[PATCH] static_call,x86: Robustify trampoline patching

Peter Zijlstra
2021-10-30T07:50:23Z

On Fri, Oct 29, 2021 at 10:03:24PM +0200, Peter Zijlstra wrote:

> So I had a bit of a peek at what clang generates:
>
> 3fa4: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 3fa7: R_X86_64_32S __SCK__x86_pmu_handle_irq
> 3fab: 48 c7 c6 00 00 00 00 mov $0x0,%rsi 3fae: R_X86_64_32S __SCT__x86_pmu_handle_irq.cfi_jt
> 3fb2: e8 00 00 00 00 call 3fb7 <init_hw_perf_events+0x1dc> 3fb3: R_X86_64_PLT32 __static_call_update-0x4
>
> So this then gives the trampoline jump table entry to
> __static_call_update(), with the result that it will rewrite the
> jump-table entry, not the trampoline!
>
> Now it so happens that the trampoline looks *exactly* like the
> jump-table entry (one jmp.d32 instruction), so in that regards it'll
> again 'work'.
>
> But this is all really, as in *really*, wrong. And I'm really sad I'm
> the one to have to discover this, even though I've mentioned
> static_call()s being tricky in previous reviews.

The below makes the clang-cfi build properly sick:

[ 0.000000] trampoline signature fail
[ 0.000000] ------------[ cut here ]------------
[ 0.000000] kernel BUG at arch/x86/kernel/static_call.c:65!

---
Subject: static_call,x86: Robustify trampoline patching

Add a few signature bytes after the static call trampoline and verify
those bytes match before patching the trampoline. This avoids patching
random other JMPs (such as CFI jump-table entries) instead.

These bytes decode as:

d: 53 push %rbx
e: 43 54 rex.XB push %r12

And happen to spell "SCT".

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/static_call.h | 1 +
arch/x86/kernel/static_call.c | 14 ++++++++++----
tools/objtool/check.c | 3 +++
3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/static_call.h b/arch/x86/include/asm/static_call.h
index cbb67b6030f9..39ebe0511869 100644
--- a/arch/x86/include/asm/static_call.h
+++ b/arch/x86/include/asm/static_call.h
@@ -27,6 +27,7 @@
".globl " STATIC_CALL_TRAMP_STR(name) " \n" \
STATIC_CALL_TRAMP_STR(name) ": \n" \
insns " \n" \
+ ".byte 0x53, 0x43, 0x54 \n" \
".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \
".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
".popsection \n")
diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index ea028e736831..9c407a33a774 100644
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -56,10 +56,15 @@ static void __ref __static_call_transform(void *insn, enum insn_type type, void
text_poke_bp(insn, code, size, emulate);
}

-static void __static_call_validate(void *insn, bool tail)
+static void __static_call_validate(void *insn, bool tail, bool tramp)
{
u8 opcode = *(u8 *)insn;

+ if (tramp && memcmp(insn+5, "SCT", 3)) {
+ pr_err("trampoline signature fail");
+ BUG();
+ }
+
if (tail) {
if (opcode == JMP32_INSN_OPCODE ||
opcode == RET_INSN_OPCODE)
@@ -74,7 +79,8 @@ static void __static_call_validate(void *insn, bool tail)
/*
* If we ever trigger this, our text is corrupt, we'll probably not live long.
*/
- WARN_ONCE(1, "unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
+ pr_err("unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
+ BUG();
}

static inline enum insn_type __sc_insn(bool null, bool tail)
@@ -97,12 +103,12 @@ void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
mutex_lock(&text_mutex);

if (tramp) {
- __static_call_validate(tramp, true);
+ __static_call_validate(tramp, true, true);
__static_call_transform(tramp, __sc_insn(!func, true), func);
}

if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
- __static_call_validate(site, tail);
+ __static_call_validate(site, tail, false);
__static_call_transform(site, __sc_insn(!func, tail), func);
}

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index fb3f251ea021..6d5951113d53 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3310,6 +3310,9 @@ static bool ignore_unreachable_insn(struct objtool_file *file, struct instructio
if (!insn->func)
return false;

+ if (insn->func->static_call_tramp)
+ return true;
+
/*
* CONFIG_UBSAN_TRAP inserts a UD2 when it sees
* __builtin_unreachable(). The BUG() macro has an unreachable() after