diff options
Diffstat (limited to 'arch/x86/lib/retpoline.S')
| -rw-r--r-- | arch/x86/lib/retpoline.S | 160 |
1 files changed, 144 insertions, 16 deletions
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S index 841955dc2573..65c5c44f006b 100644 --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -9,8 +9,9 @@ #include <asm/nospec-branch.h> #include <asm/unwind_hints.h> #include <asm/frame.h> +#include <asm/nops.h> - .section .text.__x86.indirect_thunk + .section .text..__x86.indirect_thunk .macro RETPOLINE reg ANNOTATE_INTRA_FUNCTION_CALL @@ -74,36 +75,107 @@ SYM_CODE_END(__x86_indirect_thunk_array) */ #ifdef CONFIG_RETHUNK - .section .text.__x86.return_thunk +/* + * srso_alias_untrain_ret() and srso_alias_safe_ret() are placed at + * special addresses: + * + * - srso_alias_untrain_ret() is 2M aligned + * - srso_alias_safe_ret() is also in the same 2M page but bits 2, 8, 14 + * and 20 in its virtual address are set (while those bits in the + * srso_alias_untrain_ret() function are cleared). + * + * This guarantees that those two addresses will alias in the branch + * target buffer of Zen3/4 generations, leading to any potential + * poisoned entries at that BTB slot to get evicted. + * + * As a result, srso_alias_safe_ret() becomes a safe return. + */ +#ifdef CONFIG_CPU_SRSO + .section .text..__x86.rethunk_untrain + +SYM_START(srso_alias_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE) + UNWIND_HINT_FUNC + ANNOTATE_NOENDBR + ASM_NOP2 + lfence + jmp srso_alias_return_thunk +SYM_FUNC_END(srso_alias_untrain_ret) +__EXPORT_THUNK(srso_alias_untrain_ret) + + .section .text..__x86.rethunk_safe +#else +/* dummy definition for alternatives */ +SYM_START(srso_alias_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE) + ANNOTATE_UNRET_SAFE + ret + int3 +SYM_FUNC_END(srso_alias_untrain_ret) +#endif + +SYM_START(srso_alias_safe_ret, SYM_L_GLOBAL, SYM_A_NONE) + lea 8(%_ASM_SP), %_ASM_SP + UNWIND_HINT_FUNC + ANNOTATE_UNRET_SAFE + ret + int3 +SYM_FUNC_END(srso_alias_safe_ret) + + .section .text..__x86.return_thunk + +SYM_CODE_START(srso_alias_return_thunk) + UNWIND_HINT_FUNC + ANNOTATE_NOENDBR + call srso_alias_safe_ret + ud2 +SYM_CODE_END(srso_alias_return_thunk) + +/* + * Some generic notes on the untraining sequences: + * + * They are interchangeable when it comes to flushing potentially wrong + * RET predictions from the BTB. + * + * The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the + * Retbleed sequence because the return sequence done there + * (srso_safe_ret()) is longer and the return sequence must fully nest + * (end before) the untraining sequence. Therefore, the untraining + * sequence must fully overlap the return sequence. + * + * Regarding alignment - the instructions which need to be untrained, + * must all start at a cacheline boundary for Zen1/2 generations. That + * is, instruction sequences starting at srso_safe_ret() and + * the respective instruction sequences at retbleed_return_thunk() + * must start at a cacheline boundary. + */ /* * Safety details here pertain to the AMD Zen{1,2} microarchitecture: - * 1) The RET at __x86_return_thunk must be on a 64 byte boundary, for + * 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for * alignment within the BTB. - * 2) The instruction at zen_untrain_ret must contain, and not + * 2) The instruction at retbleed_untrain_ret must contain, and not * end with, the 0xc3 byte of the RET. * 3) STIBP must be enabled, or SMT disabled, to prevent the sibling thread * from re-poisioning the BTB prediction. */ .align 64 - .skip 63, 0xcc -SYM_START(zen_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE) + .skip 64 - (retbleed_return_thunk - retbleed_untrain_ret), 0xcc +SYM_START(retbleed_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE) ANNOTATE_NOENDBR /* - * As executed from zen_untrain_ret, this is: + * As executed from retbleed_untrain_ret, this is: * * TEST $0xcc, %bl * LFENCE - * JMP __x86_return_thunk + * JMP retbleed_return_thunk * * Executing the TEST instruction has a side effect of evicting any BTB * prediction (potentially attacker controlled) attached to the RET, as - * __x86_return_thunk + 1 isn't an instruction boundary at the moment. + * retbleed_return_thunk + 1 isn't an instruction boundary at the moment. */ .byte 0xf6 /* - * As executed from __x86_return_thunk, this is a plain RET. + * As executed from retbleed_return_thunk, this is a plain RET. * * As part of the TEST above, RET is the ModRM byte, and INT3 the imm8. * @@ -115,13 +187,13 @@ SYM_START(zen_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE) * With SMT enabled and STIBP active, a sibling thread cannot poison * RET's prediction to a type of its choice, but can evict the * prediction due to competitive sharing. If the prediction is - * evicted, __x86_return_thunk will suffer Straight Line Speculation + * evicted, retbleed_return_thunk will suffer Straight Line Speculation * which will be contained safely by the INT3. */ -SYM_INNER_LABEL(__x86_return_thunk, SYM_L_GLOBAL) +SYM_INNER_LABEL(retbleed_return_thunk, SYM_L_GLOBAL) ret int3 -SYM_CODE_END(__x86_return_thunk) +SYM_CODE_END(retbleed_return_thunk) /* * Ensure the TEST decoding / BTB invalidation is complete. @@ -132,11 +204,67 @@ SYM_CODE_END(__x86_return_thunk) * Jump back and execute the RET in the middle of the TEST instruction. * INT3 is for SLS protection. */ - jmp __x86_return_thunk + jmp retbleed_return_thunk int3 -SYM_FUNC_END(zen_untrain_ret) -__EXPORT_THUNK(zen_untrain_ret) +SYM_FUNC_END(retbleed_untrain_ret) +__EXPORT_THUNK(retbleed_untrain_ret) +/* + * SRSO untraining sequence for Zen1/2, similar to retbleed_untrain_ret() + * above. On kernel entry, srso_untrain_ret() is executed which is a + * + * movabs $0xccccc30824648d48,%rax + * + * and when the return thunk executes the inner label srso_safe_ret() + * later, it is a stack manipulation and a RET which is mispredicted and + * thus a "safe" one to use. + */ + .align 64 + .skip 64 - (srso_safe_ret - srso_untrain_ret), 0xcc +SYM_START(srso_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE) + ANNOTATE_NOENDBR + .byte 0x48, 0xb8 + +/* + * This forces the function return instruction to speculate into a trap + * (UD2 in srso_return_thunk() below). This RET will then mispredict + * and execution will continue at the return site read from the top of + * the stack. + */ +SYM_INNER_LABEL(srso_safe_ret, SYM_L_GLOBAL) + lea 8(%_ASM_SP), %_ASM_SP + ret + int3 + int3 + /* end of movabs */ + lfence + call srso_safe_ret + ud2 +SYM_CODE_END(srso_safe_ret) +SYM_FUNC_END(srso_untrain_ret) +__EXPORT_THUNK(srso_untrain_ret) + +SYM_CODE_START(srso_return_thunk) + UNWIND_HINT_FUNC + ANNOTATE_NOENDBR + call srso_safe_ret + ud2 +SYM_CODE_END(srso_return_thunk) + +SYM_FUNC_START(entry_untrain_ret) + ALTERNATIVE_2 "jmp retbleed_untrain_ret", \ + "jmp srso_untrain_ret", X86_FEATURE_SRSO, \ + "jmp srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS +SYM_FUNC_END(entry_untrain_ret) +__EXPORT_THUNK(entry_untrain_ret) + +SYM_CODE_START(__x86_return_thunk) + UNWIND_HINT_FUNC + ANNOTATE_NOENDBR + ANNOTATE_UNRET_SAFE + ret + int3 +SYM_CODE_END(__x86_return_thunk) EXPORT_SYMBOL(__x86_return_thunk) #endif /* CONFIG_RETHUNK */ |