[syzbot] [bpf?] WARNING in convert_ctx_accesses

[syzbot] [bpf?] WARNING in convert_ctx_accesses

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    e8d780dcd957 Merge tag 'slab-for-6.17' of git://git.kernel..
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=151049bc580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d32de89be62206c8
dashboard link: https://syzkaller.appspot.com/bug?extid=ccac90e482b2a81d74aa
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=131049bc580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11cc2cf0580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/c62f8561b026/disk-e8d780dc.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/abb3e0f20140/vmlinux-e8d780dc.xz
kernel image: https://storage.googleapis.com/syzbot-assets/117094d1e482/bzImage-e8d780dc.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ccac90e482b2a81d74aa@syzkaller.appspotmail.com

------------[ cut here ]------------
verifier bug: error during ctx access conversion(1)
WARNING: CPU: 0 PID: 5822 at kernel/bpf/verifier.c:21448 convert_ctx_accesses+0x2045/0x2920 kernel/bpf/verifier.c:21448
Modules linked in:
CPU: 0 UID: 0 PID: 5822 Comm: syz-executor130 Not tainted 6.16.0-syzkaller-06699-ge8d780dcd957 #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:convert_ctx_accesses+0x2045/0x2920 kernel/bpf/verifier.c:21448
Code: c7 c6 a0 95 b5 8b e8 ea a5 07 00 e9 a4 f9 ff ff e8 50 6c e8 ff c6 05 16 03 bd 0e 01 90 48 c7 c7 00 96 b5 8b e8 6c 02 a7 ff 90 <0f> 0b 90 90 e9 f1 fe ff ff e8 2d 6c e8 ff 0f b6 1d f6 02 bd 0e 31
RSP: 0018:ffffc90003dcf6e8 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff817a3658
RDX: ffff8880775dc880 RSI: ffffffff817a3665 RDI: 0000000000000001
RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: dffffc0000000000
R13: ffffc90000ace048 R14: 0000000000000004 R15: ffff888023798000
FS:  0000555582c6c380(0000) GS:ffff88812471e000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000561e04e07000 CR3: 0000000071c4c000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 bpf_check+0x5960/0xc600 kernel/bpf/verifier.c:24736
 bpf_prog_load+0xe41/0x2490 kernel/bpf/syscall.c:2972
 __sys_bpf+0x4a3f/0x4de0 kernel/bpf/syscall.c:6022
 __do_sys_bpf kernel/bpf/syscall.c:6132 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:6130 [inline]
 __x64_sys_bpf+0x78/0xc0 kernel/bpf/syscall.c:6130
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f75a63f53a9
Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffeb40793f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007ffeb40795d8 RCX: 00007f75a63f53a9
RDX: 0000000000000048 RSI: 00002000000054c0 RDI: 0000000000000005
RBP: 00007f75a6468610 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffffffffffff R11: 0000000000000246 R12: 0000000000000001
R13: 00007ffeb40795c8 R14: 0000000000000001 R15: 0000000000000001
 </TASK>


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Re: [syzbot] [bpf?] WARNING in convert_ctx_accesses

[syzbot]

syzbot has bisected this issue to:

commit 0df1a55afa832f463f9ad68ddc5de92230f1bc8a
Author: Paul Chaignon <paul.chaignon@gmail.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@syzkaller.appspotmail.com
Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Re: [syzbot] [bpf?] WARNING in convert_ctx_accesses

[Paul Chaignon]

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@gmail.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@syzkaller.appspotmail.com
> Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection