1 /* SPDX-License-Identifier: GPL-2.0 */
3 #ifndef _ASM_X86_NOSPEC_BRANCH_H_
4 #define _ASM_X86_NOSPEC_BRANCH_H_
6 #include <linux/static_key.h>
7 #include <linux/objtool.h>
8 #include <linux/linkage.h>
10 #include <asm/alternative.h>
11 #include <asm/cpufeatures.h>
12 #include <asm/msr-index.h>
13 #include <asm/unwind_hints.h>
15 #define RETPOLINE_THUNK_SIZE 32
18 * Fill the CPU return stack buffer.
20 * Each entry in the RSB, if used for a speculative 'ret', contains an
21 * infinite 'pause; lfence; jmp' loop to capture speculative execution.
23 * This is required in various cases for retpoline and IBRS-based
24 * mitigations for the Spectre variant 2 vulnerability. Sometimes to
25 * eliminate potentially bogus entries from the RSB, and sometimes
26 * purely to ensure that it doesn't get empty, which on some CPUs would
27 * allow predictions from other (unwanted!) sources to be used.
29 * We define a CPP macro such that it can be used from both .S files and
30 * inline assembly. It's possible to do a .macro and then include that
31 * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
34 #define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */
37 * Google experimented with loop-unrolling and this turned out to be
38 * the optimal version - two calls, each with their own speculation
39 * trap should their return address end up getting used, in a loop.
41 #define __FILL_RETURN_BUFFER(reg, nr, sp) \
44 ANNOTATE_INTRA_FUNCTION_CALL; \
46 773: /* speculation trap */ \
52 ANNOTATE_INTRA_FUNCTION_CALL; \
54 775: /* speculation trap */ \
60 add $(BITS_PER_LONG/8) * 2, sp; \
67 * This should be used immediately before an indirect jump/call. It tells
68 * objtool the subsequent indirect jump/call is vouched safe for retpoline
71 .macro ANNOTATE_RETPOLINE_SAFE
73 .pushsection .discard.retpoline_safe
74 _ASM_PTR .Lannotate_\@
79 * (ab)use RETPOLINE_SAFE on RET to annotate away 'bare' RET instructions
80 * vs RETBleed validation.
82 #define ANNOTATE_UNRET_SAFE ANNOTATE_RETPOLINE_SAFE
85 * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
86 * indirect jmp/call which may be susceptible to the Spectre variant 2
89 .macro JMP_NOSPEC reg:req
90 #ifdef CONFIG_RETPOLINE
91 ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
92 __stringify(jmp __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
93 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), X86_FEATURE_RETPOLINE_LFENCE
99 .macro CALL_NOSPEC reg:req
100 #ifdef CONFIG_RETPOLINE
101 ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *%\reg), \
102 __stringify(call __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
103 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *%\reg), X86_FEATURE_RETPOLINE_LFENCE
110 * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
111 * monstrosity above, manually.
113 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
114 #ifdef CONFIG_RETPOLINE
115 ALTERNATIVE "jmp .Lskip_rsb_\@", "", \ftr
116 __FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)
122 * Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
123 * return thunk isn't mapped into the userspace tables (then again, AMD
124 * typically has NO_MELTDOWN).
126 * Doesn't clobber any registers but does require a stable stack.
128 * As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
129 * where we have a stack but before any RET instruction.
132 #ifdef CONFIG_RETPOLINE
133 ALTERNATIVE "", "call zen_untrain_ret", X86_FEATURE_UNRET
137 #else /* __ASSEMBLY__ */
139 #define ANNOTATE_RETPOLINE_SAFE \
141 ".pushsection .discard.retpoline_safe\n\t" \
142 _ASM_PTR " 999b\n\t" \
145 extern void __x86_return_thunk(void);
146 extern void zen_untrain_ret(void);
148 #ifdef CONFIG_RETPOLINE
150 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
153 extern retpoline_thunk_t __x86_indirect_thunk_ ## reg;
154 #include <asm/GEN-for-each-reg.h>
157 extern retpoline_thunk_t __x86_indirect_thunk_array[];
162 * Inline asm uses the %V modifier which is only in newer GCC
163 * which is ensured when CONFIG_RETPOLINE is defined.
165 # define CALL_NOSPEC \
167 ANNOTATE_RETPOLINE_SAFE \
168 "call *%[thunk_target]\n", \
169 "call __x86_indirect_thunk_%V[thunk_target]\n", \
170 X86_FEATURE_RETPOLINE, \
172 ANNOTATE_RETPOLINE_SAFE \
173 "call *%[thunk_target]\n", \
174 X86_FEATURE_RETPOLINE_LFENCE)
176 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
178 #else /* CONFIG_X86_32 */
180 * For i386 we use the original ret-equivalent retpoline, because
181 * otherwise we'll run out of registers. We don't care about CET
184 # define CALL_NOSPEC \
186 ANNOTATE_RETPOLINE_SAFE \
187 "call *%[thunk_target]\n", \
190 "901: call 903f;\n" \
195 "903: lea 4(%%esp), %%esp;\n" \
196 " pushl %[thunk_target];\n" \
199 "904: call 901b;\n", \
200 X86_FEATURE_RETPOLINE, \
202 ANNOTATE_RETPOLINE_SAFE \
203 "call *%[thunk_target]\n", \
204 X86_FEATURE_RETPOLINE_LFENCE)
206 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
208 #else /* No retpoline for C / inline asm */
209 # define CALL_NOSPEC "call *%[thunk_target]\n"
210 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
213 /* The Spectre V2 mitigation variants */
214 enum spectre_v2_mitigation {
216 SPECTRE_V2_RETPOLINE,
219 SPECTRE_V2_EIBRS_RETPOLINE,
220 SPECTRE_V2_EIBRS_LFENCE,
224 /* The indirect branch speculation control variants */
225 enum spectre_v2_user_mitigation {
226 SPECTRE_V2_USER_NONE,
227 SPECTRE_V2_USER_STRICT,
228 SPECTRE_V2_USER_STRICT_PREFERRED,
229 SPECTRE_V2_USER_PRCTL,
230 SPECTRE_V2_USER_SECCOMP,
233 /* The Speculative Store Bypass disable variants */
234 enum ssb_mitigation {
235 SPEC_STORE_BYPASS_NONE,
236 SPEC_STORE_BYPASS_DISABLE,
237 SPEC_STORE_BYPASS_PRCTL,
238 SPEC_STORE_BYPASS_SECCOMP,
241 extern char __indirect_thunk_start[];
242 extern char __indirect_thunk_end[];
244 static __always_inline
245 void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
247 asm volatile(ALTERNATIVE("", "wrmsr", %c[feature])
250 "d" ((u32)(val >> 32)),
251 [feature] "i" (feature)
255 static inline void indirect_branch_prediction_barrier(void)
257 u64 val = PRED_CMD_IBPB;
259 alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
262 /* The Intel SPEC CTRL MSR base value cache */
263 extern u64 x86_spec_ctrl_base;
264 extern void write_spec_ctrl_current(u64 val, bool force);
265 extern u64 spec_ctrl_current(void);
268 * With retpoline, we must use IBRS to restrict branch prediction
269 * before calling into firmware.
271 * (Implemented as CPP macros due to header hell.)
273 #define firmware_restrict_branch_speculation_start() \
275 u64 val = x86_spec_ctrl_base | SPEC_CTRL_IBRS; \
278 alternative_msr_write(MSR_IA32_SPEC_CTRL, val, \
279 X86_FEATURE_USE_IBRS_FW); \
282 #define firmware_restrict_branch_speculation_end() \
284 u64 val = x86_spec_ctrl_base; \
286 alternative_msr_write(MSR_IA32_SPEC_CTRL, val, \
287 X86_FEATURE_USE_IBRS_FW); \
291 DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
292 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
293 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
295 DECLARE_STATIC_KEY_FALSE(mds_user_clear);
296 DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
298 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
300 DECLARE_STATIC_KEY_FALSE(mmio_stale_data_clear);
302 #include <asm/segment.h>
305 * mds_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
307 * This uses the otherwise unused and obsolete VERW instruction in
308 * combination with microcode which triggers a CPU buffer flush when the
309 * instruction is executed.
311 static __always_inline void mds_clear_cpu_buffers(void)
313 static const u16 ds = __KERNEL_DS;
316 * Has to be the memory-operand variant because only that
317 * guarantees the CPU buffer flush functionality according to
318 * documentation. The register-operand variant does not.
319 * Works with any segment selector, but a valid writable
320 * data segment is the fastest variant.
322 * "cc" clobber is required because VERW modifies ZF.
324 asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
328 * mds_user_clear_cpu_buffers - Mitigation for MDS and TAA vulnerability
330 * Clear CPU buffers if the corresponding static key is enabled
332 static __always_inline void mds_user_clear_cpu_buffers(void)
334 if (static_branch_likely(&mds_user_clear))
335 mds_clear_cpu_buffers();
339 * mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
341 * Clear CPU buffers if the corresponding static key is enabled
343 static inline void mds_idle_clear_cpu_buffers(void)
345 if (static_branch_likely(&mds_idle_clear))
346 mds_clear_cpu_buffers();
349 #endif /* __ASSEMBLY__ */
351 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */