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 */
399