linux-kernel - Re: [PATCH v1] arch/x86/kernel: check the return value of insn_decode

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Message-ID: <CAMEuxRqUsKeqtNu1+cwW0a5o_qt56+O7B3EE2sBzsAc9gKhnsQ@mail.gmail.com>
Date:   Wed, 7 Sep 2022 01:40:08 -0700
From:   Li Zhong <floridsleeves@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        jpoimboe@...nel.org, jbaron@...mai.com, ardb@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v1] arch/x86/kernel: check the return value of insn_decode_kernel()

On Tue, Sep 6, 2022 at 6:58 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri,  2 Sep 2022 00:47:06 -0700
> Li Zhong <floridsleeves@...il.com> wrote:
>
> > @@ -20,9 +20,10 @@
> >  int arch_jump_entry_size(struct jump_entry *entry)
> >  {
> >       struct insn insn = {};
> > +     int ret;
> >
> > -     insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
> > -     BUG_ON(insn.length != 2 && insn.length != 5);
> > +     ret = insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
>
> It's highly unlikely that length will be 2 or 5 if ret is not zero (as it
> is initialized to zero going into this function).
>

In this case, I think maybe just BUG_ON(ret<0) is enough. However, the code
that decides the value of insn->length could be modified in the future. And
there are other variables insn->next_byte and insn->kaddr that are related to
insn->length, which could also be modified. So I guess we can preserve all the
assertion conditions to guarantee all assumptions are satisfied.

> > +     BUG_ON(ret < 0 || insn.length != 2 && insn.length != 5);
>
> In any case, you need parenthesis around the && condition.
>

Thanks. Will add this in v2 patch.

> -- Steve
>
>
> >
> >       return insn.length;