1 /* SPDX-License-Identifier: GPL-2.0 */
2
3 #include <linux/export.h>
4 #include <linux/stringify.h>
5 #include <linux/linkage.h>
6 #include <asm/dwarf2.h>
7 #include <asm/cpufeatures.h>
8 #include <asm/alternative.h>
9 #include <asm/asm-offsets.h>
10 #include <asm/nospec-branch.h>
11 #include <asm/unwind_hints.h>
12 #include <asm/percpu.h>
13 #include <asm/frame.h>
14 #include <asm/nops.h>
15
16 .section .text..__x86.indirect_thunk
17
18
19 .macro POLINE reg
20 ANNOTATE_INTRA_FUNCTION_CALL
21 call .Ldo_rop_\@
22 int3
23 .Ldo_rop_\@:
24 mov %\reg, (%_ASM_SP)
25 UNWIND_HINT_FUNC
26 .endm
27
28 .macro RETPOLINE reg
29 POLINE \reg
30 RET
31 .endm
32
33 .macro THUNK reg
34
35 .align RETPOLINE_THUNK_SIZE
36 SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL)
37 UNWIND_HINT_UNDEFINED
38 ANNOTATE_NOENDBR
39
40 ALTERNATIVE_2 __stringify(RETPOLINE \reg), \
41 __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg; int3), X86_FEATURE_RETPOLINE_LFENCE, \
42 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), ALT_NOT(X86_FEATURE_RETPOLINE)
43
44 .endm
45
46 /*
47 * Despite being an assembler file we can't just use .irp here
48 * because __KSYM_DEPS__ only uses the C preprocessor and would
49 * only see one instance of "__x86_indirect_thunk_\reg" rather
50 * than one per register with the correct names. So we do it
51 * the simple and nasty way...
52 *
53 * Worse, you can only have a single EXPORT_SYMBOL per line,
54 * and CPP can't insert newlines, so we have to repeat everything
55 * at least twice.
56 */
57
58 #define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)
59
60 .align RETPOLINE_THUNK_SIZE
61 SYM_CODE_START(__x86_indirect_thunk_array)
62
63 #define GEN(reg) THUNK reg
64 #include <asm/GEN-for-each-reg.h>
65 #undef GEN
66
67 .align RETPOLINE_THUNK_SIZE
68 SYM_CODE_END(__x86_indirect_thunk_array)
69
70 #define GEN(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
71 #include <asm/GEN-for-each-reg.h>
72 #undef GEN
73
74 #ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING
75 .macro CALL_THUNK reg
76 .align RETPOLINE_THUNK_SIZE
77
78 SYM_INNER_LABEL(__x86_indirect_call_thunk_\reg, SYM_L_GLOBAL)
79 UNWIND_HINT_UNDEFINED
80 ANNOTATE_NOENDBR
81
82 CALL_DEPTH_ACCOUNT
83 POLINE \reg
84 ANNOTATE_UNRET_SAFE
85 ret
86 int3
87 .endm
88
89 .align RETPOLINE_THUNK_SIZE
90 SYM_CODE_START(__x86_indirect_call_thunk_array)
91
92 #define GEN(reg) CALL_THUNK reg
93 #include <asm/GEN-for-each-reg.h>
94 #undef GEN
95
96 .align RETPOLINE_THUNK_SIZE
97 SYM_CODE_END(__x86_indirect_call_thunk_array)
98
99 #define GEN(reg) __EXPORT_THUNK(__x86_indirect_call_thunk_ ## reg)
100 #include <asm/GEN-for-each-reg.h>
101 #undef GEN
102
103 .macro JUMP_THUNK reg
104 .align RETPOLINE_THUNK_SIZE
105
106 SYM_INNER_LABEL(__x86_indirect_jump_thunk_\reg, SYM_L_GLOBAL)
107 UNWIND_HINT_UNDEFINED
108 ANNOTATE_NOENDBR
109 POLINE \reg
110 ANNOTATE_UNRET_SAFE
111 ret
112 int3
113 .endm
114
115 .align RETPOLINE_THUNK_SIZE
116 SYM_CODE_START(__x86_indirect_jump_thunk_array)
117
118 #define GEN(reg) JUMP_THUNK reg
119 #include <asm/GEN-for-each-reg.h>
120 #undef GEN
121
122 .align RETPOLINE_THUNK_SIZE
123 SYM_CODE_END(__x86_indirect_jump_thunk_array)
124
125 #define GEN(reg) __EXPORT_THUNK(__x86_indirect_jump_thunk_ ## reg)
126 #include <asm/GEN-for-each-reg.h>
127 #undef GEN
128 #endif
129
130 #ifdef CONFIG_MITIGATION_RETHUNK
131
132 /*
133 * Be careful here: that label cannot really be removed because in
134 * some configurations and toolchains, the JMP __x86_return_thunk the
135 * compiler issues is either a short one or the compiler doesn't use
136 * relocations for same-section JMPs and that breaks the returns
137 * detection logic in apply_returns() and in objtool.
138 */
139 .section .text..__x86.return_thunk
140
141 #ifdef CONFIG_MITIGATION_SRSO
142
143 /*
144 * srso_alias_untrain_ret() and srso_alias_safe_ret() are placed at
145 * special addresses:
146 *
147 * - srso_alias_untrain_ret() is 2M aligned
148 * - srso_alias_safe_ret() is also in the same 2M page but bits 2, 8, 14
149 * and 20 in its virtual address are set (while those bits in the
150 * srso_alias_untrain_ret() function are cleared).
151 *
152 * This guarantees that those two addresses will alias in the branch
153 * target buffer of Zen3/4 generations, leading to any potential
154 * poisoned entries at that BTB slot to get evicted.
155 *
156 * As a result, srso_alias_safe_ret() becomes a safe return.
157 */
158 .pushsection .text..__x86.rethunk_untrain
159 SYM_CODE_START_NOALIGN(srso_alias_untrain_ret)
160 UNWIND_HINT_FUNC
161 ANNOTATE_NOENDBR
162 ASM_NOP2
163 lfence
164 jmp srso_alias_return_thunk
165 SYM_FUNC_END(srso_alias_untrain_ret)
166 __EXPORT_THUNK(srso_alias_untrain_ret)
167 .popsection
168
169 .pushsection .text..__x86.rethunk_safe
170 SYM_CODE_START_NOALIGN(srso_alias_safe_ret)
171 lea 8(%_ASM_SP), %_ASM_SP
172 UNWIND_HINT_FUNC
173 ANNOTATE_UNRET_SAFE
174 ret
175 int3
176 SYM_FUNC_END(srso_alias_safe_ret)
177
178 SYM_CODE_START_NOALIGN(srso_alias_return_thunk)
179 UNWIND_HINT_FUNC
180 ANNOTATE_NOENDBR
181 call srso_alias_safe_ret
182 ud2
183 SYM_CODE_END(srso_alias_return_thunk)
184 .popsection
185
186 /*
187 * SRSO untraining sequence for Zen1/2, similar to retbleed_untrain_ret()
188 * above. On kernel entry, srso_untrain_ret() is executed which is a
189 *
190 * movabs $0xccccc30824648d48,%rax
191 *
192 * and when the return thunk executes the inner label srso_safe_ret()
193 * later, it is a stack manipulation and a RET which is mispredicted and
194 * thus a "safe" one to use.
195 */
196 .align 64
197 .skip 64 - (srso_safe_ret - srso_untrain_ret), 0xcc
198 SYM_CODE_START_LOCAL_NOALIGN(srso_untrain_ret)
199 ANNOTATE_NOENDBR
200 .byte 0x48, 0xb8
201
202 /*
203 * This forces the function return instruction to speculate into a trap
204 * (UD2 in srso_return_thunk() below). This RET will then mispredict
205 * and execution will continue at the return site read from the top of
206 * the stack.
207 */
208 SYM_INNER_LABEL(srso_safe_ret, SYM_L_GLOBAL)
209 lea 8(%_ASM_SP), %_ASM_SP
210 ret
211 int3
212 int3
213 /* end of movabs */
214 lfence
215 call srso_safe_ret
216 ud2
217 SYM_CODE_END(srso_safe_ret)
218 SYM_FUNC_END(srso_untrain_ret)
219
220 SYM_CODE_START(srso_return_thunk)
221 UNWIND_HINT_FUNC
222 ANNOTATE_NOENDBR
223 call srso_safe_ret
224 ud2
225 SYM_CODE_END(srso_return_thunk)
226
227 #define JMP_SRSO_UNTRAIN_RET "jmp srso_untrain_ret"
228 #else /* !CONFIG_MITIGATION_SRSO */
229 /* Dummy for the alternative in CALL_UNTRAIN_RET. */
230 SYM_CODE_START(srso_alias_untrain_ret)
231 ANNOTATE_UNRET_SAFE
232 ANNOTATE_NOENDBR
233 ret
234 int3
235 SYM_FUNC_END(srso_alias_untrain_ret)
236 __EXPORT_THUNK(srso_alias_untrain_ret)
237 #define JMP_SRSO_UNTRAIN_RET "ud2"
238 #endif /* CONFIG_MITIGATION_SRSO */
239
240 #ifdef CONFIG_MITIGATION_UNRET_ENTRY
241
242 /*
243 * Some generic notes on the untraining sequences:
244 *
245 * They are interchangeable when it comes to flushing potentially wrong
246 * RET predictions from the BTB.
247 *
248 * The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the
249 * Retbleed sequence because the return sequence done there
250 * (srso_safe_ret()) is longer and the return sequence must fully nest
251 * (end before) the untraining sequence. Therefore, the untraining
252 * sequence must fully overlap the return sequence.
253 *
254 * Regarding alignment - the instructions which need to be untrained,
255 * must all start at a cacheline boundary for Zen1/2 generations. That
256 * is, instruction sequences starting at srso_safe_ret() and
257 * the respective instruction sequences at retbleed_return_thunk()
258 * must start at a cacheline boundary.
259 */
260
261 /*
262 * Safety details here pertain to the AMD Zen{1,2} microarchitecture:
263 * 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for
264 * alignment within the BTB.
265 * 2) The instruction at retbleed_untrain_ret must contain, and not
266 * end with, the 0xc3 byte of the RET.
267 * 3) STIBP must be enabled, or SMT disabled, to prevent the sibling thread
268 * from re-poisioning the BTB prediction.
269 */
270 .align 64
271 .skip 64 - (retbleed_return_thunk - retbleed_untrain_ret), 0xcc
272 SYM_CODE_START_LOCAL_NOALIGN(retbleed_untrain_ret)
273 ANNOTATE_NOENDBR
274 /*
275 * As executed from retbleed_untrain_ret, this is:
276 *
277 * TEST $0xcc, %bl
278 * LFENCE
279 * JMP retbleed_return_thunk
280 *
281 * Executing the TEST instruction has a side effect of evicting any BTB
282 * prediction (potentially attacker controlled) attached to the RET, as
283 * retbleed_return_thunk + 1 isn't an instruction boundary at the moment.
284 */
285 .byte 0xf6
286
287 /*
288 * As executed from retbleed_return_thunk, this is a plain RET.
289 *
290 * As part of the TEST above, RET is the ModRM byte, and INT3 the imm8.
291 *
292 * We subsequently jump backwards and architecturally execute the RET.
293 * This creates a correct BTB prediction (type=ret), but in the
294 * meantime we suffer Straight Line Speculation (because the type was
295 * no branch) which is halted by the INT3.
296 *
297 * With SMT enabled and STIBP active, a sibling thread cannot poison
298 * RET's prediction to a type of its choice, but can evict the
299 * prediction due to competitive sharing. If the prediction is
300 * evicted, retbleed_return_thunk will suffer Straight Line Speculation
301 * which will be contained safely by the INT3.
302 */
303 SYM_INNER_LABEL(retbleed_return_thunk, SYM_L_GLOBAL)
304 ret
305 int3
306 SYM_CODE_END(retbleed_return_thunk)
307
308 /*
309 * Ensure the TEST decoding / BTB invalidation is complete.
310 */
311 lfence
312
313 /*
314 * Jump back and execute the RET in the middle of the TEST instruction.
315 * INT3 is for SLS protection.
316 */
317 jmp retbleed_return_thunk
318 int3
319 SYM_FUNC_END(retbleed_untrain_ret)
320
321 #define JMP_RETBLEED_UNTRAIN_RET "jmp retbleed_untrain_ret"
322 #else /* !CONFIG_MITIGATION_UNRET_ENTRY */
323 #define JMP_RETBLEED_UNTRAIN_RET "ud2"
324 #endif /* CONFIG_MITIGATION_UNRET_ENTRY */
325
326 #if defined(CONFIG_MITIGATION_UNRET_ENTRY) || defined(CONFIG_MITIGATION_SRSO)
327
328 SYM_FUNC_START(entry_untrain_ret)
329 ALTERNATIVE JMP_RETBLEED_UNTRAIN_RET, JMP_SRSO_UNTRAIN_RET, X86_FEATURE_SRSO
330 SYM_FUNC_END(entry_untrain_ret)
331 __EXPORT_THUNK(entry_untrain_ret)
332
333 #endif /* CONFIG_MITIGATION_UNRET_ENTRY || CONFIG_MITIGATION_SRSO */
334
335 #ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING
336
337 .align 64
338 SYM_FUNC_START(call_depth_return_thunk)
339 ANNOTATE_NOENDBR
340 /*
341 * Keep the hotpath in a 16byte I-fetch for the non-debug
342 * case.
343 */
344 CALL_THUNKS_DEBUG_INC_RETS
345 shlq $5, PER_CPU_VAR(pcpu_hot + X86_call_depth)
346 jz 1f
347 ANNOTATE_UNRET_SAFE
348 ret
349 int3
350 1:
351 CALL_THUNKS_DEBUG_INC_STUFFS
352 .rept 16
353 ANNOTATE_INTRA_FUNCTION_CALL
354 call 2f
355 int3
356 2:
357 .endr
358 add $(8*16), %rsp
359
360 CREDIT_CALL_DEPTH
361
362 ANNOTATE_UNRET_SAFE
363 ret
364 int3
365 SYM_FUNC_END(call_depth_return_thunk)
366
367 #endif /* CONFIG_MITIGATION_CALL_DEPTH_TRACKING */
368
369 /*
370 * This function name is magical and is used by -mfunction-return=thunk-extern
371 * for the compiler to generate JMPs to it.
372 *
373 * This code is only used during kernel boot or module init. All
374 * 'JMP __x86_return_thunk' sites are changed to something else by
375 * apply_returns().
376 *
377 * The ALTERNATIVE below adds a really loud warning to catch the case
378 * where the insufficient default return thunk ends up getting used for
379 * whatever reason like miscompilation or failure of
380 * objtool/alternatives/etc to patch all the return sites.
381 */
382 SYM_CODE_START(__x86_return_thunk)
383 UNWIND_HINT_FUNC
384 ANNOTATE_NOENDBR
385 #if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
386 defined(CONFIG_MITIGATION_SRSO) || \
387 defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
388 ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
389 "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
390 #else
391 ANNOTATE_UNRET_SAFE
392 ret
393 #endif
394 int3
395 SYM_CODE_END(__x86_return_thunk)
396 EXPORT_SYMBOL(__x86_return_thunk)
397
398 #endif /* CONFIG_MITIGATION_RETHUNK */
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