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

[Alexei Starovoitov]

On Fri, Sep 9, 2022 at 1:16 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxx>
> ---
>
>  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 */

Looks better. Ack.