Re: [syzbot] WARNING: kmalloc bug in xdp_umem_create (2)

From: Daniel Borkmann <daniel@iogearbox.net>
To: "Björn Töpel" <bjorn.topel@gmail.com>,
	syzbot <syzbot+11421fbbff99b989670e@syzkaller.appspotmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>,
	David Miller <davem@davemloft.net>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	Martin KaFai Lau <kafai@fb.com>, KP Singh <kpsingh@kernel.org>,
	Jakub Kicinski <kuba@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Karlsson, Magnus" <magnus.karlsson@intel.com>,
	Netdev <netdev@vger.kernel.org>, Song Liu <songliubraving@fb.com>,
	syzkaller-bugs@googlegroups.com, Yonghong Song <yhs@fb.com>,
	akpm@linux-foundation.org
Subject: Re: [syzbot] WARNING: kmalloc bug in xdp_umem_create (2)
Date: Tue, 7 Dec 2021 10:19:53 +0100	[thread overview]
Message-ID: <3f854ca9-f5d6-4065-c7b1-5e5b25ea742f@iogearbox.net> (raw)
In-Reply-To: <CAJ+HfNhyfsT5cS_U9EC213ducHs9k9zNxX9+abqC0kTrPbQ0gg@mail.gmail.com>

[ +Andrew ]

On 12/7/21 9:49 AM, Björn Töpel wrote:
> On Mon, 6 Dec 2021 at 11:55, syzbot
> <syzbot+11421fbbff99b989670e@syzkaller.appspotmail.com> wrote:
>>
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    a51e3ac43ddb Merge tag 'net-5.16-rc4' of git://git.kernel...
>> git tree:       net
>> console output: https://syzkaller.appspot.com/x/log.txt?x=17f04ebeb00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5b0eee8ab3ea1839
>> dashboard link: https://syzkaller.appspot.com/bug?extid=11421fbbff99b989670e
>> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+11421fbbff99b989670e@syzkaller.appspotmail.com
>>
> 
> This warning stems from mm/utils.c:
>      /* Don't even allow crazy sizes */
>      if (WARN_ON_ONCE(size > INT_MAX))
>          return NULL;
> 
> The structure that is being allocated is the page-pinning accounting.
> AF_XDP has an internal limit of U32_MAX pages, which is *a lot*, but
> still fewer than what memcg allows (PAGE_COUNTER_MAX is a
> LONG_MAX/PAGE_SIZE on 64b systems).
> 
> The (imo hacky) workaround to silence the warning is to decrease the
> U32_MAX limit to something that is less than "sizeof householding
> struct".
> 
> Note that this is a warning, and not an oops/bug.
> 
> Thoughts?

This is coming from 7661809d493b ("mm: don't allow oversized kvmalloc() calls").
There was a recent discussion on this topic here [0]; this adds another instance.

Iff removal would not be an option, could we maybe add a __GFP_LARGE flag to tag
these instances that it is indeed intended that large allocs are allowed (and they
would thus bypass this warning)?

Thanks,
Daniel

   [0] https://lore.kernel.org/bpf/20211201202905.b9892171e3f5b9a60f9da251@linux-foundation.org/