From: Zijing Yin Date: Tue, 19 May 2026 17:26:33 +0000 (-0700) Subject: phonet/pep: disable BH around forwarded sk_receive_skb() X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=dbc81608e3a653dea6cf403f20cae35468b8ab9c;p=thirdparty%2Fkernel%2Flinux.git phonet/pep: disable BH around forwarded sk_receive_skb() The networking receive path is usually run from softirq context, but protocols that take the socket lock may have packets stored in the backlog and processed later from process context. In that case release_sock() -> __release_sock() drops the slock with spin_unlock_bh() and then calls sk->sk_backlog_rcv() with bottom halves enabled. Typical sk_backlog_rcv handlers process the socket whose backlog is being drained, so the BH state at entry is irrelevant for the slocks they touch. pep_do_rcv() is different: when the inbound skb targets an existing PEP pipe, it forwards the skb to a different *child* socket via sk_receive_skb(). That helper takes the child slock with bh_lock_sock_nested(), which is just spin_lock_nested() and assumes BH is already off. The same child slock therefore ends up acquired with BH on (process path) and with BH off (softirq path): process context softirq context --------------- --------------- release_sock(listener) __netif_receive_skb() __release_sock() phonet_rcv() spin_unlock_bh() __sk_receive_skb(listener) [BH now ENABLED] [BH already disabled] sk_backlog_rcv: sk_backlog_rcv: pep_do_rcv() pep_do_rcv() sk_receive_skb(child) sk_receive_skb(child) bh_lock_sock_nested(child) bh_lock_sock_nested(child) => SOFTIRQ-ON-W => IN-SOFTIRQ-W Lockdep flags this as inconsistent lock state, and it can become a real self-deadlock if a softirq on the same CPU tries to receive to the same child socket while its slock is held in the BH-enabled path: WARNING: inconsistent lock state inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage. (slock-AF_PHONET/1){+.?.}-{3:3}, at: __sk_receive_skb+0x1cf/0x900 __sk_receive_skb net/core/sock.c:563 sk_receive_skb include/net/sock.h:2022 [inline] pep_do_rcv net/phonet/pep.c:675 sk_backlog_rcv include/net/sock.h:1190 __release_sock net/core/sock.c:3216 release_sock net/core/sock.c:3815 pep_sock_accept net/phonet/pep.c:879 Wrap the forwarded sk_receive_skb() in local_bh_disable() / local_bh_enable() so the child slock is always acquired with BH off. local_bh_disable() nests safely on the softirq path. Discovered via in-house syzkaller fuzzing; the same root cause also on the linux-6.1.y syzbot dashboard as extid 44f0626dd6284f02663c. Reproduced under KASAN + LOCKDEP + PROVE_LOCKING, reproducer: https://pastebin.com/A3t8xzCR Fixes: 9641458d3ec4 ("Phonet: Pipe End Point for Phonet Pipes protocol") Link: https://syzkaller.appspot.com/bug?extid=44f0626dd6284f02663c Cc: stable@vger.kernel.org Signed-off-by: Zijing Yin Acked-by: Rémi Denis-Courmont Reported-by: syzbot+9f4a135646b66c509935@syzkaller.appspotmail.com Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260519172635.86304-1-yzjaurora@gmail.com Signed-off-by: Jakub Kicinski --- diff --git a/net/phonet/pep.c b/net/phonet/pep.c index 4dbf0914df7d..706927139393 100644 --- a/net/phonet/pep.c +++ b/net/phonet/pep.c @@ -671,8 +671,23 @@ static int pep_do_rcv(struct sock *sk, struct sk_buff *skb) /* Look for an existing pipe handle */ sknode = pep_find_pipe(&pn->hlist, &dst, pipe_handle); - if (sknode) - return sk_receive_skb(sknode, skb, 1); + if (sknode) { + int rc; + + /* pep_do_rcv() runs from two contexts: from softirq via + * phonet_rcv() -> __sk_receive_skb() with BH disabled, + * and from process context via + * release_sock() -> __release_sock(), which drops + * the listener slock with spin_unlock_bh() before draining + * the backlog. The child pipe slock is taken below via + * bh_lock_sock_nested(), which does not itself disable BH, so + * disable BH here to keep both acquire contexts consistent. + */ + local_bh_disable(); + rc = sk_receive_skb(sknode, skb, 1); + local_bh_enable(); + return rc; + } switch (hdr->message_id) { case PNS_PEP_CONNECT_REQ: