netdev - Re: WARNING: kmalloc bug in xdp_umem

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Date:   Sun, 10 Jun 2018 15:03:14 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     penguin-kernel@...ove.sakura.ne.jp
Cc:     dvyukov@...gle.com,
        syzbot+4abadc5d69117b346506@...kaller.appspotmail.com,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>, syzkaller-bugs@...glegroups.com
Subject: Re: WARNING: kmalloc bug in xdp_umem_create

Den sön 10 juni 2018 kl 14:53 skrev Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp>:
>
> On 2018/06/10 20:52, Dmitry Vyukov wrote:
> > On Sun, Jun 10, 2018 at 11:31 AM, Björn Töpel <bjorn.topel@...il.com> wrote:
> >> Den sön 10 juni 2018 kl 04:53 skrev Tetsuo Handa
> >> <penguin-kernel@...ove.sakura.ne.jp>:
> >>>
> >>> On 2018/06/10 7:47, syzbot wrote:
> >>>> Hello,
> >>>>
> >>>> syzbot found the following crash on:
> >>>>
> >>>> HEAD commit:    7d3bf613e99a Merge tag 'libnvdimm-for-4.18' of git://git.k..
> >>>> git tree:       upstream
> >>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1073f68f800000
> >>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=f04d8d0a2afb789a
> >>>> dashboard link: https://syzkaller.appspot.com/bug?extid=4abadc5d69117b346506
> >>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> >>>> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=13c9756f800000
> >>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16366f9f800000
> >>>>
> >>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> >>>> Reported-by: syzbot+4abadc5d69117b346506@...kaller.appspotmail.com
> >>>>
> >>>> random: sshd: uninitialized urandom read (32 bytes read)
> >>>> random: sshd: uninitialized urandom read (32 bytes read)
> >>>> random: sshd: uninitialized urandom read (32 bytes read)
> >>>> random: sshd: uninitialized urandom read (32 bytes read)
> >>>> random: sshd: uninitialized urandom read (32 bytes read)
> >>>> WARNING: CPU: 1 PID: 4537 at mm/slab_common.c:996 kmalloc_slab+0x56/0x70 mm/slab_common.c:996
> >>>> Kernel panic - not syncing: panic_on_warn set ...
> >>>
> >>> syzbot gave up upon kmalloc(), but actually error handling path has
> >>> NULL pointer dereference bug.
> >>>
> >>
> >> Thanks Tetsuo! This crash has been fixed by Daniel Borkmann in commit
> >> c09290c56376 ("bpf, xdp: fix crash in xdp_umem_unaccount_pages").
> >
> > Let's tell syzbot about this:
> >
> > #syz fix: bpf, xdp: fix crash in xdp_umem_unaccount_pages
> >
> >
> Excuse me, but that patch fixes NULL pointer dereference which occurs after kmalloc()'s
> "WARNING: CPU: 1 PID: 4537 at mm/slab_common.c:996 kmalloc_slab+0x56/0x70 mm/slab_common.c:996"
> message. That is, "Too large memory allocation" itself is not yet fixed.

The code relies on that the sl{u,a,o}b layer says no, and the
setsockopt bails out. The warning could be opted out using
__GFP_NOWARN. Is there another preferred way? Two get_user_pages
calls, where the first call would set pages to NULL just to fault the
region? Walk the process' VMAs? Something else?


Björn