/* SPDX-License-Identifier: GPL-2.0 */ #include <linux/export.h> #include <linux/stringify.h> #include <linux/linkage.h> #include <asm/dwarf2.h> #include <asm/cpufeatures.h> #include <asm/alternative.h> #include <asm/asm-offsets.h> #include <asm/nospec-branch.h> #include <asm/unwind_hints.h> #include <asm/percpu.h> #include <asm/frame.h> #include <asm/nops.h> .section .text..__x86.indirect_thunk .macro POLINE reg ANNOTATE_INTRA_FUNCTION_CALL call .Ldo_rop_\@ int3 .Ldo_rop_\@: mov %\reg, (%_ASM_SP) UNWIND_HINT_FUNC .endm .macro RETPOLINE reg POLINE \reg RET .endm .macro THUNK reg .align RETPOLINE_THUNK_SIZE SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL) UNWIND_HINT_UNDEFINED ANNOTATE_NOENDBR 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) .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(__x86_indirect_thunk_ ## reg) #include <asm/GEN-for-each-reg.h> #undef GEN #ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING .macro CALL_THUNK reg .align RETPOLINE_THUNK_SIZE SYM_INNER_LABEL(__x86_indirect_call_thunk_\reg, SYM_L_GLOBAL) UNWIND_HINT_UNDEFINED ANNOTATE_NOENDBR CALL_DEPTH_ACCOUNT POLINE \reg ANNOTATE_UNRET_SAFE ret int3 .endm .align RETPOLINE_THUNK_SIZE SYM_CODE_START(__x86_indirect_call_thunk_array) #define GEN(reg) CALL_THUNK reg #include <asm/GEN-for-each-reg.h> #undef GEN .align RETPOLINE_THUNK_SIZE SYM_CODE_END(__x86_indirect_call_thunk_array) #define GEN(reg) __EXPORT_THUNK(__x86_indirect_call_thunk_ ## reg) #include <asm/GEN-for-each-reg.h> #undef GEN .macro JUMP_THUNK reg .align RETPOLINE_THUNK_SIZE SYM_INNER_LABEL(__x86_indirect_jump_thunk_\reg, SYM_L_GLOBAL) UNWIND_HINT_UNDEFINED ANNOTATE_NOENDBR POLINE \reg ANNOTATE_UNRET_SAFE ret int3 .endm .align RETPOLINE_THUNK_SIZE SYM_CODE_START(__x86_indirect_jump_thunk_array) #define GEN(reg) JUMP_THUNK reg #include <asm/GEN-for-each-reg.h> #undef GEN .align RETPOLINE_THUNK_SIZE SYM_CODE_END(__x86_indirect_jump_thunk_array) #define GEN(reg) __EXPORT_THUNK(__x86_indirect_jump_thunk_ ## reg) #include <asm/GEN-for-each-reg.h> #undef GEN #endif #ifdef CONFIG_MITIGATION_RETHUNK /* * Be careful here: that label cannot really be removed because in * some configurations and toolchains, the JMP __x86_return_thunk the * compiler issues is either a short one or the compiler doesn't use * relocations for same-section JMPs and that breaks the returns * detection logic in apply_returns() and in objtool. */ .section .text..__x86.return_thunk #ifdef CONFIG_MITIGATION_SRSO /* * 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. */ .pushsection .text..__x86.rethunk_untrain SYM_CODE_START_NOALIGN(srso_alias_untrain_ret) 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) .popsection .pushsection .text..__x86.rethunk_safe SYM_CODE_START_NOALIGN(srso_alias_safe_ret) lea 8(%_ASM_SP), %_ASM_SP UNWIND_HINT_FUNC ANNOTATE_UNRET_SAFE ret int3 SYM_FUNC_END(srso_alias_safe_ret) SYM_CODE_START_NOALIGN(srso_alias_return_thunk) UNWIND_HINT_FUNC ANNOTATE_NOENDBR call srso_alias_safe_ret ud2 SYM_CODE_END(srso_alias_return_thunk) .popsection /* * 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_CODE_START_LOCAL_NOALIGN(srso_untrain_ret) 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) SYM_CODE_START(srso_return_thunk) UNWIND_HINT_FUNC ANNOTATE_NOENDBR call srso_safe_ret ud2 SYM_CODE_END(srso_return_thunk) #define JMP_SRSO_UNTRAIN_RET "jmp srso_untrain_ret" #else /* !CONFIG_MITIGATION_SRSO */ /* Dummy for the alternative in CALL_UNTRAIN_RET. */ SYM_CODE_START(srso_alias_untrain_ret) ANNOTATE_UNRET_SAFE ANNOTATE_NOENDBR ret int3 SYM_FUNC_END(srso_alias_untrain_ret) __EXPORT_THUNK(srso_alias_untrain_ret) #define JMP_SRSO_UNTRAIN_RET "ud2" #endif /* CONFIG_MITIGATION_SRSO */ #ifdef CONFIG_MITIGATION_UNRET_ENTRY /* * 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_CODE_START_LOCAL_NOALIGN(retbleed_untrain_ret) ANNOTATE_NOENDBR /* * 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) #define JMP_RETBLEED_UNTRAIN_RET "jmp retbleed_untrain_ret" #else /* !CONFIG_MITIGATION_UNRET_ENTRY */ #define JMP_RETBLEED_UNTRAIN_RET "ud2" #endif /* CONFIG_MITIGATION_UNRET_ENTRY */ #if defined(CONFIG_MITIGATION_UNRET_ENTRY) || defined(CONFIG_MITIGATION_SRSO) SYM_FUNC_START(entry_untrain_ret) ALTERNATIVE JMP_RETBLEED_UNTRAIN_RET, JMP_SRSO_UNTRAIN_RET, X86_FEATURE_SRSO SYM_FUNC_END(entry_untrain_ret) __EXPORT_THUNK(entry_untrain_ret) #endif /* CONFIG_MITIGATION_UNRET_ENTRY || CONFIG_MITIGATION_SRSO */ #ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING .align 64 SYM_FUNC_START(call_depth_return_thunk) ANNOTATE_NOENDBR /* * Keep the hotpath in a 16byte I-fetch for the non-debug * case. */ CALL_THUNKS_DEBUG_INC_RETS shlq $5, PER_CPU_VAR(pcpu_hot + X86_call_depth) jz 1f ANNOTATE_UNRET_SAFE ret int3 1: CALL_THUNKS_DEBUG_INC_STUFFS .rept 16 ANNOTATE_INTRA_FUNCTION_CALL call 2f int3 2: .endr add $(8*16), %rsp CREDIT_CALL_DEPTH ANNOTATE_UNRET_SAFE ret int3 SYM_FUNC_END(call_depth_return_thunk) #endif /* CONFIG_MITIGATION_CALL_DEPTH_TRACKING */ /* * This function name is magical and is used by -mfunction-return=thunk-extern * for the compiler to generate JMPs to it. * * This code is only used during kernel boot or module init. All * 'JMP __x86_return_thunk' sites are changed to something else by * apply_returns(). * * The ALTERNATIVE below adds a really loud warning to catch the case * where the insufficient default return thunk ends up getting used for * whatever reason like miscompilation or failure of * objtool/alternatives/etc to patch all the return sites. */ SYM_CODE_START(__x86_return_thunk) UNWIND_HINT_FUNC ANNOTATE_NOENDBR #if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \ defined(CONFIG_MITIGATION_SRSO) || \ defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING) ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \ "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS #else ANNOTATE_UNRET_SAFE ret #endif int3 SYM_CODE_END(__x86_return_thunk) EXPORT_SYMBOL(__x86_return_thunk) #endif /* CONFIG_MITIGATION_RETHUNK */
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