linux-kernel - Re: [syzbot] [bpf?] WARNING in convert_ctx

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Message-ID: <aIt1MuiclSoonZLe@mail.gmail.com>
Date: Thu, 31 Jul 2025 15:53:43 +0200
From: Paul Chaignon <paul.chaignon@...il.com>
To: syzbot <syzbot+ccac90e482b2a81d74aa@...kaller.appspotmail.com>
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
	daniel@...earbox.net, eddyz87@...il.com, haoluo@...gle.com,
	john.fastabend@...il.com, jolsa@...nel.org, kpsingh@...nel.org,
	linux-kernel@...r.kernel.org, martin.lau@...ux.dev, sdf@...ichev.me,
	song@...nel.org, syzkaller-bugs@...glegroups.com,
	yonghong.song@...ux.dev
Subject: Re: [syzbot] [bpf?] WARNING in convert_ctx_accesses

This is happening because flow_dissector_is_valid_access matches on
ranges (bpf_ctx_range) whereas flow_dissector_convert_ctx_access matches
on offsets (offsetof). Hence an access to
offsetof(struct __sk_buff, data_end) + 1 is considered valid and then
fails during convertion.

I'll send a fix asap.

Paul

On Thu, Jul 31, 2025 at 06:43:05AM -0700, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit 0df1a55afa832f463f9ad68ddc5de92230f1bc8a
> Author: Paul Chaignon <paul.chaignon@...il.com>
> Date:   Tue Jul 1 18:36:15 2025 +0000
> 
>     bpf: Warn on internal verifier errors
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=17d6aca2580000
> start commit:   e8d780dcd957 Merge tag 'slab-for-6.17' of git://git.kernel..
> git tree:       upstream
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=1436aca2580000
> console output: https://syzkaller.appspot.com/x/log.txt?x=1036aca2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d32de89be62206c8
> dashboard link: https://syzkaller.appspot.com/bug?extid=ccac90e482b2a81d74aa
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=131049bc580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11cc2cf0580000
> 
> Reported-by: syzbot+ccac90e482b2a81d74aa@...kaller.appspotmail.com
> Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection