linux-kernel - Re: WARNING: kmalloc bug in xdp_umem

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Date:   Mon, 11 Jun 2018 07:49:41 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        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 <syzkaller-bugs@...glegroups.com>
Subject: Re: WARNING: kmalloc bug in xdp_umem_create

On Sun, Jun 10, 2018 at 3:03 PM, Björn Töpel <bjorn.topel@...il.com> wrote:
>> 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?

Hi Björn,

Yes, either __GFP_NOWARN for allocations with user-controllable size
or stricter custom limit (if we don't want current sla/u/ob
implementation details to be part of public kernel interface).