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

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Date:   Tue, 12 Jun 2018 14:08:52 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>,
        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

On 06/10/2018 03:03 PM, Björn Töpel wrote:
> 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?

(Now resolved as well.)

#syz fix: xsk: silence warning on memory allocation failure