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

Alexei Starovoitov
2022-09-08T14:02:06Z

On Thu, Sep 8, 2022 at 3:07 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Sep 08, 2022 at 11:30:41AM +0200, Peter Zijlstra wrote:
> > Let me go do a patch.
>
> ---
> Subject: x86,retpoline: Be sure to emit INT3 after JMP *%\reg
>
> 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 | 3 ++-
> 2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 62f6b8b7c4a5..68d84cf8e001 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -453,6 +453,15 @@ static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> 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;
>
> 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.