[syzbot] [net?] [bpf?] general protection fault in bq_flush_to_queue (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: aa833fc394ba drm/xe: Fix duplicated put due to merge resol..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12f27cc2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=bb66b4eefaf3f448
dashboard link: https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=148d701a580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=166e7192580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/262740328d6d/disk-aa833fc3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/bf629be0ab14/vmlinux-aa833fc3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b71e61abc00a/bzImage-aa833fc3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2b3391...@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 6085 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:__list_del include/linux/list.h:204 [inline]
RIP: 0010:__list_del_clearprev include/linux/list.h:217 [inline]
RIP: 0010:bq_flush_to_queue+0x46f/0x580 kernel/bpf/cpumap.c:740
Code: 35 00 4d 8b 26 4d 8d 74 24 08 4c 89 f0 48 c1 e8 03 42 80 3c 38 00 74 08 4c 89 f7 e8 0b f2 35 00 49 89 1e 48 89 d8 48 c1 e8 03 <42> 80 3c 38 00 74 08 48 89 df e8 f2 f1 35 00 4c 89 23 48 8b 04 24
RSP: 0018:ffffc90004207650 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000004 RDI: 00000000ffffffff
RBP: ffff888030b0c400 R08: ffff888024b2802b R09: 1ffff11004965005
R10: dffffc0000000000 R11: ffffed1004965006 R12: ffffc90003ad79e0
R13: 0000000000000008 R14: ffffc90003ad79e8 R15: dffffc0000000000
FS: 00007f6b36bed6c0(0000) GS:ffff888126d5e000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6b36becf98 CR3: 000000003f6c6000 CR4: 00000000003526f0
Call Trace:
<TASK>
__cpu_map_flush+0x5d/0xd0 kernel/bpf/cpumap.c:808
xdp_do_flush+0x13c/0x1d0 net/core/filter.c:4348
xdp_test_run_batch net/bpf/test_run.c:348 [inline]
bpf_test_run_xdp_live+0x154f/0x1b20 net/bpf/test_run.c:379
bpf_prog_test_run_xdp+0x7c0/0x10e0 net/bpf/test_run.c:1388
bpf_prog_test_run+0x2cd/0x340 kernel/bpf/syscall.c:4703
__sys_bpf+0x562/0x860 kernel/bpf/syscall.c:6182
__do_sys_bpf kernel/bpf/syscall.c:6274 [inline]
__se_sys_bpf kernel/bpf/syscall.c:6272 [inline]
__x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6272
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6b3759f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f6b36bed038 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007f6b377f6090 RCX: 00007f6b3759f749
RDX: 0000000000000050 RSI: 0000200000000000 RDI: 000000000000000a
RBP: 00007f6b37623f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6b377f6128 R14: 00007f6b377f6090 R15: 00007ffda92d99b8
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__list_del include/linux/list.h:204 [inline]
RIP: 0010:__list_del_clearprev include/linux/list.h:217 [inline]
RIP: 0010:bq_flush_to_queue+0x46f/0x580 kernel/bpf/cpumap.c:740
Code: 35 00 4d 8b 26 4d 8d 74 24 08 4c 89 f0 48 c1 e8 03 42 80 3c 38 00 74 08 4c 89 f7 e8 0b f2 35 00 49 89 1e 48 89 d8 48 c1 e8 03 <42> 80 3c 38 00 74 08 48 89 df e8 f2 f1 35 00 4c 89 23 48 8b 04 24
RSP: 0018:ffffc90004207650 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000004 RDI: 00000000ffffffff
RBP: ffff888030b0c400 R08: ffff888024b2802b R09: 1ffff11004965005
R10: dffffc0000000000 R11: ffffed1004965006 R12: ffffc90003ad79e0
R13: 0000000000000008 R14: ffffc90003ad79e8 R15: dffffc0000000000
FS: 00007f6b36bed6c0(0000) GS:ffff888126d5e000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6b36becf98 CR3: 000000003f6c6000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
0: 35 00 4d 8b 26 xor $0x268b4d00,%eax
5: 4d 8d 74 24 08 lea 0x8(%r12),%r14
a: 4c 89 f0 mov %r14,%rax
d: 48 c1 e8 03 shr $0x3,%rax
11: 42 80 3c 38 00 cmpb $0x0,(%rax,%r15,1)
16: 74 08 je 0x20
18: 4c 89 f7 mov %r14,%rdi
1b: e8 0b f2 35 00 call 0x35f22b
20: 49 89 1e mov %rbx,(%r14)
23: 48 89 d8 mov %rbx,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 38 00 cmpb $0x0,(%rax,%r15,1) <-- trapping instruction
2f: 74 08 je 0x39
31: 48 89 df mov %rbx,%rdi
34: e8 f2 f1 35 00 call 0x35f22b
39: 4c 89 23 mov %r12,(%rbx)
3c: 48 8b 04 24 mov (%rsp),%rax


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+2b3391...@syzkaller.appspotmail.com
Tested-by: syzbot+2b3391...@syzkaller.appspotmail.com

Tested on:

commit: 8f0b4cce Linux 6.19-rc1
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=137ab91a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=412fc3ec22077a03

dashboard link: https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=10be172c580000

Note: testing is done by a robot and is best-effort only.

[Alexei Starovoitov]

On Mon, Dec 15, 2025 at 7:14 PM <dongl...@126.com> wrote:
>
> From: DLpang <dongl...@126.com>
>
> #syz test
>
> Hi,
>
> This patch fixes a NULL pointer dereference in the BPF subsystem that occurs
> when __list_del_clearprev() is called on an already-cleared flush_node list_head.
>
> The fix includes two parts:
> 1. Properly initialize the flush_node list_head during per-CPU bulk queue allocation
> using INIT_LIST_HEAD(&bq->flush_node)
> 2. Add defensive checks before calling __list_del_clearprev() to ensure the node
> is actually in the list by checking if (bq->flush_node.prev)
>
> According to the __list_del_clearprev documentation in include/linux/list.h,
> 'The code that uses this needs to check the node 'prev' pointer instead of calling list_empty()'.
>
> This patch fixes the following syzbot-reported issue:
> https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
>
> Reported-by: syzbot+2b3391...@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
> Signed-off-by: DLpang <dongl...@126.com>

If you're going to throw AI slop at us, please do your homework
and root cause the issue.
Nothing in this AI generated commit log explains the bug.

pw-bot: cr

[dongl...@126.com]

From: DLpang <dongl...@126.com>

#syz test

Hi,

This patch fixes a NULL pointer dereference in the BPF subsystem that occurs
when __list_del_clearprev() is called on an already-cleared flush_node list_head.

The fix includes two parts:
1. Properly initialize the flush_node list_head during per-CPU bulk queue allocation
using INIT_LIST_HEAD(&bq->flush_node)
2. Add defensive checks before calling __list_del_clearprev() to ensure the node
is actually in the list by checking if (bq->flush_node.prev)

According to the __list_del_clearprev documentation in include/linux/list.h,
'The code that uses this needs to check the node 'prev' pointer instead of calling list_empty()'.

This patch fixes the following syzbot-reported issue:
https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91

Reported-by: syzbot+2b3391...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
Signed-off-by: DLpang <dongl...@126.com>

---
kernel/bpf/cpumap.c | 4 +++-
kernel/bpf/devmap.c | 3 ++-
net/xdp/xsk.c | 3 ++-
3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index 703e5df1f4ef..248336df591a 100644
--- a/kernel/bpf/cpumap.c
+++ b/kernel/bpf/cpumap.c
@@ -450,6 +450,7 @@ __cpu_map_entry_alloc(struct bpf_map *map, struct bpf_cpumap_val *value,

for_each_possible_cpu(i) {
bq = per_cpu_ptr(rcpu->bulkq, i);
+ INIT_LIST_HEAD(&bq->flush_node);
bq->obj = rcpu;
}

@@ -737,7 +738,8 @@ static void bq_flush_to_queue(struct xdp_bulk_queue *bq)
bq->count = 0;
spin_unlock(&q->producer_lock);

- __list_del_clearprev(&bq->flush_node);
+ if (bq->flush_node.prev)
+ __list_del_clearprev(&bq->flush_node);

/* Feedback loop via tracepoints */
trace_xdp_cpumap_enqueue(rcpu->map_id, processed, drops, to_cpu);
diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 2625601de76e..7a7347e709cc 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -428,7 +428,8 @@ void __dev_flush(struct list_head *flush_list)
bq_xmit_all(bq, XDP_XMIT_FLUSH);
bq->dev_rx = NULL;
bq->xdp_prog = NULL;
- __list_del_clearprev(&bq->flush_node);
+ if (bq->flush_node.prev)
+ __list_del_clearprev(&bq->flush_node);
}
}

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index f093c3453f64..052b8583542d 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -406,7 +406,8 @@ void __xsk_map_flush(struct list_head *flush_list)

list_for_each_entry_safe(xs, tmp, flush_list, flush_node) {
xsk_flush(xs);
- __list_del_clearprev(&xs->flush_node);
+ if (xs->flush_node.prev)
+ __list_del_clearprev(&xs->flush_node);
}
}

--
2.43.0