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

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Message-ID: <Yx37QmkpvPoyHFMN@hirez.programming.kicks-ass.net>
Date:   Sun, 11 Sep 2022 17:14:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...e.de>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH] x86,retpoline: Be sure to emit INT3 after JMP *%\reg

On Fri, Sep 09, 2022 at 09:48:09AM -0700, Josh Poimboeuf wrote:
> On Fri, Sep 09, 2022 at 10:16:13AM +0200, Peter Zijlstra wrote:
> > +++ 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 */
> >  	}
> 
> Hm, if you have retpolines disabled at runtime, why would you want this.

Because I don't think eIBRS guarantees it will not SLS.