From: Peter Zijlstra peterz@infradead.org
commit d43490d0ab824023e11d0b57d0aeec17a6e0ca13 upstream.
Use the existing configurable return thunk. There is absolute no justification for having created this __x86_return_thunk alternative.
To clarify, the whole thing looks like:
Zen3/4 does:
srso_alias_untrain_ret: nop2 lfence jmp srso_alias_return_thunk int3
srso_alias_safe_ret: // aliasses srso_alias_untrain_ret just so add $8, %rsp ret int3
srso_alias_return_thunk: call srso_alias_safe_ret ud2
While Zen1/2 does:
srso_untrain_ret: movabs $foo, %rax lfence call srso_safe_ret (jmp srso_return_thunk ?) int3
srso_safe_ret: // embedded in movabs instruction add $8,%rsp ret int3
srso_return_thunk: call srso_safe_ret ud2
While retbleed does:
zen_untrain_ret: test $0xcc, %bl lfence jmp zen_return_thunk int3
zen_return_thunk: // embedded in the test instruction ret int3
Where Zen1/2 flush the BTB entry using the instruction decoder trick (test,movabs) Zen3/4 use BTB aliasing. SRSO adds a return sequence (srso_safe_ret()) which forces the function return instruction to speculate into a trap (UD2). This RET will then mispredict and execution will continue at the return site read from the top of the stack.
Pick one of three options at boot (evey function can only ever return once).
[ bp: Fixup commit message uarch details and add them in a comment in the code too. Add a comment about the srso_select_mitigation() dependency on retbleed_select_mitigation(). Add moar ifdeffery for 32-bit builds. Add a dummy srso_untrain_ret_alias() definition for 32-bit alternatives needing the symbol. ]
Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation") Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Link: https://lore.kernel.org/r/20230814121148.842775684@infradead.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/nospec-branch.h | 5 +++ arch/x86/kernel/cpu/bugs.c | 17 ++++++++-- arch/x86/kernel/vmlinux.lds.S | 2 - arch/x86/lib/retpoline.S | 58 +++++++++++++++++++++++++---------- tools/objtool/arch/x86/decode.c | 2 - 5 files changed, 63 insertions(+), 21 deletions(-)
--- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -201,9 +201,14 @@ extern void __x86_return_thunk(void); static inline void __x86_return_thunk(void) {} #endif
+extern void zen_return_thunk(void); +extern void srso_return_thunk(void); +extern void srso_alias_return_thunk(void); + extern void zen_untrain_ret(void); extern void srso_untrain_ret(void); extern void srso_untrain_ret_alias(void); + extern void entry_ibpb(void);
#ifdef CONFIG_RETPOLINE --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -157,8 +157,13 @@ void __init cpu_select_mitigations(void) l1tf_select_mitigation(); md_clear_select_mitigation(); srbds_select_mitigation(); - gds_select_mitigation(); + + /* + * srso_select_mitigation() depends and must run after + * retbleed_select_mitigation(). + */ srso_select_mitigation(); + gds_select_mitigation(); }
/* @@ -978,6 +983,9 @@ do_cmd_auto: setup_force_cpu_cap(X86_FEATURE_RETHUNK); setup_force_cpu_cap(X86_FEATURE_UNRET);
+ if (IS_ENABLED(CONFIG_RETHUNK)) + x86_return_thunk = zen_return_thunk; + if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD && boot_cpu_data.x86_vendor != X86_VENDOR_HYGON) pr_err(RETBLEED_UNTRAIN_MSG); @@ -2352,10 +2360,13 @@ static void __init srso_select_mitigatio */ setup_force_cpu_cap(X86_FEATURE_RETHUNK);
- if (boot_cpu_data.x86 == 0x19) + if (boot_cpu_data.x86 == 0x19) { setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS); - else + x86_return_thunk = srso_alias_return_thunk; + } else { setup_force_cpu_cap(X86_FEATURE_SRSO); + x86_return_thunk = srso_return_thunk; + } srso_mitigation = SRSO_MITIGATION_SAFE_RET; } else { pr_err("WARNING: kernel not compiled with CPU_SRSO.\n"); --- a/arch/x86/kernel/vmlinux.lds.S +++ b/arch/x86/kernel/vmlinux.lds.S @@ -518,7 +518,7 @@ INIT_PER_CPU(irq_stack_backing_store); #endif
#ifdef CONFIG_RETHUNK -. = ASSERT((__ret & 0x3f) == 0, "__ret not cacheline-aligned"); +. = ASSERT((zen_return_thunk & 0x3f) == 0, "zen_return_thunk not cacheline-aligned"); . = ASSERT((srso_safe_ret & 0x3f) == 0, "srso_safe_ret not cacheline-aligned"); #endif
--- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -93,21 +93,26 @@ SYM_CODE_END(__x86_indirect_thunk_array) .section .text.__x86.rethunk_untrain
SYM_START(srso_untrain_ret_alias, SYM_L_GLOBAL, SYM_A_NONE) + UNWIND_HINT_FUNC ASM_NOP2 lfence - jmp __x86_return_thunk + jmp srso_alias_return_thunk SYM_FUNC_END(srso_untrain_ret_alias) __EXPORT_THUNK(srso_untrain_ret_alias)
.section .text.__x86.rethunk_safe +#else +/* dummy definition for alternatives */ +SYM_START(srso_untrain_ret_alias, SYM_L_GLOBAL, SYM_A_NONE) + ANNOTATE_UNRET_SAFE + ret + int3 +SYM_FUNC_END(srso_untrain_ret_alias) #endif
-/* Needs a definition for the __x86_return_thunk alternative below. */ SYM_START(srso_safe_ret_alias, SYM_L_GLOBAL, SYM_A_NONE) -#ifdef CONFIG_CPU_SRSO add $8, %_ASM_SP UNWIND_HINT_FUNC -#endif ANNOTATE_UNRET_SAFE ret int3 @@ -115,9 +120,16 @@ SYM_FUNC_END(srso_safe_ret_alias)
.section .text.__x86.return_thunk
+SYM_CODE_START(srso_alias_return_thunk) + UNWIND_HINT_FUNC + ANNOTATE_NOENDBR + call srso_safe_ret_alias + ud2 +SYM_CODE_END(srso_alias_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 + * 1) The RET at zen_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. @@ -125,7 +137,7 @@ SYM_FUNC_END(srso_safe_ret_alias) * from re-poisioning the BTB prediction. */ .align 64 - .skip 64 - (__ret - zen_untrain_ret), 0xcc + .skip 64 - (zen_return_thunk - zen_untrain_ret), 0xcc SYM_FUNC_START_NOALIGN(zen_untrain_ret);
/* @@ -133,16 +145,16 @@ SYM_FUNC_START_NOALIGN(zen_untrain_ret); * * TEST $0xcc, %bl * LFENCE - * JMP __x86_return_thunk + * JMP zen_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. + * zen_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 zen_return_thunk, this is a plain RET. * * As part of the TEST above, RET is the ModRM byte, and INT3 the imm8. * @@ -154,13 +166,13 @@ SYM_FUNC_START_NOALIGN(zen_untrain_ret); * 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, zen_return_thunk will suffer Straight Line Speculation * which will be contained safely by the INT3. */ -SYM_INNER_LABEL(__ret, SYM_L_GLOBAL) +SYM_INNER_LABEL(zen_return_thunk, SYM_L_GLOBAL) ret int3 -SYM_CODE_END(__ret) +SYM_CODE_END(zen_return_thunk)
/* * Ensure the TEST decoding / BTB invalidation is complete. @@ -171,7 +183,7 @@ SYM_CODE_END(__ret) * Jump back and execute the RET in the middle of the TEST instruction. * INT3 is for SLS protection. */ - jmp __ret + jmp zen_return_thunk int3 SYM_FUNC_END(zen_untrain_ret) __EXPORT_THUNK(zen_untrain_ret) @@ -191,12 +203,19 @@ __EXPORT_THUNK(zen_untrain_ret) 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) add $8, %_ASM_SP ret int3 int3 int3 + /* end of movabs */ lfence call srso_safe_ret ud2 @@ -204,12 +223,19 @@ SYM_CODE_END(srso_safe_ret) SYM_FUNC_END(srso_untrain_ret) __EXPORT_THUNK(srso_untrain_ret)
-SYM_CODE_START(__x86_return_thunk) +SYM_CODE_START(srso_return_thunk) UNWIND_HINT_FUNC ANNOTATE_NOENDBR - ALTERNATIVE_2 "jmp __ret", "call srso_safe_ret", X86_FEATURE_SRSO, \ - "call srso_safe_ret_alias", X86_FEATURE_SRSO_ALIAS + call srso_safe_ret ud2 +SYM_CODE_END(srso_return_thunk) + +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)
--- a/tools/objtool/arch/x86/decode.c +++ b/tools/objtool/arch/x86/decode.c @@ -655,5 +655,5 @@ bool arch_is_rethunk(struct symbol *sym) return !strcmp(sym->name, "__x86_return_thunk") || !strcmp(sym->name, "srso_untrain_ret") || !strcmp(sym->name, "srso_safe_ret") || - !strcmp(sym->name, "__ret"); + !strcmp(sym->name, "zen_return_thunk"); }