[syzbot] [net?] possible deadlock in __sk_receive_skb

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 97e797263a5e Add linux-next specific files for 20260420
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=17c62c36580000
kernel config: https://syzkaller.appspot.com/x/.config?x=354b135d724a721f
dashboard link: https://syzkaller.appspot.com/bug?extid=9f4a135646b66c509935
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10f622d2580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12f88e6a580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/bfe08255b02b/disk-97e79726.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/70874e341331/vmlinux-97e79726.xz
kernel image: https://storage.googleapis.com/syzbot-assets/68352f7fca94/bzImage-97e79726.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+9f4a13...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.17/6130 is trying to acquire lock:
ffff88803976da20 (slock-AF_PHONET/1){+.+.}-{3:3}, at: __sk_receive_skb+0x1bf/0x9e0 net/core/sock.c:563

but task is already holding lock:
ffff88803976e2e0 (slock-AF_PHONET){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
ffff88803976e2e0 (slock-AF_PHONET){+...}-{3:3}, at: __sk_receive_skb+0x1f1/0x9e0 net/core/sock.c:565

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (slock-AF_PHONET){+...}-{3:3}:
rt_spin_lock+0x83/0x400 kernel/locking/spinlock_rt.c:56
spin_lock include/linux/spinlock_rt.h:45 [inline]
__sk_receive_skb+0x1f1/0x9e0 net/core/sock.c:565
sk_receive_skb include/net/sock.h:2022 [inline]
phonet_rcv+0x781/0xc40 net/phonet/af_phonet.c:-1
__netif_receive_skb_one_core net/core/dev.c:6210 [inline]
__netif_receive_skb net/core/dev.c:6323 [inline]
process_backlog+0x5e1/0xc60 net/core/dev.c:6674
__napi_poll+0xab/0x550 net/core/dev.c:7738
napi_poll net/core/dev.c:7801 [inline]
net_rx_action+0x696/0xe00 net/core/dev.c:7958
handle_softirqs+0x1de/0x6d0 kernel/softirq.c:626
__do_softirq kernel/softirq.c:660 [inline]
__local_bh_enable_ip+0x170/0x2b0 kernel/softirq.c:302
local_bh_enable include/linux/bottom_half.h:33 [inline]
netif_rx+0xb9/0xf0 net/core/dev.c:5776
pn_send+0x62a/0x8e0 net/phonet/af_phonet.c:188
pn_skb_send+0x218/0x530 net/phonet/af_phonet.c:275
pep_indicate net/phonet/pep.c:123 [inline]
pipe_snd_status+0x1f1/0x320 net/phonet/pep.c:221
pipe_grant_credits net/phonet/pep.c:244 [inline]
pipe_do_rcv+0xf15/0x16a0 net/phonet/pep.c:433
sk_backlog_rcv include/net/sock.h:1190 [inline]
__sk_receive_skb+0x962/0x9e0 net/core/sock.c:572
sk_receive_skb include/net/sock.h:2022 [inline]
pep_do_rcv+0x685/0xaa0 net/phonet/pep.c:675
sk_backlog_rcv include/net/sock.h:1190 [inline]
__release_sock+0x2a9/0x3d0 net/core/sock.c:3216
release_sock+0x1be/0x290 net/core/sock.c:3815
pep_sock_accept+0xd47/0x11e0 net/phonet/pep.c:879
pn_socket_accept+0xc1/0x310 net/phonet/socket.c:303
do_accept+0x6ca/0x930 net/socket.c:2062
__sys_accept4_file net/socket.c:2096 [inline]
__sys_accept4+0x139/0x230 net/socket.c:2118
__do_sys_accept4 net/socket.c:2125 [inline]
__se_sys_accept4 net/socket.c:2122 [inline]
__x64_sys_accept4+0x9a/0xb0 net/socket.c:2122
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (slock-AF_PHONET/1){+.+.}-{3:3}:
check_prev_add kernel/locking/lockdep.c:3167 [inline]
check_prevs_add kernel/locking/lockdep.c:3286 [inline]
validate_chain kernel/locking/lockdep.c:3910 [inline]
__lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5239
lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5870
rt_spin_lock_nested+0x81/0x3f0 kernel/locking/spinlock_rt.c:64
__sk_receive_skb+0x1bf/0x9e0 net/core/sock.c:563
sk_receive_skb include/net/sock.h:2022 [inline]
pep_do_rcv+0x685/0xaa0 net/phonet/pep.c:675
sk_backlog_rcv include/net/sock.h:1190 [inline]
__sk_receive_skb+0x962/0x9e0 net/core/sock.c:572
sk_receive_skb include/net/sock.h:2022 [inline]
phonet_rcv+0x781/0xc40 net/phonet/af_phonet.c:-1
__netif_receive_skb_one_core net/core/dev.c:6210 [inline]
__netif_receive_skb net/core/dev.c:6323 [inline]
process_backlog+0x5e1/0xc60 net/core/dev.c:6674
__napi_poll+0xab/0x550 net/core/dev.c:7738
napi_poll net/core/dev.c:7801 [inline]
net_rx_action+0x696/0xe00 net/core/dev.c:7958
handle_softirqs+0x1de/0x6d0 kernel/softirq.c:626
__do_softirq kernel/softirq.c:660 [inline]
__local_bh_enable_ip+0x170/0x2b0 kernel/softirq.c:302
local_bh_enable include/linux/bottom_half.h:33 [inline]
netif_rx+0xb9/0xf0 net/core/dev.c:5776
pn_send+0x62a/0x8e0 net/phonet/af_phonet.c:188
pn_skb_send+0x218/0x530 net/phonet/af_phonet.c:275
pipe_skb_send+0x2f7/0x540 net/phonet/pep.c:1130
pep_sendmsg+0x9ca/0xb00 net/phonet/pep.c:1206
pn_socket_sendmsg+0x1e5/0x250 net/phonet/socket.c:424
sock_sendmsg_nosec+0x112/0x150 net/socket.c:797
__sock_sendmsg net/socket.c:812 [inline]
sock_write_iter+0x308/0x410 net/socket.c:1269
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x629/0xba0 fs/read_write.c:688
ksys_write+0x156/0x270 fs/read_write.c:740
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(slock-AF_PHONET);
lock(slock-AF_PHONET/1);
lock(slock-AF_PHONET);
lock(slock-AF_PHONET/1);

*** DEADLOCK ***

6 locks held by syz.0.17/6130:
#0: ffff88803aded218 (sk_lock-AF_PHONET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1713 [inline]
#0: ffff88803aded218 (sk_lock-AF_PHONET){+.+.}-{0:0}, at: pep_sendmsg+0x7b6/0xb00 net/phonet/pep.c:1199
#1: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
#2: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#2: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#2: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: process_backlog+0x271/0xc60 net/core/dev.c:6673
#3: ffff88803976e2e0 (slock-AF_PHONET){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
#3: ffff88803976e2e0 (slock-AF_PHONET){+...}-{3:3}, at: __sk_receive_skb+0x1f1/0x9e0 net/core/sock.c:565
#4: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#4: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#4: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
#4: ffffffff8e3c8140 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1e0/0x400 kernel/locking/spinlock_rt.c:57
#5: ffff88803976e398 (sk_lock-AF_PHONET){+.+.}-{0:0}, at: sk_receive_skb include/net/sock.h:2022 [inline]
#5: ffff88803976e398 (sk_lock-AF_PHONET){+.+.}-{0:0}, at: phonet_rcv+0x781/0xc40 net/phonet/af_phonet.c:-1

stack backtrace:
CPU: 0 UID: 0 PID: 6130 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2045
check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2177
check_prev_add kernel/locking/lockdep.c:3167 [inline]
check_prevs_add kernel/locking/lockdep.c:3286 [inline]
validate_chain kernel/locking/lockdep.c:3910 [inline]
__lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5239
lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5870
rt_spin_lock_nested+0x81/0x3f0 kernel/locking/spinlock_rt.c:64
__sk_receive_skb+0x1bf/0x9e0 net/core/sock.c:563
sk_receive_skb include/net/sock.h:2022 [inline]
pep_do_rcv+0x685/0xaa0 net/phonet/pep.c:675
sk_backlog_rcv include/net/sock.h:1190 [inline]
__sk_receive_skb+0x962/0x9e0 net/core/sock.c:572
sk_receive_skb include/net/sock.h:2022 [inline]
phonet_rcv+0x781/0xc40 net/phonet/af_phonet.c:-1
__netif_receive_skb_one_core net/core/dev.c:6210 [inline]
__netif_receive_skb net/core/dev.c:6323 [inline]
process_backlog+0x5e1/0xc60 net/core/dev.c:6674
__napi_poll+0xab/0x550 net/core/dev.c:7738
napi_poll net/core/dev.c:7801 [inline]
net_rx_action+0x696/0xe00 net/core/dev.c:7958
handle_softirqs+0x1de/0x6d0 kernel/softirq.c:626
__do_softirq kernel/softirq.c:660 [inline]
__local_bh_enable_ip+0x170/0x2b0 kernel/softirq.c:302
local_bh_enable include/linux/bottom_half.h:33 [inline]
netif_rx+0xb9/0xf0 net/core/dev.c:5776
pn_send+0x62a/0x8e0 net/phonet/af_phonet.c:188
pn_skb_send+0x218/0x530 net/phonet/af_phonet.c:275
pipe_skb_send+0x2f7/0x540 net/phonet/pep.c:1130
pep_sendmsg+0x9ca/0xb00 net/phonet/pep.c:1206
pn_socket_sendmsg+0x1e5/0x250 net/phonet/socket.c:424
sock_sendmsg_nosec+0x112/0x150 net/socket.c:797
__sock_sendmsg net/socket.c:812 [inline]
sock_write_iter+0x308/0x410 net/socket.c:1269
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x629/0xba0 fs/read_write.c:688
ksys_write+0x156/0x270 fs/read_write.c:740
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fd74410c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fd743745028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fd744386090 RCX: 00007fd74410c819
RDX: 00000000000003db RSI: 0000200000000480 RDI: 0000000000000006
RBP: 00007fd7441a2c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fd744386128 R14: 00007fd744386090 R15: 00007fff7efc5418
</TASK>


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