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

[Daniel Borkmann]

[ +Andrew ]

On 12/7/21 9:49 AM, Björn Töpel wrote:
> On Mon, 6 Dec 2021 at 11:55, syzbot
> <syzbot+11421fbbff99b989670e@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> 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@xxxxxxxxxxxxxxxxxxxx/