMENT_CALL_DEPTH                                \
 102         sarq    $5, PER_CPU_VAR(pcpu_hot + X86_call_depth);     \
 103         CALL_THUNKS_DEBUG_INC_CALLS
 104
 105 #else
 106 #define CREDIT_CALL_DEPTH
 107 #define ASM_CREDIT_CALL_DEPTH
 108 #define RESET_CALL_DEPTH
 109 #define INCREMENT_CALL_DEPTH
 110 #define ASM_INCREMENT_CALL_DEPTH
 111 #define RESET_CALL_DEPTH_FROM_CALL
 112 #endif
 113
 114 /*
 115  * Fill the CPU return stack buffer.
 116  *
 117  * Each entry in the RSB, if used for a speculative 'ret', contains an
 118  * infinite 'pause; lfence; jmp' loop to capture speculative execution.
 119  *
 120  * This is required in various cases for retpoline and IBRS-based
 121  * mitigations for the Spectre variant 2 vulnerability. Sometimes to
 122  * eliminate potentially bogus entries from the RSB, and sometimes
 123  * purely to ensure that it doesn't get empty, which on some CPUs would
 124  * allow predictions from other (unwanted!) sources to be used.
 125  *
 126  * We define a CPP macro such that it can be used from both .S files and
 127  * inline assembly. It's possible to do a .macro and then include that
 128  * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
 129  */
 130
 131 #define RETPOLINE_THUNK_SIZE    32
 132 #define RSB_CLEAR_LOOPS         32      /* To forcibly overwrite all entries */
 133
 134 /*
 135  * Common helper for __FILL_RETURN_BUFFER and __FILL_ONE_RETURN.
 136  */
 137 #define __FILL_RETURN_SLOT                      \
 138         ANNOTATE_INTRA_FUNCTION_CALL;           \
 139         call    772f;                           \
 140         int3;                                   \
 141 772:
 142
 143 /*
 144  * Stuff the entire RSB.
 145  *
 146  * Google experimented with loop-unrolling and this turned out to be
 147  * the optimal version - two calls, each with their own speculation
 148  * trap should their return address end up getting used, in a loop.
 149  */
 150 #ifdef CONFIG_X86_64
 151 #define __FILL_RETURN_BUFFER(reg, nr)                   \
 152         mov     $(nr/2), reg;                           \
 153 771:                                                    \
 154         __FILL_RETURN_SLOT                              \
 155         __FILL_RETURN_SLOT                              \
 156         add     $(BITS_PER_LONG/8) * 2, %_ASM_SP;       \
 157         dec     reg;                                    \
 158         jnz     771b;                                   \
 159         /* barrier for jnz misprediction */             \
 160         lfence;                                         \
 161         ASM_CREDIT_CALL_DEPTH                           \
 162         CALL_THUNKS_DEBUG_INC_CTXSW
 163 #else
 164 /*
 165  * i386 doesn't unconditionally have LFENCE, as such it can't
 166  * do a loop.
 167  */
 168 #define __FILL_RETURN_BUFFER(reg, nr)                   \
 169         .rept nr;                                       \
 170         __FILL_RETURN_SLOT;                             \
 171         .endr;                                          \
 172         add     $(BITS_PER_LONG/8) * nr, %_ASM_SP;
 173 #endif
 174
 175 /*
 176  * Stuff a single RSB slot.
 177  *
 178  * To mitigate Post-Barrier RSB speculation, one CALL instruction must be
 179  * forced to retire before letting a RET instruction execute.
 180  *
 181  * On PBRSB-vulnerable CPUs, it is not safe for a RET to be executed
 182  * before this point.
 183  */
 184 #define __FILL_ONE_RETURN                               \
 185         __FILL_RETURN_SLOT                              \
 186         add     $(BITS_PER_LONG/8), %_ASM_SP;           \
 187         lfence;
 188
 189 #ifdef __ASSEMBLY__
 190
 191 /*
 192  * This should be used immediately before an indirect jump/call. It tells
 193  * objtool the subsequent indirect jump/call is vouched safe for retpoline
 194  * builds.
 195  */
 196 .macro ANNOTATE_RETPOLINE_SAFE
 197 .Lhere_\@:
 198         .pushsection .discard.retpoline_safe
 199         .long .Lhere_\@
 200         .popsection
 201 .endm
 202
 203 /*
 204  * (ab)use RETPOLINE_SAFE on RET to annotate away 'bare' RET instructions
 205  * vs RETBleed validation.
 206  */
 207 #define ANNOTATE_UNRET_SAFE ANNOTATE_RETPOLINE_SAFE
 208
 209 /*
 210  * Abuse ANNOTATE_RETPOLINE_SAFE on a NOP to indicate UNRET_END, should
 211  * eventually turn into its own annotation.
 212  */
 213 .macro VALIDATE_UNRET_END
 214 #if defined(CONFIG_NOINSTR_VALIDATION) && \
 215         (defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_SRSO))
 216         ANNOTATE_RETPOLINE_SAFE
 217         nop
 218 #endif
 219 .endm
 220
 221 /*
 222  * Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call
 223  * to the retpoline thunk with a CS prefix when the register requires
 224  * a RAX prefix byte to encode. Also see apply_retpolines().
 225  */
 226 .macro __CS_PREFIX reg:req
 227         .irp rs,r8,r9,r10,r11,r12,r13,r14,r15
 228         .ifc \reg,\rs
 229         .byte 0x2e
 230         .endif
 231         .endr
 232 .endm
 233
 234 /*
 235  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
 236  * indirect jmp/call which may be susceptible to the Spectre variant 2
 237  * attack.
 238  *
 239  * NOTE: these do not take kCFI into account and are thus not comparable to C
 240  * indirect calls, take care when using. The target of these should be an ENDBR
 241  * instruction irrespective of kCFI.
 242  */
 243 .macro JMP_NOSPEC reg:req
 244 #ifdef CONFIG_RETPOLINE
 245         __CS_PREFIX \reg
 246         jmp     __x86_indirect_thunk_\reg
 247 #else
 248         jmp     *%\reg
 249         int3
 250 #endif
 251 .endm
 252
 253 .macro CALL_NOSPEC reg:req
 254 #ifdef CONFIG_RETPOLINE
 255         __CS_PREFIX \reg
 256         call    __x86_indirect_thunk_\reg
 257 #else
 258         call    *%\reg
 259 #endif
 260 .endm
 261
 262  /*
 263   * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
 264   * monstrosity above, manually.
 265   */
 266 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2=ALT_NOT(X86_FEATURE_ALWAYS)
 267         ALTERNATIVE_2 "jmp .Lskip_rsb_\@", \
 268                 __stringify(__FILL_RETURN_BUFFER(\reg,\nr)), \ftr, \
 269                 __stringify(nop;nop;__FILL_ONE_RETURN), \ftr2
 270
 271 .Lskip_rsb_\@:
 272 .endm
 273
 274 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_SRSO)
 275 #define CALL_UNTRAIN_RET        "call entry_untrain_ret"
 276 #else
 277 #define CALL_UNTRAIN_RET        ""
 278 #endif
 279
 280 /*
 281  * Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
 282  * return thunk isn't mapped into the userspace tables (then again, AMD
 283  * typically has NO_MELTDOWN).
 284  *
 285  * While retbleed_untrain_ret() doesn't clobber anything but requires stack,
 286  * entry_ibpb() will clobber AX, CX, DX.
 287  *
 288  * As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
 289  * where we have a stack but before any RET instruction.
 290  */
 291 .macro __UNTRAIN_RET ibpb_feature, call_depth_insns
 292 #if defined(CONFIG_RETHUNK) || defined(CONFIG_CPU_IBPB_ENTRY)
 293         VALIDATE_UNRET_END
 294         ALTERNATIVE_3 "",                                               \
 295                       CALL_UNTRAIN_RET, X86_FEATURE_UNRET,              \
 296                       "call entry_ibpb", \ibpb_feature,                 \
 297                      __stringify(\call_depth_insns), X86_FEATURE_CALL_DEPTH
 298 #endif
 299 .endm
 300
 301 #define UNTRAIN_RET \
 302         __UNTRAIN_RET X86_FEATURE_ENTRY_IBPB, __stringify(RESET_CALL_DEPTH)
 303
 304 #define UNTRAIN_RET_VM \
 305         __UNTRAIN_RET X86_FEATURE_IBPB_ON_VMEXIT, __stringify(RESET_CALL_DEPTH)
 306
 307 #define UNTRAIN_RET_FROM_CALL \
 308         __UNTRAIN_RET X86_FEATURE_ENTRY_IBPB, __stringify(RESET_CALL_DEPTH_FROM_CALL)
 309
 310
 311 .macro CALL_DEPTH_ACCOUNT
 312 #ifdef CONFIG_CALL_DEPTH_TRACKING
 313         ALTERNATIVE "",                                                 \
 314                     __stringify(ASM_INCREMENT_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
 315 #endif
 316 .endm
 317
 318 /*
 319  * Macro to execute VERW instruction that mitigate transient data sampling
 320  * attacks such as MDS. On affected systems a microcode update overloaded VERW
 321  * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
 322  *
 323  * Note: Only the memory operand variant of VERW clears the CPU buffers.
 324  */
 325 .macro CLEAR_CPU_BUFFERS
 326         ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
 327 .endm
 328
 329 #else /* __ASSEMBLY__ */
 330
 331 #define ANNOTATE_RETPOLINE_SAFE                                 \
 332         "999:\n\t"                                              \
 333         ".pushsection .discard.retpoline_safe\n\t"              \
 334         ".long 999b\n\t"                                        \
 335         ".popsection\n\t"
 336
 337 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
 338 extern retpoline_thunk_t __x86_indirect_thunk_array[];
 339 extern retpoline_thunk_t __x86_indirect_call_thunk_array[];
 340 extern retpoline_thunk_t __x86_indirect_jump_thunk_array[];
 341
 342 #ifdef CONFIG_RETHUNK
 343 extern void __x86_return_thunk(void);
 344 #else
 345 static inline void __x86_return_thunk(void) {}
 346 #endif
 347
 348 #ifdef CONFIG_CPU_UNRET_ENTRY
 349 extern void retbleed_return_thunk(void);
 350 #else
 351 static inline void retbleed_return_thunk(void) {}
 352 #endif
 353
 354 #ifdef CONFIG_CPU_SRSO
 355 extern void srso_return_thunk(void);
 356 extern void srso_alias_return_thunk(void);
 357 #else
 358 static inline void srso_return_thunk(void) {}
 359 static inline void srso_alias_return_thunk(void) {}
 360 #endif
 361
 362 extern void retbleed_return_thunk(void);
 363 extern void srso_return_thunk(void);
 364 extern void srso_alias_return_thunk(void);
 365
 366 extern void entry_untrain_ret(void);
 367 extern void entry_ibpb(void);
 368
 369 extern void (*x86_return_thunk)(void);
 370
 371 #ifdef CONFIG_CALL_DEPTH_TRACKING
 372 extern void call_depth_return_thunk(void);
 373
 374 #define CALL_DEPTH_ACCOUNT                                      \
 375         ALTERNATIVE("",                                         \
 376                     __stringify(INCREMENT_CALL_DEPTH),          \
 377                     X86_FEATURE_CALL_DEPTH)
 378
 379 #ifdef CONFIG_CALL_THUNKS_DEBUG
 380 DECLARE_PER_CPU(u64, __x86_call_count);
 381 DECLARE_PER_CPU(u64, __x86_ret_count);
 382 DECLARE_PER_CPU(u64, __x86_stuffs_count);
 383 DECLARE_PER_CPU(u64, __x86_ctxsw_count);
 384 #endif
 385 #else /* !CONFIG_CALL_DEPTH_TRACKING */
 386
 387 static inline void call_depth_return_thunk(void) {}
 388 #define CALL_DEPTH_ACCOUNT ""
 389
 390 #endif /* CONFIG_CALL_DEPTH_TRACKING */
 391
 392 #ifdef CONFIG_RETPOLINE
 393
 394 #define GEN(reg) \
 395         extern retpoline_thunk_t __x86_indirect_thunk_ ## reg;
 396 #include <asm/GEN-for-each-reg.h>
 397 #undef GEN
 398
 399 #define GEN(reg)                                                \
 400         extern retpoline_thunk_t __x86_indirect_call_thunk_ ## reg;
 401 #include <asm/GEN-for-each-reg.h>
 402 #undef GEN
 403
 404 #define GEN(reg)                                                \
 405         extern retpoline_thunk_t __x86_indirect_jump_thunk_ ## reg;
 406 #include <asm/GEN-for-each-reg.h>
 407 #undef GEN
 408
 409 #ifdef CONFIG_X86_64
 410
 411 /*
 412  * Inline asm uses the %V modifier which is only in newer GCC
 413  * which is ensured when CONFIG_RETPOLINE is defined.
 414  */
 415 # define CALL_NOSPEC                                            \
 416         ALTERNATIVE_2(                                          \
 417         ANNOTATE_RETPOLINE_SAFE                                 \
 418         "call *%[thunk_target]\n",                              \
 419         "call __x86_indirect_thunk_%V[thunk_target]\n",         \
 420         X86_FEATURE_RETPOLINE,                                  \
 421         "lfence;\n"                                             \
 422         ANNOTATE_RETPOLINE_SAFE                                 \
 423         "call *%[thunk_target]\n",                              \
 424         X86_FEATURE_RETPOLINE_LFENCE)
 425
 426 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 427
 428 #else /* CONFIG_X86_32 */
 429 /*
 430  * For i386 we use the original ret-equivalent retpoline, because
 431  * otherwise we'll run out of registers. We don't care about CET
 432  * here, anyway.
 433  */
 434 # define CALL_NOSPEC                                            \
 435         ALTERNATIVE_2(                                          \
 436         ANNOTATE_RETPOLINE_SAFE                                 \
 437         "call *%[thunk_target]\n",                              \
 438         "       jmp    904f;\n"                                 \
 439         "       .align 16\n"                                    \
 440         "901:   call   903f;\n"                                 \
 441         "902:   pause;\n"                                       \
 442         "       lfence;\n"                                      \
 443         "       jmp    902b;\n"                                 \
 444         "       .align 16\n"                                    \
 445         "903:   lea    4(%%esp), %%esp;\n"                      \
 446         "       pushl  %[thunk_target];\n"                      \
 447         "       ret;\n"                                         \
 448         "       .align 16\n"                                    \
 449         "904:   call   901b;\n",                                \
 450         X86_FEATURE_RETPOLINE,                                  \
 451         "lfence;\n"                                             \
 452         ANNOTATE_RETPOLINE_SAFE                                 \
 453         "call *%[thunk_target]\n",                              \
 454         X86_FEATURE_RETPOLINE_LFENCE)
 455
 456 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 457 #endif
 458 #else /* No retpoline for C / inline asm */
 459 # define CALL_NOSPEC "call *%[thunk_target]\n"
 460 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 461 #endif
 462
 463 /* The Spectre V2 mitigation variants */
 464 enum spectre_v2_mitigation {
 465         SPECTRE_V2_NONE,
 466         SPECTRE_V2_RETPOLINE,
 467         SPECTRE_V2_LFENCE,
 468         SPECTRE_V2_EIBRS,
 469         SPECTRE_V2_EIBRS_RETPOLINE,
 470         SPECTRE_V2_EIBRS_LFENCE,
 471         SPECTRE_V2_IBRS,
 472 };
 473
 474 /* The indirect branch speculation control variants */
 475 enum spectre_v2_user_mitigation {
 476         SPECTRE_V2_USER_NONE,
 477         SPECTRE_V2_USER_STRICT,
 478         SPECTRE_V2_USER_STRICT_PREFERRED,
 479         SPECTRE_V2_USER_PRCTL,
 480         SPECTRE_V2_USER_SECCOMP,
 481 };
 482
 483 /* The Speculative Store Bypass disable variants */
 484 enum ssb_mitigation {
 485         SPEC_STORE_BYPASS_NONE,
 486         SPEC_STORE_BYPASS_DISABLE,
 487         SPEC_STORE_BYPASS_PRCTL,
 488         SPEC_STORE_BYPASS_SECCOMP,
 489 };
 490
 491 static __always_inline
 492 void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 493 {
 494         asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
 495                 : : "c" (msr),
 496                     "a" ((u32)val),
 497                     "d" ((u32)(val >> 32)),
 498                     [feature] "i" (feature)
 499                 : "memory");
 500 }
 501
 502 extern u64 x86_pred_cmd;
 503
 504 static inline void indirect_branch_prediction_barrier(void)
 505 {
 506         alternative_msr_write(MSR_IA32_PRED_CMD, x86_pred_cmd, X86_FEATURE_USE_IBPB);
 507 }
 508
 509 /* The Intel SPEC CTRL MSR base value cache */
 510 extern u64 x86_spec_ctrl_base;
 511 DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
 512 extern void update_spec_ctrl_cond(u64 val);
 513 extern u64 spec_ctrl_current(void);
 514
 515 /*
 516  * With retpoline, we must use IBRS to restrict branch prediction
 517  * before calling into firmware.
 518  *
 519  * (Implemented as CPP macros due to header hell.)
 520  */
 521 #define firmware_restrict_branch_speculation_start()                    \
 522 do {                                                                    \
 523         preempt_disable();                                              \
 524         alternative_msr_write(MSR_IA32_SPEC_CTRL,                       \
 525                               spec_ctrl_current() | SPEC_CTRL_IBRS,     \
 526                               X86_FEATURE_USE_IBRS_FW);                 \
 527         alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB,         \
 528                               X86_FEATURE_USE_IBPB_FW);                 \
 529 } while (0)
 530
 531 #define firmware_restrict_branch_speculation_end()                      \
 532 do {                                                                    \
 533         alternative_msr_write(MSR_IA32_SPEC_CTRL,                       \
 534                               spec_ctrl_current(),                      \
 535                               X86_FEATURE_USE_IBRS_FW);                 \
 536         preempt_enable();                                               \
 537 } while (0)
 538
 539 DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
 540 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 541 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 542
 543 DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
 544
 545 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 546
 547 DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
 548
 549 extern u16 mds_verw_sel;
 550
 551 #include <asm/segment.h>
 552
 553 /**
 554  * mds_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
 555  *
 556  * This uses the otherwise unused and obsolete VERW instruction in
 557  * combination with microcode which triggers a CPU buffer flush when the
 558  * instruction is executed.
 559  */
 560 static __always_inline void mds_clear_cpu_buffers(void)
 561 {
 562         static const u16 ds = __KERNEL_DS;
 563
 564         /*
 565          * Has to be the memory-operand variant because only that
 566          * guarantees the CPU buffer flush functionality according to
 567          * documentation. The register-operand variant does not.
 568          * Works with any segment selector, but a valid writable
 569          * data segment is the fastest variant.
 570          *
 571          * "cc" clobber is required because VERW modifies ZF.
 572          */
 573         asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
 574 }
 575
 576 /**
 577  * mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
 578  *
 579  * Clear CPU buffers if the corresponding static key is enabled
 580  */
 581 static __always_inline void mds_idle_clear_cpu_buffers(void)
 582 {
 583         if (static_branch_likely(&mds_idle_clear))
 584                 mds_clear_cpu_buffers();
 585 }
 586
 587 #endif /* __ASSEMBLY__ */
 588
 589 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */