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 * extremly 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 it's 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 /*
275 * The CALL to srso_alias_untrain_ret() must be patched in directly at
276 * the spot where untraining must be done, ie., srso_alias_untrain_ret()
277 * must be the target of a CALL instruction instead of indirectly
278 * jumping to a wrapper which then calls it. Therefore, this macro is
279 * called outside of __UNTRAIN_RET below, for the time being, before the
280 * kernel can support nested alternatives with arbitrary nesting.
281 */
282 .macro CALL_UNTRAIN_RET
283 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_SRSO)
284 ALTERNATIVE_2 "", "call entry_untrain_ret", X86_FEATURE_UNRET, \
285 "call srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
286 #endif
287 .endm
288
289 /*
290 * Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
291 * return thunk isn't mapped into the userspace tables (then again, AMD
292 * typically has NO_MELTDOWN).
293 *
294 * While retbleed_untrain_ret() doesn't clobber anything but requires stack,
295 * entry_ibpb() will clobber AX, CX, DX.
296 *
297 * As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
298 * where we have a stack but before any RET instruction.
299 */
300 .macro __UNTRAIN_RET ibpb_feature, call_depth_insns
301 #if defined(CONFIG_RETHUNK) || defined(CONFIG_CPU_IBPB_ENTRY)
302 VALIDATE_UNRET_END
303 CALL_UNTRAIN_RET
304 ALTERNATIVE_2 "", \
305 "call entry_ibpb", \ibpb_feature, \
306 __stringify(\call_depth_insns), X86_FEATURE_CALL_DEPTH
307 #endif
308 .endm
309
310 #define UNTRAIN_RET \
311 __UNTRAIN_RET X86_FEATURE_ENTRY_IBPB, __stringify(RESET_CALL_DEPTH)
312
313 #define UNTRAIN_RET_VM \
314 __UNTRAIN_RET X86_FEATURE_IBPB_ON_VMEXIT, __stringify(RESET_CALL_DEPTH)
315
316 #define UNTRAIN_RET_FROM_CALL \
317 __UNTRAIN_RET X86_FEATURE_ENTRY_IBPB, __stringify(RESET_CALL_DEPTH_FROM_CALL)
318
319
320 .macro CALL_DEPTH_ACCOUNT
321 #ifdef CONFIG_CALL_DEPTH_TRACKING
322 ALTERNATIVE "", \
323 __stringify(ASM_INCREMENT_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
324 #endif
325 .endm
326
327 /*
328 * Macro to execute VERW instruction that mitigate transient data sampling
329 * attacks such as MDS. On affected systems a microcode update overloaded VERW
330 * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
331 *
332 * Note: Only the memory operand variant of VERW clears the CPU buffers.
333 */
334 .macro CLEAR_CPU_BUFFERS
335 ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
336 .endm
337
338 #ifdef CONFIG_X86_64
339 .macro CLEAR_BRANCH_HISTORY
340 ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP
341 .endm
342
343 .macro CLEAR_BRANCH_HISTORY_VMEXIT
344 ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT
345 .endm
346 #else
347 #define CLEAR_BRANCH_HISTORY
348 #define CLEAR_BRANCH_HISTORY_VMEXIT
349 #endif
350
351 #else /* __ASSEMBLY__ */
352
353 #define ANNOTATE_RETPOLINE_SAFE \
354 "999:\n\t" \
355 ".pushsection .discard.retpoline_safe\n\t" \
356 ".long 999b\n\t" \
357 ".popsection\n\t"
358
359 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
360 extern retpoline_thunk_t __x86_indirect_thunk_array[];
361 extern retpoline_thunk_t __x86_indirect_call_thunk_array[];
362 extern retpoline_thunk_t __x86_indirect_jump_thunk_array[];
363
364 #ifdef CONFIG_RETHUNK
365 extern void __x86_return_thunk(void);
366 #else
__x86_return_thunk(void)367 static inline void __x86_return_thunk(void) {}
368 #endif
369
370 #ifdef CONFIG_CPU_UNRET_ENTRY
371 extern void retbleed_return_thunk(void);
372 #else
retbleed_return_thunk(void)373 static inline void retbleed_return_thunk(void) {}
374 #endif
375
376 extern void srso_alias_untrain_ret(void);
377
378 #ifdef CONFIG_CPU_SRSO
379 extern void srso_return_thunk(void);
380 extern void srso_alias_return_thunk(void);
381 #else
srso_return_thunk(void)382 static inline void srso_return_thunk(void) {}
srso_alias_return_thunk(void)383 static inline void srso_alias_return_thunk(void) {}
384 #endif
385
386 extern void retbleed_return_thunk(void);
387 extern void srso_return_thunk(void);
388 extern void srso_alias_return_thunk(void);
389
390 extern void retbleed_untrain_ret(void);
391 extern void srso_untrain_ret(void);
392 extern void srso_alias_untrain_ret(void);
393
394 extern void entry_untrain_ret(void);
395 extern void entry_ibpb(void);
396
397 #ifdef CONFIG_X86_64
398 extern void clear_bhb_loop(void);
399 #endif
400
401 extern void (*x86_return_thunk)(void);
402
403 #ifdef CONFIG_CALL_DEPTH_TRACKING
404 extern void __x86_return_skl(void);
405
x86_set_skl_return_thunk(void)406 static inline void x86_set_skl_return_thunk(void)
407 {
408 x86_return_thunk = &__x86_return_skl;
409 }
410
411 #define CALL_DEPTH_ACCOUNT \
412 ALTERNATIVE("", \
413 __stringify(INCREMENT_CALL_DEPTH), \
414 X86_FEATURE_CALL_DEPTH)
415
416 #ifdef CONFIG_CALL_THUNKS_DEBUG
417 DECLARE_PER_CPU(u64, __x86_call_count);
418 DECLARE_PER_CPU(u64, __x86_ret_count);
419 DECLARE_PER_CPU(u64, __x86_stuffs_count);
420 DECLARE_PER_CPU(u64, __x86_ctxsw_count);
421 #endif
422 #else
x86_set_skl_return_thunk(void)423 static inline void x86_set_skl_return_thunk(void) {}
424
425 #define CALL_DEPTH_ACCOUNT ""
426
427 #endif
428
429 #ifdef CONFIG_RETPOLINE
430
431 #define GEN(reg) \
432 extern retpoline_thunk_t __x86_indirect_thunk_ ## reg;
433 #include <asm/GEN-for-each-reg.h>
434 #undef GEN
435
436 #define GEN(reg) \
437 extern retpoline_thunk_t __x86_indirect_call_thunk_ ## reg;
438 #include <asm/GEN-for-each-reg.h>
439 #undef GEN
440
441 #define GEN(reg) \
442 extern retpoline_thunk_t __x86_indirect_jump_thunk_ ## reg;
443 #include <asm/GEN-for-each-reg.h>
444 #undef GEN
445
446 #ifdef CONFIG_X86_64
447
448 /*
449 * Inline asm uses the %V modifier which is only in newer GCC
450 * which is ensured when CONFIG_RETPOLINE is defined.
451 */
452 # define CALL_NOSPEC \
453 ALTERNATIVE_2( \
454 ANNOTATE_RETPOLINE_SAFE \
455 "call *%[thunk_target]\n", \
456 "call __x86_indirect_thunk_%V[thunk_target]\n", \
457 X86_FEATURE_RETPOLINE, \
458 "lfence;\n" \
459 ANNOTATE_RETPOLINE_SAFE \
460 "call *%[thunk_target]\n", \
461 X86_FEATURE_RETPOLINE_LFENCE)
462
463 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
464
465 #else /* CONFIG_X86_32 */
466 /*
467 * For i386 we use the original ret-equivalent retpoline, because
468 * otherwise we'll run out of registers. We don't care about CET
469 * here, anyway.
470 */
471 # define CALL_NOSPEC \
472 ALTERNATIVE_2( \
473 ANNOTATE_RETPOLINE_SAFE \
474 "call *%[thunk_target]\n", \
475 " jmp 904f;\n" \
476 " .align 16\n" \
477 "901: call 903f;\n" \
478 "902: pause;\n" \
479 " lfence;\n" \
480 " jmp 902b;\n" \
481 " .align 16\n" \
482 "903: lea 4(%%esp), %%esp;\n" \
483 " pushl %[thunk_target];\n" \
484 " ret;\n" \
485 " .align 16\n" \
486 "904: call 901b;\n", \
487 X86_FEATURE_RETPOLINE, \
488 "lfence;\n" \
489 ANNOTATE_RETPOLINE_SAFE \
490 "call *%[thunk_target]\n", \
491 X86_FEATURE_RETPOLINE_LFENCE)
492
493 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
494 #endif
495 #else /* No retpoline for C / inline asm */
496 # define CALL_NOSPEC "call *%[thunk_target]\n"
497 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
498 #endif
499
500 /* The Spectre V2 mitigation variants */
501 enum spectre_v2_mitigation {
502 SPECTRE_V2_NONE,
503 SPECTRE_V2_RETPOLINE,
504 SPECTRE_V2_LFENCE,
505 SPECTRE_V2_EIBRS,
506 SPECTRE_V2_EIBRS_RETPOLINE,
507 SPECTRE_V2_EIBRS_LFENCE,
508 SPECTRE_V2_IBRS,
509 };
510
511 /* The indirect branch speculation control variants */
512 enum spectre_v2_user_mitigation {
513 SPECTRE_V2_USER_NONE,
514 SPECTRE_V2_USER_STRICT,
515 SPECTRE_V2_USER_STRICT_PREFERRED,
516 SPECTRE_V2_USER_PRCTL,
517 SPECTRE_V2_USER_SECCOMP,
518 };
519
520 /* The Speculative Store Bypass disable variants */
521 enum ssb_mitigation {
522 SPEC_STORE_BYPASS_NONE,
523 SPEC_STORE_BYPASS_DISABLE,
524 SPEC_STORE_BYPASS_PRCTL,
525 SPEC_STORE_BYPASS_SECCOMP,
526 };
527
528 static __always_inline
alternative_msr_write(unsigned int msr,u64 val,unsigned int feature)529 void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
530 {
531 asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
532 : : "c" (msr),
533 "a" ((u32)val),
534 "d" ((u32)(val >> 32)),
535 [feature] "i" (feature)
536 : "memory");
537 }
538
539 extern u64 x86_pred_cmd;
540
indirect_branch_prediction_barrier(void)541 static inline void indirect_branch_prediction_barrier(void)
542 {
543 alternative_msr_write(MSR_IA32_PRED_CMD, x86_pred_cmd, X86_FEATURE_USE_IBPB);
544 }
545
546 /* The Intel SPEC CTRL MSR base value cache */
547 extern u64 x86_spec_ctrl_base;
548 DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
549 extern void update_spec_ctrl_cond(u64 val);
550 extern u64 spec_ctrl_current(void);
551
552 /*
553 * With retpoline, we must use IBRS to restrict branch prediction
554 * before calling into firmware.
555 *
556 * (Implemented as CPP macros due to header hell.)
557 */
558 #define firmware_restrict_branch_speculation_start() \
559 do { \
560 preempt_disable(); \
561 alternative_msr_write(MSR_IA32_SPEC_CTRL, \
562 spec_ctrl_current() | SPEC_CTRL_IBRS, \
563 X86_FEATURE_USE_IBRS_FW); \
564 alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, \
565 X86_FEATURE_USE_IBPB_FW); \
566 } while (0)
567
568 #define firmware_restrict_branch_speculation_end() \
569 do { \
570 alternative_msr_write(MSR_IA32_SPEC_CTRL, \
571 spec_ctrl_current(), \
572 X86_FEATURE_USE_IBRS_FW); \
573 preempt_enable(); \
574 } while (0)
575
576 DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
577 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
578 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
579
580 DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
581
582 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
583
584 DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
585
586 extern u16 mds_verw_sel;
587
588 #include <asm/segment.h>
589
590 /**
591 * mds_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
592 *
593 * This uses the otherwise unused and obsolete VERW instruction in
594 * combination with microcode which triggers a CPU buffer flush when the
595 * instruction is executed.
596 */
mds_clear_cpu_buffers(void)597 static __always_inline void mds_clear_cpu_buffers(void)
598 {
599 static const u16 ds = __KERNEL_DS;
600
601 /*
602 * Has to be the memory-operand variant because only that
603 * guarantees the CPU buffer flush functionality according to
604 * documentation. The register-operand variant does not.
605 * Works with any segment selector, but a valid writable
606 * data segment is the fastest variant.
607 *
608 * "cc" clobber is required because VERW modifies ZF.
609 */
610 asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
611 }
612
613 /**
614 * mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
615 *
616 * Clear CPU buffers if the corresponding static key is enabled
617 */
mds_idle_clear_cpu_buffers(void)618 static __always_inline void mds_idle_clear_cpu_buffers(void)
619 {
620 if (static_branch_likely(&mds_idle_clear))
621 mds_clear_cpu_buffers();
622 }
623
624 #endif /* __ASSEMBLY__ */
625
626 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */
627