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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Wed, 8 Jun 2022 17:05:58 -0700
Message-ID: <CABCJKueigY+oJ0HTV_YXKcTZG35ACxPE_YpKp_ZURgzxNYQ4WA@mail.gmail.com>
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups
To:     =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= <maskray@google.com>
Cc:     Peter Collingbourne <pcc@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Joao Moreira <joao@overdrivepizza.com>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        hjl.tools@gmail.com, Josh Poimboeuf <jpoimboe@redhat.com>,
        andrew.cooper3@citrix.com, LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        llvm@lists.linux.dev
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On Wed, Jun 8, 2022 at 10:53 AM F=C4=81ng-ru=C3=AC S=C3=B2ng <maskray@googl=
e.com> wrote:
>
> Hi Peter,
>
> On Tue, Mar 1, 2022 at 7:06 PM Peter Collingbourne <pcc@google.com> wrote=
:
> >
> > Hi Peter,
> > One issue with this call sequence is that:
> >
> > On Fri, Feb 11, 2022 at 02:38:03PM +0100, Peter Zijlstra wrote:
> > > caller:
> > >       cmpl    $0xdeadbeef, -0x4(%rax)         # 7 bytes
> >
> > Because this instruction ends in the constant 0xdeadbeef, it may
> > be used as a "gadget" that would effectively allow branching to an
> > arbitrary address in %rax if the attacker can arrange to set ZF=3D1.
>
> Do you mind elaborating how this instruction can be used as a gadget?
> How does it look like?

With the offset of -4, the je instruction here can be an indirect call
target because it's preceded by a valid type hash at the end of the
cmpl instruction. If we change the offset to -6, only the ud2
instruction is a potential call target in this sequence, which will be
less useful to an attacker.

> The information will be useful to the summary of Sami's KCFI LLVM
> patch: https://reviews.llvm.org/D119296

I'll add more information about the X86 preamble to the description.

Sami