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From:   Eric Dumazet <edumazet@google.com>
Date:   Fri, 14 Aug 2020 12:12:40 -0700
Message-ID: <CANn89iLdozfCse24+G65+XKE25MzOQkcN+eMM_n-pkTjVQUJhQ@mail.gmail.com>
Subject: Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Jann Horn <jannh@google.com>,
        syzbot <syzkaller@googlegroups.com>,
        Andy Lutomirski <luto@kernel.org>,
        "Chang S . Bae" <chang.seok.bae@intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Markus T Metzger <markus.t.metzger@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ravi Shankar <ravi.v.shankar@intel.com>,
        Rik van Riel <riel@surriel.com>,
        Thomas Gleixner <tglx@linutronix.de>
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On Fri, Aug 14, 2020 at 12:03 PM Andy Lutomirski <luto@amacapital.net> wrot=
e:
>
>
>
> > On Aug 14, 2020, at 11:16 AM, Eric Dumazet <edumazet@google.com> wrote:
> >
> > =EF=BB=BFsyzbot found its way in 86_fsgsbase_read_task() [1]
> >
> > Fix is to make sure ldt pointer is not NULL
>
> Acked-by: Andy Lutomirski <luto@kernel.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=E2=80=99ll see if I can whip up a test case too.
>

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