Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

ppp: do not assume bh is held in ppp_channel_bridge_input()

Networking receive path is usually handled from BH handler. However, some protocols need to acquire the socket lock, and packets might be stored in the socket backlog is the socket was owned by a user process.

In this case, release_sock(), __release_sock(), and sk_backlog_rcv() might call the sk->sk_backlog_rcv() handler in process context.

sybot caught ppp was not considering this case in ppp_channel_bridge_input() :

WARNING: inconsistent lock state 6.11.0-rc7-syzkaller-g5f5673607153 #0 Not tainted -------------------------------- inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage. ksoftirqd/1/24 [HC0[0]:SC1[1]:HE1:SE0] takes: ffff0000db7f11e0 (&pch->downl){+.?.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline] ffff0000db7f11e0 (&pch->downl){+.?.}-{2:2}, at: ppp_channel_bridge_input drivers/net/ppp/ppp_generic.c:2272 [inline] ffff0000db7f11e0 (&pch->downl){+.?.}-{2:2}, at: ppp_input+0x16c/0x854 drivers/net/ppp/ppp_generic.c:2304 {SOFTIRQ-ON-W} state was registered at: lock_acquire+0x240/0x728 kernel/locking/lockdep.c:5759 __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline] _raw_spin_lock+0x48/0x60 kernel/locking/spinlock.c:154 spin_lock include/linux/spinlock.h:351 [inline] ppp_channel_bridge_input drivers/net/ppp/ppp_generic.c:2272 [inline] ppp_input+0x16c/0x854 drivers/net/ppp/ppp_generic.c:2304 p…