linux-kernel - Re: [syzbot] WARNING: kmalloc bug in xdp_umem

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Message-ID: <CAJ+HfNhyfsT5cS_U9EC213ducHs9k9zNxX9+abqC0kTrPbQ0gg@mail.gmail.com>
Date:   Tue, 7 Dec 2021 09:49:04 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     syzbot <syzbot+11421fbbff99b989670e@...kaller.appspotmail.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        KP Singh <kpsingh@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Netdev <netdev@...r.kernel.org>,
        Song Liu <songliubraving@...com>,
        syzkaller-bugs@...glegroups.com, Yonghong Song <yhs@...com>
Subject: Re: [syzbot] WARNING: kmalloc bug in xdp_umem_create (2)

On Mon, 6 Dec 2021 at 11:55, syzbot
<syzbot+11421fbbff99b989670e@...kaller.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@...kaller.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?


Björn