linux-kernel - Re: BUG: null pointer dereference in seccomp

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Message-ID: <CAG48ez3bxCBtWbMG7eooSivNQtb+kTixk6Z2EaEQbmLPB+G=bg@mail.gmail.com>
Date: Thu, 29 Aug 2024 22:09:51 +0200
From: Jann Horn <jannh@...gle.com>
To: Juefei Pu <juefei.pu@...il.ucr.edu>
Cc: Kees Cook <kees@...nel.org>, luto@...capital.net, wad@...omium.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, 
	bpf@...r.kernel.org
Subject: Re: BUG: null pointer dereference in seccomp

On Thu, Aug 29, 2024 at 2:38 AM Kees Cook <kees@...nel.org> wrote:
> On Tue, Aug 27, 2024 at 09:09:49PM -0700, Juefei Pu wrote:
> > Hello,
> > We found the following null-pointer-dereference issue using syzkaller
> > on Linux v6.10.
>
> In seccomp! Yikes.
>
> > Unfortunately, the syzkaller failed to generate a reproducer.
>
> That's a bummer.
>
> > But at least we have the report:
> >
> > Oops: general protection fault, probably for non-canonical address
> > 0xdffffc0000000006: 0000 [#1] PREEMPT SMP KASAN PTI
> > KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
> > CPU: 0 PID: 4493 Comm: systemd-journal Not tainted 6.10.0 #13
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> > RIP: 0010:__bpf_prog_run include/linux/filter.h:691 [inline]

So, the issue can't be that we went through the seccomp filter setup
path properly and the bpf_prog was never actually installed, because
in that case we would've crashed in seccomp_cache_prepare_bitmap().

I'm pretty confused how we could've possibly gotten into this situation...

What kind of test setup are you using - is this QEMU in TCG mode or in
KVM mode? Can you share your vmlinux binary, so we can see if there's
a clue left in the register state of the crash?