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x86,static_call: Use alternative RET encoding

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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>
This commit is contained in:
Peter Zijlstra 2022-06-14 23:15:39 +02:00 committed by Borislav Petkov
parent d9e9d23006
commit ee88d363d1
3 changed files with 64 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
arch/x86/include/asm/static_call.h
View File

@ -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 */

12
arch/x86/kernel/alternative.c
View File

@ -539,18 +539,22 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end)
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",

38
arch/x86/kernel/static_call.c
View File

@ -11,6 +11,13 @@ enum insn_type {
RET = 3, /* tramp / site cond-tail-call */
};
/*
* 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
*/
@ -43,6 +50,9 @@ static void __ref __static_call_transform(void *insn, enum insn_type type, void
break;
case RET:
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 *insn, bool tail, bool tramp)
{
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 *site, void *tramp, void *func, bool tail)
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