| From foo@baz Tue Jul 12 05:03:58 PM CEST 2022 |
| From: Peter Zijlstra <peterz@infradead.org> |
| Date: Tue, 14 Jun 2022 23:15:39 +0200 |
| Subject: x86,static_call: Use alternative RET encoding |
| |
| From: Peter Zijlstra <peterz@infradead.org> |
| |
| commit ee88d363d15617ff50ac24fab0ffec11113b2aeb upstream. |
| |
| In addition to teaching static_call about the new way to spell 'RET', |
| there is an added complication in that static_call() is allowed to |
| rewrite text before it is known which particular spelling is required. |
| |
| In order to deal with this; have a static_call specific fixup in the |
| apply_return() 'alternative' patching routine that will rewrite the |
| static_call trampoline to match the definite sequence. |
| |
| This in turn creates the problem of uniquely identifying static call |
| trampolines. Currently trampolines are 8 bytes, the first 5 being the |
| jmp.d32/ret sequence and the final 3 a byte sequence that spells out |
| 'SCT'. |
| |
| This sequence is used in __static_call_validate() to ensure it is |
| patching a trampoline and not a random other jmp.d32. That is, |
| false-positives shouldn't be plenty, but aren't a big concern. |
| |
| OTOH the new __static_call_fixup() must not have false-positives, and |
| 'SCT' decodes to the somewhat weird but semi plausible sequence: |
| |
| push %rbx |
| rex.XB push %r12 |
| |
| Additionally, there are SLS concerns with immediate jumps. Combined it |
| seems like a good moment to change the signature to a single 3 byte |
| trap instruction that is unique to this usage and will not ever get |
| generated by accident. |
| |
| As such, change the signature to: '0x0f, 0xb9, 0xcc', which decodes |
| to: |
| |
| ud1 %esp, %ecx |
| |
| 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> |
| Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| arch/x86/include/asm/static_call.h | 19 ++++++++++++++++- |
| arch/x86/kernel/alternative.c | 12 +++++++---- |
| arch/x86/kernel/static_call.c | 40 +++++++++++++++++++++++++++++++++++-- |
| 3 files changed, 64 insertions(+), 7 deletions(-) |
| |
| --- a/arch/x86/include/asm/static_call.h |
| +++ b/arch/x86/include/asm/static_call.h |
| @@ -21,6 +21,16 @@ |
| * relative displacement across sections. |
| */ |
| |
| +/* |
| + * The trampoline is 8 bytes and of the general form: |
| + * |
| + * jmp.d32 \func |
| + * ud1 %esp, %ecx |
| + * |
| + * That trailing #UD provides both a speculation stop and serves as a unique |
| + * 3 byte signature identifying static call trampolines. Also see tramp_ud[] |
| + * and __static_call_fixup(). |
| + */ |
| #define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, insns) \ |
| asm(".pushsection .static_call.text, \"ax\" \n" \ |
| ".align 4 \n" \ |
| @@ -28,7 +38,7 @@ |
| STATIC_CALL_TRAMP_STR(name) ": \n" \ |
| ANNOTATE_NOENDBR \ |
| insns " \n" \ |
| - ".byte 0x53, 0x43, 0x54 \n" \ |
| + ".byte 0x0f, 0xb9, 0xcc \n" \ |
| ".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \ |
| ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \ |
| ".popsection \n") |
| @@ -36,8 +46,13 @@ |
| #define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \ |
| __ARCH_DEFINE_STATIC_CALL_TRAMP(name, ".byte 0xe9; .long " #func " - (. + 4)") |
| |
| +#ifdef CONFIG_RETPOLINE |
| +#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \ |
| + __ARCH_DEFINE_STATIC_CALL_TRAMP(name, "jmp __x86_return_thunk") |
| +#else |
| #define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \ |
| __ARCH_DEFINE_STATIC_CALL_TRAMP(name, "ret; int3; nop; nop; nop") |
| +#endif |
| |
| #define ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name) \ |
| ARCH_DEFINE_STATIC_CALL_TRAMP(name, __static_call_return0) |
| @@ -48,4 +63,6 @@ |
| ".long " STATIC_CALL_KEY_STR(name) " - . \n" \ |
| ".popsection \n") |
| |
| +extern bool __static_call_fixup(void *tramp, u8 op, void *dest); |
| + |
| #endif /* _ASM_STATIC_CALL_H */ |
| --- a/arch/x86/kernel/alternative.c |
| +++ b/arch/x86/kernel/alternative.c |
| @@ -539,18 +539,22 @@ void __init_or_module noinline apply_ret |
| s32 *s; |
| |
| for (s = start; s < end; s++) { |
| - void *addr = (void *)s + *s; |
| + void *dest = NULL, *addr = (void *)s + *s; |
| struct insn insn; |
| int len, ret; |
| u8 bytes[16]; |
| - u8 op1; |
| + u8 op; |
| |
| ret = insn_decode_kernel(&insn, addr); |
| if (WARN_ON_ONCE(ret < 0)) |
| continue; |
| |
| - op1 = insn.opcode.bytes[0]; |
| - if (WARN_ON_ONCE(op1 != JMP32_INSN_OPCODE)) |
| + op = insn.opcode.bytes[0]; |
| + if (op == JMP32_INSN_OPCODE) |
| + dest = addr + insn.length + insn.immediate.value; |
| + |
| + if (__static_call_fixup(addr, op, dest) || |
| + WARN_ON_ONCE(dest != &__x86_return_thunk)) |
| continue; |
| |
| DPRINTK("return thunk at: %pS (%px) len: %d to: %pS", |
| --- a/arch/x86/kernel/static_call.c |
| +++ b/arch/x86/kernel/static_call.c |
| @@ -12,6 +12,13 @@ enum insn_type { |
| }; |
| |
| /* |
| + * ud1 %esp, %ecx - a 3 byte #UD that is unique to trampolines, chosen such |
| + * that there is no false-positive trampoline identification while also being a |
| + * speculation stop. |
| + */ |
| +static const u8 tramp_ud[] = { 0x0f, 0xb9, 0xcc }; |
| + |
| +/* |
| * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax |
| */ |
| static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 }; |
| @@ -43,7 +50,10 @@ static void __ref __static_call_transfor |
| break; |
| |
| case RET: |
| - code = &retinsn; |
| + if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) |
| + code = text_gen_insn(JMP32_INSN_OPCODE, insn, &__x86_return_thunk); |
| + else |
| + code = &retinsn; |
| break; |
| } |
| |
| @@ -60,7 +70,7 @@ static void __static_call_validate(void |
| { |
| u8 opcode = *(u8 *)insn; |
| |
| - if (tramp && memcmp(insn+5, "SCT", 3)) { |
| + if (tramp && memcmp(insn+5, tramp_ud, 3)) { |
| pr_err("trampoline signature fail"); |
| BUG(); |
| } |
| @@ -115,3 +125,29 @@ void arch_static_call_transform(void *si |
| mutex_unlock(&text_mutex); |
| } |
| EXPORT_SYMBOL_GPL(arch_static_call_transform); |
| + |
| +#ifdef CONFIG_RETPOLINE |
| +/* |
| + * This is called by apply_returns() to fix up static call trampolines, |
| + * specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as |
| + * having a return trampoline. |
| + * |
| + * The problem is that static_call() is available before determining |
| + * X86_FEATURE_RETHUNK and, by implication, running alternatives. |
| + * |
| + * This means that __static_call_transform() above can have overwritten the |
| + * return trampoline and we now need to fix things up to be consistent. |
| + */ |
| +bool __static_call_fixup(void *tramp, u8 op, void *dest) |
| +{ |
| + if (memcmp(tramp+5, tramp_ud, 3)) { |
| + /* Not a trampoline site, not our problem. */ |
| + return false; |
| + } |
| + |
| + if (op == RET_INSN_OPCODE || dest == &__x86_return_thunk) |
| + __static_call_transform(tramp, RET, NULL); |
| + |
| + return true; |
| +} |
| +#endif |