linux-kernel - Re: [syzbot] [bpf?] possible deadlock in __bpf_ringbuf

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Message-ID: <CAADnVQ+p76vYLjs9zivq694PSqiPPjv7LJOSXCHsLGuMwgG1jw@mail.gmail.com>
Date: Mon, 25 Aug 2025 19:23:50 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Leon Hwang <hffilwlqm@...il.com>
Cc: syzbot <syzbot+fa5c2814795b5adca240@...kaller.appspotmail.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Eduard <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>, 
	John Fastabend <john.fastabend@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	KP Singh <kpsingh@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Network Development <netdev@...r.kernel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Song Liu <song@...nel.org>, 
	syzkaller-bugs <syzkaller-bugs@...glegroups.com>, Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [syzbot] [bpf?] possible deadlock in __bpf_ringbuf_reserve (2)

On Mon, Aug 25, 2025 at 7:20 PM Leon Hwang <hffilwlqm@...il.com> wrote:
>
>
>
> On 26/8/25 01:39, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    dd9de524183a xsk: Fix immature cq descriptor production
> > git tree:       bpf
> > console output: https://syzkaller.appspot.com/x/log.txt?x=102da862580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=c321f33e4545e2a1
> > dashboard link: https://syzkaller.appspot.com/bug?extid=fa5c2814795b5adca240
> > compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=142da862580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1588aef0580000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/5a3389c1558f/disk-dd9de524.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/c97133192a27/vmlinux-dd9de524.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/3ae5a1a88637/bzImage-dd9de524.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+fa5c2814795b5adca240@...kaller.appspotmail.com
> >
> > ============================================
> > WARNING: possible recursive locking detected
> > syzkaller #0 Not tainted
> > --------------------------------------------
> > syz-execprog/5866 is trying to acquire lock:
> > ffffc900048c10d8 (&rb->spinlock){-.-.}-{2:2}, at: __bpf_ringbuf_reserve+0x1c7/0x5a0 kernel/bpf/ringbuf.c:423
> >
> > but task is already holding lock:
> > ffffc900048e90d8 (&rb->spinlock){-.-.}-{2:2}, at: __bpf_ringbuf_reserve+0x1c7/0x5a0 kernel/bpf/ringbuf.c:423
> >
> > other info that might help us debug this:
> >  Possible unsafe locking scenario:
> >
> >        CPU0
> >        ----
> >   lock(&rb->spinlock);
> >   lock(&rb->spinlock);
> >
> >  *** DEADLOCK ***
> >
> >  May be due to missing lock nesting notation
>
> Confirmed.
>
> I can reproduce this deadlock issue and will work on a fix.

Don't.

It's due to revert.
Once rqspinlock is fixed the revert will be reverted.