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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
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1cd5f059d9
Paolo reported that the instruction sequence that is used to replace:
call __static_call_return0
namely:
66 66 48 31 c0 data16 data16 xor %rax,%rax
decodes to something else on i386, namely:
66 66 48 data16 dec %ax
31 c0 xor %eax,%eax
Which is a nonsensical sequence that happens to have the same outcome.
*However* an important distinction is that it consists of 2
instructions which is a problem when the thing needs to be overwriten
with a regular call instruction again.
As such, replace the instruction with something that decodes the same
on both i386 and x86_64.
Fixes: 3f2a8fc4b1 ("static_call/x86: Add __static_call_return0()")
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220318204419.GT8939@worktop.programming.kicks-ass.net
117 lines
2.5 KiB
C
117 lines
2.5 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/static_call.h>
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#include <linux/memory.h>
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#include <linux/bug.h>
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#include <asm/text-patching.h>
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enum insn_type {
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CALL = 0, /* site call */
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NOP = 1, /* site cond-call */
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JMP = 2, /* tramp / site tail-call */
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RET = 3, /* tramp / site cond-tail-call */
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};
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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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static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
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static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
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static void __ref __static_call_transform(void *insn, enum insn_type type, void *func)
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{
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const void *emulate = NULL;
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int size = CALL_INSN_SIZE;
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const void *code;
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switch (type) {
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case CALL:
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code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
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if (func == &__static_call_return0) {
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emulate = code;
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code = &xor5rax;
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}
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break;
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case NOP:
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code = x86_nops[5];
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break;
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case JMP:
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code = text_gen_insn(JMP32_INSN_OPCODE, insn, func);
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break;
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case RET:
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code = &retinsn;
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break;
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}
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if (memcmp(insn, code, size) == 0)
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return;
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if (unlikely(system_state == SYSTEM_BOOTING))
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return text_poke_early(insn, code, size);
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text_poke_bp(insn, code, size, emulate);
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}
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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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pr_err("trampoline signature fail");
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BUG();
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}
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if (tail) {
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if (opcode == JMP32_INSN_OPCODE ||
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opcode == RET_INSN_OPCODE)
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return;
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} else {
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if (opcode == CALL_INSN_OPCODE ||
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!memcmp(insn, x86_nops[5], 5) ||
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!memcmp(insn, xor5rax, 5))
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return;
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}
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/*
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* If we ever trigger this, our text is corrupt, we'll probably not live long.
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*/
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pr_err("unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
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BUG();
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}
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static inline enum insn_type __sc_insn(bool null, bool tail)
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{
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/*
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* Encode the following table without branches:
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*
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* tail null insn
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* -----+-------+------
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* 0 | 0 | CALL
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* 0 | 1 | NOP
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* 1 | 0 | JMP
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* 1 | 1 | RET
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*/
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return 2*tail + null;
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}
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void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
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{
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mutex_lock(&text_mutex);
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if (tramp) {
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__static_call_validate(tramp, true, true);
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__static_call_transform(tramp, __sc_insn(!func, true), func);
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}
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if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
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__static_call_validate(site, tail, false);
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__static_call_transform(site, __sc_insn(!func, tail), func);
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}
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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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