linux-kernel - Re: [RFC] Have insn decoder functions return success/failure

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Message-ID: <20201022080405.GS2628@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Oct 2020 10:04:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>, x86-ml <x86@...nel.org>,
        Joerg Roedel <jroedel@...e.de>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Have insn decoder functions return success/failure

On Wed, Oct 21, 2020 at 06:45:58PM +0200, Borislav Petkov wrote:
> On Wed, Oct 21, 2020 at 11:26:13PM +0900, Masami Hiramatsu wrote:
> > Hmm, I meant someone might think it can be used for filtering the
> > instruction something like,
> > 
> > insn_init(insn, buf, buflen, 1);
> > ret = insn_get_length(insn);
> > if (!ret) {
> > 	/* OK, this is safe */
> > 	patch_text(buf, trampoline);
> > }
> > 
> > No, we need another validator for such usage.
> 
> Well, I think calling insn_get_length() should give you only the
> *length* of the insn and nothing else - I mean that is what the function
> is called. And I believe current use is wrong.
> 
> Examples:
> 
> arch/x86/kernel/kprobes/core.c:
>                 insn_get_length(&insn);
> 
>                 /*
>                  * Another debugging subsystem might insert this breakpoint.
>                  * In that case, we can't recover it.
>                  */
>                 if (insn.opcode.bytes[0] == INT3_INSN_OPCODE)
> 
> So this has called get_length but it is far from clear that after that
> call, the opcode bytes in insn.opcode.bytes are there.

Given the trainwreck called x86-instruction-encoding, it's impossible to
not have decoded the opcode and still know the length. Therefore, if you
know the length, you also have the opcode. Hm?