diff options
Diffstat (limited to 'arch/x86/lib/retpoline.S')
| -rw-r--r-- | arch/x86/lib/retpoline.S | 133 |
1 files changed, 84 insertions, 49 deletions
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S index 9556ff5f4773..1221bb099afb 100644 --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -28,45 +28,13 @@ .macro THUNK reg - .align 32 - -SYM_FUNC_START(__x86_indirect_thunk_\reg) - - ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \ - __stringify(RETPOLINE \reg), X86_FEATURE_RETPOLINE, \ - __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg; int3), X86_FEATURE_RETPOLINE_LFENCE - -SYM_FUNC_END(__x86_indirect_thunk_\reg) - -.endm - -/* - * This generates .altinstr_replacement symbols for use by objtool. They, - * however, must not actually live in .altinstr_replacement since that will be - * discarded after init, but module alternatives will also reference these - * symbols. - * - * Their names matches the "__x86_indirect_" prefix to mark them as retpolines. - */ -.macro ALT_THUNK reg - - .align 1 - -SYM_FUNC_START_NOALIGN(__x86_indirect_alt_call_\reg) - ANNOTATE_RETPOLINE_SAFE -1: call *%\reg -2: .skip 5-(2b-1b), 0x90 -SYM_FUNC_END(__x86_indirect_alt_call_\reg) - -STACK_FRAME_NON_STANDARD(__x86_indirect_alt_call_\reg) - -SYM_FUNC_START_NOALIGN(__x86_indirect_alt_jmp_\reg) - ANNOTATE_RETPOLINE_SAFE -1: jmp *%\reg -2: .skip 5-(2b-1b), 0x90 -SYM_FUNC_END(__x86_indirect_alt_jmp_\reg) + .align RETPOLINE_THUNK_SIZE +SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL) + UNWIND_HINT_EMPTY -STACK_FRAME_NON_STANDARD(__x86_indirect_alt_jmp_\reg) + ALTERNATIVE_2 __stringify(RETPOLINE \reg), \ + __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg; int3), X86_FEATURE_RETPOLINE_LFENCE, \ + __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), ALT_NOT(X86_FEATURE_RETPOLINE) .endm @@ -85,22 +53,89 @@ STACK_FRAME_NON_STANDARD(__x86_indirect_alt_jmp_\reg) #define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym) #define EXPORT_THUNK(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg) -#undef GEN + .align RETPOLINE_THUNK_SIZE +SYM_CODE_START(__x86_indirect_thunk_array) + #define GEN(reg) THUNK reg #include <asm/GEN-for-each-reg.h> - #undef GEN + + .align RETPOLINE_THUNK_SIZE +SYM_CODE_END(__x86_indirect_thunk_array) + #define GEN(reg) EXPORT_THUNK(reg) #include <asm/GEN-for-each-reg.h> - #undef GEN -#define GEN(reg) ALT_THUNK reg -#include <asm/GEN-for-each-reg.h> -#undef GEN -#define GEN(reg) __EXPORT_THUNK(__x86_indirect_alt_call_ ## reg) -#include <asm/GEN-for-each-reg.h> +/* + * This function name is magical and is used by -mfunction-return=thunk-extern + * for the compiler to generate JMPs to it. + */ +#ifdef CONFIG_RETHUNK -#undef GEN -#define GEN(reg) __EXPORT_THUNK(__x86_indirect_alt_jmp_ ## reg) -#include <asm/GEN-for-each-reg.h> + .section .text.__x86.return_thunk + +/* + * 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 + * alignment within the BTB. + * 2) The instruction at zen_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_FUNC_START_NOALIGN(zen_untrain_ret); + + /* + * As executed from zen_untrain_ret, this is: + * + * TEST $0xcc, %bl + * LFENCE + * JMP __x86_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. + */ + .byte 0xf6 + + /* + * As executed from __x86_return_thunk, this is a plain RET. + * + * As part of the TEST above, RET is the ModRM byte, and INT3 the imm8. + * + * We subsequently jump backwards and architecturally execute the RET. + * This creates a correct BTB prediction (type=ret), but in the + * meantime we suffer Straight Line Speculation (because the type was + * no branch) which is halted by the INT3. + * + * 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 + * which will be contained safely by the INT3. + */ +SYM_INNER_LABEL(__x86_return_thunk, SYM_L_GLOBAL) + ret + int3 +SYM_CODE_END(__x86_return_thunk) + + /* + * Ensure the TEST decoding / BTB invalidation is complete. + */ + lfence + + /* + * Jump back and execute the RET in the middle of the TEST instruction. + * INT3 is for SLS protection. + */ + jmp __x86_return_thunk + int3 +SYM_FUNC_END(zen_untrain_ret) +__EXPORT_THUNK(zen_untrain_ret) + +EXPORT_SYMBOL(__x86_return_thunk) + +#endif /* CONFIG_RETHUNK */ |