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

[Peter Zijlstra <peterz@infradead.org>]

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;