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

[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+2b3391f44313b3983e91@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 syzkaller@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

[PATCH] bpf: Fix NULL deref in __list_del_clearprev for flush_node

[donglaipang]

From: DLpang <donglaipang@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+2b3391f44313b3983e91@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
Signed-off-by: DLpang <donglaipang@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

Re: [PATCH] bpf: Fix NULL deref in __list_del_clearprev for flush_node

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 2052 bytes --]

> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index 04171fbc39cb..6ab6eae31989 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;
>  	}
>

[ ... ]

> 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);
                ^^^^

The devmap code adds the same defensive check but doesn't initialize
flush_node the way cpumap does. Looking at dev_map_notification(), when
NETDEV_REGISTER occurs, it calls:

    alloc_percpu(struct xdp_dev_bulk_queue)

And then only initializes the dev field:

    per_cpu_ptr(netdev->xdp_bulkq, cpu)->dev = netdev;

Without INIT_LIST_HEAD(&bq->flush_node), if alloc_percpu() returns
memory with non-zero bytes, bq->flush_node.prev could contain garbage.

The enqueue path in bq_enqueue() uses:

    if (!bq->dev_rx)
        list_add(&bq->flush_node, flush_list);

So if dev_rx contains non-zero garbage, the node never gets added to the
list. But this flush code checks bq->flush_node.prev which could also be
garbage, causing __list_del_clearprev() to be called with invalid
pointers.

Does devmap need INIT_LIST_HEAD(&bq->flush_node) similar to cpumap? The
commit message states that initialization is part of the fix, but only
cpumap gets it.

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20255377282

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

[syzbot]

Hello,

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

Reported-by: syzbot+2b3391f44313b3983e91@syzkaller.appspotmail.com
Tested-by: syzbot+2b3391f44313b3983e91@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.

Re: [PATCH] bpf: Fix NULL deref in __list_del_clearprev for flush_node

[Alexei Starovoitov]

On Mon, Dec 15, 2025 at 7:14 PM <donglaipang@126.com> wrote:
>
> From: DLpang <donglaipang@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+2b3391f44313b3983e91@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2b3391f44313b3983e91
> Signed-off-by: DLpang <donglaipang@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