Re: [PATCH] x86,retpoline: Be sure to emit INT3 after JMP *%\reg

Peter Zijlstra
2022-09-09T08:16:38Z

On Thu, Sep 08, 2022 at 07:01:12AM -0700, Alexei Starovoitov wrote:

> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index c1f6c1c51d99..37f821dee68f 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -419,7 +419,8 @@ static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
> > OPTIMIZER_HIDE_VAR(reg);
> > emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
> > } else {
> > - EMIT2(0xFF, 0xE0 + reg);
> > + EMIT2(0xFF, 0xE0 + reg); /* jmp *%\reg */
> > + EMIT1(0xCC); /* int3 */
>
> Hmm. Why is this unconditional?
> Shouldn't it be guarded with CONFIG_xx or cpu_feature_enabled ?
> People that don't care about hw speculation vulnerabilities
> shouldn't pay the price of increased code size.

Sure, like so then?

---
Subject: x86,retpoline: Be sure to emit INT3 after JMP *%\reg
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 8 Sep 2022 12:04:50 +0200

Both AMD and Intel recommend using INT3 after an indirect JMP. Make sure
to emit one when rewriting the retpoline JMP irrespective of compiler
SLS options or even CONFIG_SLS.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---

arch/x86/kernel/alternative.c | 9 +++++++++
arch/x86/net/bpf_jit_comp.c | 4 +++-
2 files changed, 12 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -453,6 +453,15 @@ static int patch_retpoline(void *addr, s
return ret;
i += ret;

+ /*
+ * The compiler is supposed to EMIT an INT3 after every unconditional
+ * JMP instruction due to AMD BTC. However, if the compiler is too old
+ * or SLS isn't enabled, we still need an INT3 after indirect JMPs
+ * even on Intel.
+ */
+ if (op == JMP32_INSN_OPCODE && i < insn->length)
+ bytes[i++] = INT3_INSN_OPCODE;
+
for (; i < insn->length;)
bytes[i++] = BYTES_NOP1;

--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -419,7 +419,9 @@ static void emit_indirect_jump(u8 **ppro
OPTIMIZER_HIDE_VAR(reg);
emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
} else {
- EMIT2(0xFF, 0xE0 + reg);
+ EMIT2(0xFF, 0xE0 + reg); /* jmp *%\reg */
+ if (IS_ENABLED(CONFIG_RETPOLINE) || IS_ENABLED(CONFIG_SLS))
+ EMIT1(0xCC); /* int3 */
}

*pprog = prog;