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1 /* SPDX-License-Identifier: GPL-2.0 */
3 #ifndef _ASM_X86_NOSPEC_BRANCH_H_
4 #define _ASM_X86_NOSPEC_BRANCH_H_
6 #include <asm/alternative.h>
7 #include <asm/alternative-asm.h>
8 #include <asm/cpufeatures.h>
9 #include <asm/msr-index.h>
12 * Fill the CPU return stack buffer.
14 * Each entry in the RSB, if used for a speculative 'ret', contains an
15 * infinite 'pause; lfence; jmp' loop to capture speculative execution.
17 * This is required in various cases for retpoline and IBRS-based
18 * mitigations for the Spectre variant 2 vulnerability. Sometimes to
19 * eliminate potentially bogus entries from the RSB, and sometimes
20 * purely to ensure that it doesn't get empty, which on some CPUs would
21 * allow predictions from other (unwanted!) sources to be used.
23 * We define a CPP macro such that it can be used from both .S files and
24 * inline assembly. It's possible to do a .macro and then include that
25 * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
28 #define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */
29 #define RSB_FILL_LOOPS 16 /* To avoid underflow */
32 * Google experimented with loop-unrolling and this turned out to be
33 * the optimal version — two calls, each with their own speculation
34 * trap should their return address end up getting used, in a loop.
36 #define __FILL_RETURN_BUFFER(reg, nr, sp) \
37 mov $(nr/2), reg; \
38 771: \
39 call 772f; \
40 773: /* speculation trap */ \
41 pause; \
42 lfence; \
43 jmp 773b; \
44 772: \
45 call 774f; \
46 775: /* speculation trap */ \
47 pause; \
48 lfence; \
49 jmp 775b; \
50 774: \
51 dec reg; \
52 jnz 771b; \
53 add $(BITS_PER_LONG/8) * nr, sp;
55 #ifdef __ASSEMBLY__
58 * This should be used immediately before a retpoline alternative. It tells
59 * objtool where the retpolines are so that it can make sense of the control
60 * flow by just reading the original instruction(s) and ignoring the
61 * alternatives.
63 .macro ANNOTATE_NOSPEC_ALTERNATIVE
64 .Lannotate_\@:
65 .pushsection .discard.nospec
66 .long .Lannotate_\@ - .
67 .popsection
68 .endm
71 * This should be used immediately before an indirect jump/call. It tells
72 * objtool the subsequent indirect jump/call is vouched safe for retpoline
73 * builds.
75 .macro ANNOTATE_RETPOLINE_SAFE
76 .Lannotate_\@:
77 .pushsection .discard.retpoline_safe
78 _ASM_PTR .Lannotate_\@
79 .popsection
80 .endm
83 * These are the bare retpoline primitives for indirect jmp and call.
84 * Do not use these directly; they only exist to make the ALTERNATIVE
85 * invocation below less ugly.
87 .macro RETPOLINE_JMP reg:req
88 call .Ldo_rop_\@
89 .Lspec_trap_\@:
90 pause
91 lfence
92 jmp .Lspec_trap_\@
93 .Ldo_rop_\@:
94 mov \reg, (%_ASM_SP)
95 ret
96 .endm
99 * This is a wrapper around RETPOLINE_JMP so the called function in reg
100 * returns to the instruction after the macro.
102 .macro RETPOLINE_CALL reg:req
103 jmp .Ldo_call_\@
104 .Ldo_retpoline_jmp_\@:
105 RETPOLINE_JMP \reg
106 .Ldo_call_\@:
107 call .Ldo_retpoline_jmp_\@
108 .endm
111 * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
112 * indirect jmp/call which may be susceptible to the Spectre variant 2
113 * attack.
115 .macro JMP_NOSPEC reg:req
116 #ifdef CONFIG_RETPOLINE
117 ANNOTATE_NOSPEC_ALTERNATIVE
118 ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *\reg), \
119 __stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
120 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
121 #else
122 jmp *\reg
123 #endif
124 .endm
126 .macro CALL_NOSPEC reg:req
127 #ifdef CONFIG_RETPOLINE
128 ANNOTATE_NOSPEC_ALTERNATIVE
129 ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *\reg), \
130 __stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
131 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *\reg), X86_FEATURE_RETPOLINE_AMD
132 #else
133 call *\reg
134 #endif
135 .endm
138 * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
139 * monstrosity above, manually.
141 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
142 #ifdef CONFIG_RETPOLINE
143 ANNOTATE_NOSPEC_ALTERNATIVE
144 ALTERNATIVE "jmp .Lskip_rsb_\@", \
145 __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)) \
146 \ftr
147 .Lskip_rsb_\@:
148 #endif
149 .endm
151 #else /* __ASSEMBLY__ */
153 #define ANNOTATE_NOSPEC_ALTERNATIVE \
154 "999:\n\t" \
155 ".pushsection .discard.nospec\n\t" \
156 ".long 999b - .\n\t" \
157 ".popsection\n\t"
159 #define ANNOTATE_RETPOLINE_SAFE \
160 "999:\n\t" \
161 ".pushsection .discard.retpoline_safe\n\t" \
162 _ASM_PTR " 999b\n\t" \
163 ".popsection\n\t"
165 #if defined(CONFIG_X86_64) && defined(RETPOLINE)
168 * Since the inline asm uses the %V modifier which is only in newer GCC,
169 * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
171 # define CALL_NOSPEC \
172 ANNOTATE_NOSPEC_ALTERNATIVE \
173 ALTERNATIVE( \
174 ANNOTATE_RETPOLINE_SAFE \
175 "call *%[thunk_target]\n", \
176 "call __x86_indirect_thunk_%V[thunk_target]\n", \
177 X86_FEATURE_RETPOLINE)
178 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
180 #elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
182 * For i386 we use the original ret-equivalent retpoline, because
183 * otherwise we'll run out of registers. We don't care about CET
184 * here, anyway.
186 # define CALL_NOSPEC ALTERNATIVE("call *%[thunk_target]\n", \
187 " jmp 904f;\n" \
188 " .align 16\n" \
189 "901: call 903f;\n" \
190 "902: pause;\n" \
191 " lfence;\n" \
192 " jmp 902b;\n" \
193 " .align 16\n" \
194 "903: addl $4, %%esp;\n" \
195 " pushl %[thunk_target];\n" \
196 " ret;\n" \
197 " .align 16\n" \
198 "904: call 901b;\n", \
199 X86_FEATURE_RETPOLINE)
201 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
202 #else /* No retpoline for C / inline asm */
203 # define CALL_NOSPEC "call *%[thunk_target]\n"
204 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
205 #endif
207 /* The Spectre V2 mitigation variants */
208 enum spectre_v2_mitigation {
209 SPECTRE_V2_NONE,
210 SPECTRE_V2_RETPOLINE_MINIMAL,
211 SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
212 SPECTRE_V2_RETPOLINE_GENERIC,
213 SPECTRE_V2_RETPOLINE_AMD,
214 SPECTRE_V2_IBRS,
217 extern char __indirect_thunk_start[];
218 extern char __indirect_thunk_end[];
221 * On VMEXIT we must ensure that no RSB predictions learned in the guest
222 * can be followed in the host, by overwriting the RSB completely. Both
223 * retpoline and IBRS mitigations for Spectre v2 need this; only on future
224 * CPUs with IBRS_ALL *might* it be avoided.
226 static inline void vmexit_fill_RSB(void)
228 #ifdef CONFIG_RETPOLINE
229 unsigned long loops;
231 asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
232 ALTERNATIVE("jmp 910f",
233 __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
234 X86_FEATURE_RETPOLINE)
235 "910:"
236 : "=r" (loops), ASM_CALL_CONSTRAINT
237 : : "memory" );
238 #endif
241 #define alternative_msr_write(_msr, _val, _feature) \
242 asm volatile(ALTERNATIVE("", \
243 "movl %[msr], %%ecx\n\t" \
244 "movl %[val], %%eax\n\t" \
245 "movl $0, %%edx\n\t" \
246 "wrmsr", \
247 _feature) \
248 : : [msr] "i" (_msr), [val] "i" (_val) \
249 : "eax", "ecx", "edx", "memory")
251 static inline void indirect_branch_prediction_barrier(void)
253 alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB,
254 X86_FEATURE_USE_IBPB);
258 * With retpoline, we must use IBRS to restrict branch prediction
259 * before calling into firmware.
261 * (Implemented as CPP macros due to header hell.)
263 #define firmware_restrict_branch_speculation_start() \
264 do { \
265 preempt_disable(); \
266 alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS, \
267 X86_FEATURE_USE_IBRS_FW); \
268 } while (0)
270 #define firmware_restrict_branch_speculation_end() \
271 do { \
272 alternative_msr_write(MSR_IA32_SPEC_CTRL, 0, \
273 X86_FEATURE_USE_IBRS_FW); \
274 preempt_enable(); \
275 } while (0)
277 #endif /* __ASSEMBLY__ */
280 * Below is used in the eBPF JIT compiler and emits the byte sequence
281 * for the following assembly:
283 * With retpolines configured:
285 * callq do_rop
286 * spec_trap:
287 * pause
288 * lfence
289 * jmp spec_trap
290 * do_rop:
291 * mov %rax,(%rsp)
292 * retq
294 * Without retpolines configured:
296 * jmp *%rax
298 #ifdef CONFIG_RETPOLINE
299 # define RETPOLINE_RAX_BPF_JIT_SIZE 17
300 # define RETPOLINE_RAX_BPF_JIT() \
301 EMIT1_off32(0xE8, 7); /* callq do_rop */ \
302 /* spec_trap: */ \
303 EMIT2(0xF3, 0x90); /* pause */ \
304 EMIT3(0x0F, 0xAE, 0xE8); /* lfence */ \
305 EMIT2(0xEB, 0xF9); /* jmp spec_trap */ \
306 /* do_rop: */ \
307 EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */ \
308 EMIT1(0xC3); /* retq */
309 #else
310 # define RETPOLINE_RAX_BPF_JIT_SIZE 2
311 # define RETPOLINE_RAX_BPF_JIT() \
312 EMIT2(0xFF, 0xE0); /* jmp *%rax */
313 #endif
315 #endif /* _ASM_X86_NOSPEC_BRANCH_H_ */