linux-kernel - Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read

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Message-ID: <CANn89iLdozfCse24+G65+XKE25MzOQkcN+eMM_n-pkTjVQUJhQ@mail.gmail.com>
Date:   Fri, 14 Aug 2020 12:12:40 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jann Horn <jannh@...gle.com>,
        syzbot <syzkaller@...glegroups.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Chang S . Bae" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Markus T Metzger <markus.t.metzger@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

On Fri, Aug 14, 2020 at 12:03 PM Andy Lutomirski <luto@...capital.net> wrote:
>
>
>
> > On Aug 14, 2020, at 11:16 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > syzbot found its way in 86_fsgsbase_read_task() [1]
> >
> > Fix is to make sure ldt pointer is not NULL
>
> Acked-by: Andy Lutomirski <luto@...nel.org>
>
> Maybe add something like this to the changelog:
>
> This can happen if ptrace() or sigreturn() pokes an LDT selector into FS or GS for a task with no LDT and something tries to read the base before a return to usermode notices the bad selector and fixes it.
>
> I’ll see if I can whip up a test case too.
>

Jann has a repro if needed (and syzbot also had one)