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x86,static_call: Use alternative RET encoding
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>
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@ -21,6 +21,16 @@
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* relative displacement across sections.
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*/
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/*
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* The trampoline is 8 bytes and of the general form:
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*
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* jmp.d32 \func
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* ud1 %esp, %ecx
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*
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* That trailing #UD provides both a speculation stop and serves as a unique
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* 3 byte signature identifying static call trampolines. Also see tramp_ud[]
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* and __static_call_fixup().
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*/
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#define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, insns) \
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asm(".pushsection .static_call.text, \"ax\" \n" \
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".align 4 \n" \
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@ -28,7 +38,7 @@
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STATIC_CALL_TRAMP_STR(name) ": \n" \
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ANNOTATE_NOENDBR \
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insns " \n" \
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".byte 0x53, 0x43, 0x54 \n" \
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".byte 0x0f, 0xb9, 0xcc \n" \
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".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \
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".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
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".popsection \n")
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@ -36,8 +46,13 @@
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#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \
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__ARCH_DEFINE_STATIC_CALL_TRAMP(name, ".byte 0xe9; .long " #func " - (. + 4)")
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#ifdef CONFIG_RETPOLINE
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#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \
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__ARCH_DEFINE_STATIC_CALL_TRAMP(name, "jmp __x86_return_thunk")
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#else
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#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \
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__ARCH_DEFINE_STATIC_CALL_TRAMP(name, "ret; int3; nop; nop; nop")
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#endif
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#define ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name) \
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ARCH_DEFINE_STATIC_CALL_TRAMP(name, __static_call_return0)
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@ -48,4 +63,6 @@
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".long " STATIC_CALL_KEY_STR(name) " - . \n" \
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".popsection \n")
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extern bool __static_call_fixup(void *tramp, u8 op, void *dest);
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#endif /* _ASM_STATIC_CALL_H */
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@ -539,18 +539,22 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end)
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s32 *s;
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for (s = start; s < end; s++) {
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void *addr = (void *)s + *s;
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void *dest = NULL, *addr = (void *)s + *s;
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struct insn insn;
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int len, ret;
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u8 bytes[16];
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u8 op1;
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u8 op;
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ret = insn_decode_kernel(&insn, addr);
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if (WARN_ON_ONCE(ret < 0))
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continue;
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op1 = insn.opcode.bytes[0];
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if (WARN_ON_ONCE(op1 != JMP32_INSN_OPCODE))
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op = insn.opcode.bytes[0];
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if (op == JMP32_INSN_OPCODE)
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dest = addr + insn.length + insn.immediate.value;
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if (__static_call_fixup(addr, op, dest) ||
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WARN_ON_ONCE(dest != &__x86_return_thunk))
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continue;
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DPRINTK("return thunk at: %pS (%px) len: %d to: %pS",
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@ -11,6 +11,13 @@ enum insn_type {
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RET = 3, /* tramp / site cond-tail-call */
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};
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/*
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* ud1 %esp, %ecx - a 3 byte #UD that is unique to trampolines, chosen such
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* that there is no false-positive trampoline identification while also being a
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* speculation stop.
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*/
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static const u8 tramp_ud[] = { 0x0f, 0xb9, 0xcc };
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/*
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* cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
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*/
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@ -43,6 +50,9 @@ static void __ref __static_call_transform(void *insn, enum insn_type type, void
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break;
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case RET:
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if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
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code = text_gen_insn(JMP32_INSN_OPCODE, insn, &__x86_return_thunk);
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else
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code = &retinsn;
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break;
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}
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@ -60,7 +70,7 @@ static void __static_call_validate(void *insn, bool tail, bool tramp)
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{
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u8 opcode = *(u8 *)insn;
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if (tramp && memcmp(insn+5, "SCT", 3)) {
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if (tramp && memcmp(insn+5, tramp_ud, 3)) {
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pr_err("trampoline signature fail");
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BUG();
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}
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@ -115,3 +125,29 @@ void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
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mutex_unlock(&text_mutex);
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}
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EXPORT_SYMBOL_GPL(arch_static_call_transform);
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#ifdef CONFIG_RETPOLINE
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/*
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* This is called by apply_returns() to fix up static call trampolines,
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* specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as
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* having a return trampoline.
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*
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* The problem is that static_call() is available before determining
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* X86_FEATURE_RETHUNK and, by implication, running alternatives.
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*
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* This means that __static_call_transform() above can have overwritten the
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* return trampoline and we now need to fix things up to be consistent.
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*/
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bool __static_call_fixup(void *tramp, u8 op, void *dest)
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{
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if (memcmp(tramp+5, tramp_ud, 3)) {
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/* Not a trampoline site, not our problem. */
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return false;
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}
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if (op == RET_INSN_OPCODE || dest == &__x86_return_thunk)
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__static_call_transform(tramp, RET, NULL);
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return true;
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}
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#endif
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