[PATCH 5.18 14/70] x86,static_call: Use alternative RET encoding

Greg Kroah-Hartman
2022-07-22T09:11:54Z

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