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Diffstat (limited to 'arch/x86/lib/retpoline.S')
| -rw-r--r-- | arch/x86/lib/retpoline.S | 267 |
1 files changed, 267 insertions, 0 deletions
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S new file mode 100644 index 000000000..6f5321b36 --- /dev/null +++ b/arch/x86/lib/retpoline.S @@ -0,0 +1,267 @@ +/* SPDX-License-Identifier: GPL-2.0 */ + +#include <linux/stringify.h> +#include <linux/linkage.h> +#include <asm/dwarf2.h> +#include <asm/cpufeatures.h> +#include <asm/alternative.h> +#include <asm/export.h> +#include <asm/nospec-branch.h> +#include <asm/unwind_hints.h> +#include <asm/frame.h> +#include <asm/nops.h> + + .section .text..__x86.indirect_thunk + +.macro RETPOLINE reg + ANNOTATE_INTRA_FUNCTION_CALL + call .Ldo_rop_\@ +.Lspec_trap_\@: + UNWIND_HINT_EMPTY + pause + lfence + jmp .Lspec_trap_\@ +.Ldo_rop_\@: + mov %\reg, (%_ASM_SP) + UNWIND_HINT_FUNC + RET +.endm + +.macro THUNK reg + + .align RETPOLINE_THUNK_SIZE +SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL) + UNWIND_HINT_EMPTY + + 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 + +/* + * Despite being an assembler file we can't just use .irp here + * because __KSYM_DEPS__ only uses the C preprocessor and would + * only see one instance of "__x86_indirect_thunk_\reg" rather + * than one per register with the correct names. So we do it + * the simple and nasty way... + * + * Worse, you can only have a single EXPORT_SYMBOL per line, + * and CPP can't insert newlines, so we have to repeat everything + * at least twice. + */ + +#define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym) +#define EXPORT_THUNK(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg) + + .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 + +/* + * This function name is magical and is used by -mfunction-return=thunk-extern + * for the compiler to generate JMPs to it. + */ +#ifdef CONFIG_RETHUNK + +/* + * 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 + 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 retbleed_return_thunk must be on a 64 byte boundary, for + * alignment within the BTB. + * 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 64 - (retbleed_return_thunk - retbleed_untrain_ret), 0xcc +SYM_FUNC_START_NOALIGN(retbleed_untrain_ret); + + /* + * As executed from retbleed_untrain_ret, this is: + * + * TEST $0xcc, %bl + * LFENCE + * 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 + * retbleed_return_thunk + 1 isn't an instruction boundary at the moment. + */ + .byte 0xf6 + + /* + * 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. + * + * 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, retbleed_return_thunk will suffer Straight Line Speculation + * which will be contained safely by the INT3. + */ +SYM_INNER_LABEL(retbleed_return_thunk, SYM_L_GLOBAL) + ret + int3 +SYM_CODE_END(retbleed_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 retbleed_return_thunk + int3 +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) + .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 */ |