linux-kernel - Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an indirect jump

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Message-ID: <20230706071705.GD2833176@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Jul 2023 09:17:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Petr Pavlu <petr.pavlu@...e.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, samitolvanen@...gle.com, x86@...nel.org,
        linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an
 indirect jump

On Thu, Jul 06, 2023 at 09:47:23AM +0900, Masami Hiramatsu wrote:

> > > If I understand correctly, all indirect jump will be replaced with JMP_NOSPEC.
> > > If you read the insn_jump_into_range, I onlu jecks the jump code, not call.
> > > So the functions only have indirect call still allow optprobe.
> > 
> > With the introduction of kCFI JMP_NOSPEC is no longer an equivalent to a
> > C indirect jump.
> 
> If I understand correctly, kCFI is enabled by CFI_CLANG, and clang is not
> using jump-tables by default, so we can focus on gcc. In that case
> current check still work, correct?

IIRC clang can use jump tables, but like GCC needs RETPOLINE=n and
IBT=n, so effectively nobody has them.

The reason I did mention kCFI though is that kCFI has a larger 'indirect
jump' sequence, and I'm not sure we've thought about what can go
sideways if that's optprobed.

I suspect the UD2 that's in there will go 'funny' if it's relocated into
an optprobe, as in, it'll not be recognised as a CFI fail.