From: Peter Zijlstra peterz@infradead.org
commit a149180fbcf336e97ce4eb2cdc13672727feb94d upstream.
Note: needs to be in a section distinct from Retpolines such that the Retpoline RET substitution cannot possibly use immediate jumps.
ORC unwinding for zen_untrain_ret() and __x86_return_thunk() is a little tricky but works due to the fact that zen_untrain_ret() doesn't have any stack ops and as such will emit a single ORC entry at the start (+0x3f).
Meanwhile, unwinding an IP, including the __x86_return_thunk() one (+0x40) will search for the largest ORC entry smaller or equal to the IP, these will find the one ORC entry (+0x3f) and all works.
[ Alexandre: SVM part. ] [ bp: Build fix, massages. ]
Suggested-by: Andrew Cooper Andrew.Cooper3@citrix.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Borislav Petkov bp@suse.de Reviewed-by: Josh Poimboeuf jpoimboe@kernel.org Signed-off-by: Borislav Petkov bp@suse.de [cascardo: conflicts at arch/x86/entry/entry_64_compat.S] [cascardo: there is no ANNOTATE_NOENDBR] [cascardo: objtool commit 34c861e806478ac2ea4032721defbf1d6967df08 missing] [cascardo: conflict fixup] Signed-off-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com [bwh: Backported to 5.10: SEV-ES is not supported, so drop the change in arch/x86/kvm/svm/vmenter.S] Signed-off-by: Ben Hutchings ben@decadent.org.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/entry/entry_64.S | 6 ++ arch/x86/entry/entry_64_compat.S | 4 + arch/x86/include/asm/cpufeatures.h | 1 arch/x86/include/asm/disabled-features.h | 3 - arch/x86/include/asm/nospec-branch.h | 17 ++++++++ arch/x86/kernel/vmlinux.lds.S | 2 arch/x86/kvm/svm/vmenter.S | 9 ++++ arch/x86/lib/retpoline.S | 63 +++++++++++++++++++++++++++++-- tools/objtool/check.c | 20 ++++++++- 9 files changed, 117 insertions(+), 8 deletions(-)
--- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -102,6 +102,7 @@ SYM_CODE_START(entry_SYSCALL_64) movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
SYM_INNER_LABEL(entry_SYSCALL_64_safe_stack, SYM_L_GLOBAL) + UNTRAIN_RET
/* Construct struct pt_regs on stack */ pushq $__USER_DS /* pt_regs->ss */ @@ -675,6 +676,7 @@ native_irq_return_ldt: pushq %rdi /* Stash user RDI */ swapgs /* to kernel GS */ SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi /* to kernel CR3 */ + UNTRAIN_RET
movq PER_CPU_VAR(espfix_waddr), %rdi movq %rax, (0*8)(%rdi) /* user RAX */ @@ -910,6 +912,7 @@ SYM_CODE_START_LOCAL(paranoid_entry) * be retrieved from a kernel internal table. */ SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14 + UNTRAIN_RET
/* * Handling GSBASE depends on the availability of FSGSBASE. @@ -1022,6 +1025,7 @@ SYM_CODE_START_LOCAL(error_entry) FENCE_SWAPGS_USER_ENTRY /* We have user CR3. Change to kernel CR3. */ SWITCH_TO_KERNEL_CR3 scratch_reg=%rax + UNTRAIN_RET
.Lerror_entry_from_usermode_after_swapgs: /* Put us onto the real thread stack. */ @@ -1077,6 +1081,7 @@ SYM_CODE_START_LOCAL(error_entry) SWAPGS FENCE_SWAPGS_USER_ENTRY SWITCH_TO_KERNEL_CR3 scratch_reg=%rax + UNTRAIN_RET
/* * Pretend that the exception came from user mode: set up pt_regs @@ -1171,6 +1176,7 @@ SYM_CODE_START(asm_exc_nmi) movq %rsp, %rdx movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp UNWIND_HINT_IRET_REGS base=%rdx offset=8 + UNTRAIN_RET pushq 5*8(%rdx) /* pt_regs->ss */ pushq 4*8(%rdx) /* pt_regs->rsp */ pushq 3*8(%rdx) /* pt_regs->flags */ --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -14,6 +14,7 @@ #include <asm/irqflags.h> #include <asm/asm.h> #include <asm/smap.h> +#include <asm/nospec-branch.h> #include <linux/linkage.h> #include <linux/err.h>
@@ -71,6 +72,7 @@ SYM_CODE_START(entry_SYSENTER_compat) pushq $__USER32_CS /* pt_regs->cs */ pushq $0 /* pt_regs->ip = 0 (placeholder) */ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL) + UNTRAIN_RET
/* * User tracing code (ptrace or signal handlers) might assume that @@ -211,6 +213,7 @@ SYM_CODE_START(entry_SYSCALL_compat) movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
SYM_INNER_LABEL(entry_SYSCALL_compat_safe_stack, SYM_L_GLOBAL) + UNTRAIN_RET
/* Construct struct pt_regs on stack */ pushq $__USER32_DS /* pt_regs->ss */ @@ -377,6 +380,7 @@ SYM_CODE_START(entry_INT80_compat) pushq (%rdi) /* pt_regs->di */ .Lint80_keep_stack:
+ UNTRAIN_RET pushq %rsi /* pt_regs->si */ xorl %esi, %esi /* nospec si */ pushq %rdx /* pt_regs->dx */ --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -297,6 +297,7 @@ #define X86_FEATURE_RETPOLINE (11*32+12) /* "" Generic Retpoline mitigation for Spectre variant 2 */ #define X86_FEATURE_RETPOLINE_LFENCE (11*32+13) /* "" Use LFENCE for Spectre variant 2 */ #define X86_FEATURE_RETHUNK (11*32+14) /* "" Use REturn THUNK */ +#define X86_FEATURE_UNRET (11*32+15) /* "" AMD BTB untrain return */
/* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */ #define X86_FEATURE_AVX512_BF16 (12*32+ 5) /* AVX512 BFLOAT16 instructions */ --- a/arch/x86/include/asm/disabled-features.h +++ b/arch/x86/include/asm/disabled-features.h @@ -61,7 +61,8 @@ #else # define DISABLE_RETPOLINE ((1 << (X86_FEATURE_RETPOLINE & 31)) | \ (1 << (X86_FEATURE_RETPOLINE_LFENCE & 31)) | \ - (1 << (X86_FEATURE_RETHUNK & 31))) + (1 << (X86_FEATURE_RETHUNK & 31)) | \ + (1 << (X86_FEATURE_UNRET & 31))) #endif
/* Force disable because it's broken beyond repair */ --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -112,6 +112,22 @@ #endif .endm
+/* + * Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the + * return thunk isn't mapped into the userspace tables (then again, AMD + * typically has NO_MELTDOWN). + * + * Doesn't clobber any registers but does require a stable stack. + * + * As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point + * where we have a stack but before any RET instruction. + */ +.macro UNTRAIN_RET +#ifdef CONFIG_RETPOLINE + ALTERNATIVE "", "call zen_untrain_ret", X86_FEATURE_UNRET +#endif +.endm + #else /* __ASSEMBLY__ */
#define ANNOTATE_RETPOLINE_SAFE \ @@ -121,6 +137,7 @@ ".popsection\n\t"
extern void __x86_return_thunk(void); +extern void zen_untrain_ret(void);
#ifdef CONFIG_RETPOLINE
--- a/arch/x86/kernel/vmlinux.lds.S +++ b/arch/x86/kernel/vmlinux.lds.S @@ -142,7 +142,7 @@ SECTIONS
#ifdef CONFIG_RETPOLINE __indirect_thunk_start = .; - *(.text.__x86.indirect_thunk) + *(.text.__x86.*) __indirect_thunk_end = .; #endif } :text =0xcccc --- a/arch/x86/kvm/svm/vmenter.S +++ b/arch/x86/kvm/svm/vmenter.S @@ -129,6 +129,15 @@ SYM_FUNC_START(__svm_vcpu_run) #endif
/* + * Mitigate RETBleed for AMD/Hygon Zen uarch. RET should be + * untrained as soon as we exit the VM and are back to the + * kernel. This should be done before re-enabling interrupts + * because interrupt handlers won't sanitize 'ret' if the return is + * from the kernel. + */ + UNTRAIN_RET + + /* * Clear all general purpose registers except RSP and RAX to prevent * speculative use of the guest's values, even those that are reloaded * via the stack. In theory, an L1 cache miss when restoring registers --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -71,10 +71,67 @@ SYM_CODE_END(__x86_indirect_thunk_array) * This function name is magical and is used by -mfunction-return=thunk-extern * for the compiler to generate JMPs to it. */ -SYM_CODE_START(__x86_return_thunk) - UNWIND_HINT_EMPTY + .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)
-__EXPORT_THUNK(__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) --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -1082,7 +1082,7 @@ static void add_retpoline_call(struct ob annotate_call_site(file, insn, false); }
-static void add_return_call(struct objtool_file *file, struct instruction *insn) +static void add_return_call(struct objtool_file *file, struct instruction *insn, bool add) { /* * Return thunk tail calls are really just returns in disguise, @@ -1092,7 +1092,7 @@ static void add_return_call(struct objto insn->retpoline_safe = true;
/* Skip the non-text sections, specially .discard ones */ - if (insn->sec->text) + if (add && insn->sec->text) list_add_tail(&insn->call_node, &file->return_thunk_list); }
@@ -1121,7 +1121,7 @@ static int add_jump_destinations(struct add_retpoline_call(file, insn); continue; } else if (reloc->sym->return_thunk) { - add_return_call(file, insn); + add_return_call(file, insn, true); continue; } else if (insn->func) { /* internal or external sibling call (with reloc) */ @@ -1138,6 +1138,7 @@ static int add_jump_destinations(struct
insn->jump_dest = find_insn(file, dest_sec, dest_off); if (!insn->jump_dest) { + struct symbol *sym = find_symbol_by_offset(dest_sec, dest_off);
/* * This is a special case where an alt instruction @@ -1147,6 +1148,19 @@ static int add_jump_destinations(struct if (!strcmp(insn->sec->name, ".altinstr_replacement")) continue;
+ /* + * This is a special case for zen_untrain_ret(). + * It jumps to __x86_return_thunk(), but objtool + * can't find the thunk's starting RET + * instruction, because the RET is also in the + * middle of another instruction. Objtool only + * knows about the outer instruction. + */ + if (sym && sym->return_thunk) { + add_return_call(file, insn, false); + continue; + } + WARN_FUNC("can't find jump dest instruction at %s+0x%lx", insn->sec, insn->offset, dest_sec->name, dest_off);