Re: [PATCH net] phonet/pep: disable BH around forwarded sk_receive_skb()

["Rémi Denis-Courmont" <remi@remlab.net>]

Le 19 mai 2026 20:26:33 GMT+03:00, Zijing Yin <yzjaurora@gmail.com> a écrit :
>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 <yzjaurora@gmail.com>

Acked-by: Rémi Denis-Courmont <remi@remlab.net>