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Date: Sun, 20 Nov 2022 14:43:06 +0300 From: Kirill Tkhai <tkhai@...ru> To: Kuniyuki Iwashima <kuniyu@...zon.com> Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, netdev@...r.kernel.org, pabeni@...hat.com Subject: Re: [PATCH net] unix: Fix race in SOCK_SEQPACKET's unix_dgram_sendmsg() On 20.11.2022 12:46, Kirill Tkhai wrote: > On 20.11.2022 12:09, Kuniyuki Iwashima wrote: >> From: Kirill Tkhai <tkhai@...ru> >> Date: Sun, 20 Nov 2022 02:16:47 +0300 >>> There is a race resulting in alive SOCK_SEQPACKET socket >>> may change its state from TCP_ESTABLISHED to TCP_CLOSE: >>> >>> unix_release_sock(peer) unix_dgram_sendmsg(sk) >>> sock_orphan(peer) >>> sock_set_flag(peer, SOCK_DEAD) >>> sock_alloc_send_pskb() >>> if !(sk->sk_shutdown & SEND_SHUTDOWN) >>> OK >>> if sock_flag(peer, SOCK_DEAD) >>> sk->sk_state = TCP_CLOSE >>> sk->sk_shutdown = SHUTDOWN_MASK >>> >>> >>> After that socket sk remains almost normal: it is able to connect, listen, accept >>> and recvmsg, while it can't sendmsg. >> >> nit: Then, also recvmsg() fails with -ENOTCONN. And after connect(), even >> both of recvmsg() and sendmsg() does not fail. > > Possible, I wrote not clear. I mean sendmsg fails after connect, while recvmsg does not fail :) > Here is sendmsg abort path: > > unix_dgram_sendmsg() > sock_alloc_send_pskb() > if (sk->sk_shutdown & SEND_SHUTDOWN) > FAIL > >> static int unix_seqpacket_recvmsg(struct socket *sock, struct msghdr *msg, >> size_t size, int flags) >> { >> struct sock *sk = sock->sk; >> >> if (sk->sk_state != TCP_ESTABLISHED) >> return -ENOTCONN; >> >> return unix_dgram_recvmsg(sock, msg, size, flags); >> } >> >> >>> >>> Since this is the only possibility for alive SOCK_SEQPACKET to change >>> the state in such way, we should better fix this strange and potentially >>> danger corner case. >>> >>> Also, move TCP_CLOSE assignment for SOCK_DGRAM sockets under state lock. >>> >>> Signed-off-by: Kirill Tkhai <tkhai@...ru> >> >> Fixes tag is needed: >> >> Fixes: 83301b5367a9 ("af_unix: Set TCP_ESTABLISHED for datagram sockets too") >>> Before this commit, there was no state change and SEQPACKET sk also went >> through the same path. The bug was introduced because the commit did not >> consider SEAPACKET. >> >> So, I think the fix should be like below, then we can free the peer faster. >> Note unix_dgram_peer_wake_disconnect_wakeup() is dgram-specific too. >> >> ---8<--- >> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c >> index b3545fc68097..be40023a61fb 100644 >> --- a/net/unix/af_unix.c >> +++ b/net/unix/af_unix.c >> @@ -2001,11 +2001,14 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg, >> err = 0; >> if (unix_peer(sk) == other) { >> unix_peer(sk) = NULL; >> - unix_dgram_peer_wake_disconnect_wakeup(sk, other); >> + >> + if (sk->sk_type == SOCK_DGRAM) { >> + unix_dgram_peer_wake_disconnect_wakeup(sk, other); >> + sk->sk_state = TCP_CLOSE; >> + } >> >> unix_state_unlock(sk); >> >> - sk->sk_state = TCP_CLOSE; >> unix_dgram_disconnected(sk, other); >> sock_put(other); >> err = -ECONNREFUSED; >> ---8<--- >> >> Also, it's better to mention that moving TCP_CLOSE under the lock resolves >> another rare race with unix_dgram_connect() for DGRAM sk: >> >> unix_state_unlock(sk); >> <--------------------------> connect() could set TCP_ESTABLISHED here. >> sk->sk_state = TCP_CLOSE; > > Sounds good, I'll test this variant. Thanks! Thinking again, I'd argue about disconnecting from peer (unix_peer(sk) = NULL) here. Normally, unix_dgram_sendmsg() never came here like I wrote: unix_dgram_sendmsg() sock_alloc_send_pskb() if (sk->sk_shutdown & SEND_SHUTDOWN) FAIL with EPIPE So, this optimization will work very rare, it fact there will not real performance profit. But this creates a cornet case (safe but corner), which seems not good. Corner cases are not good in general. I'd leave an original variant. What do you think about this? Kirill >>> --- >>> net/unix/af_unix.c | 11 +++++++++-- >>> 1 file changed, 9 insertions(+), 2 deletions(-) >>> >>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c >>> index b3545fc68097..6fd745cfc492 100644 >>> --- a/net/unix/af_unix.c >>> +++ b/net/unix/af_unix.c >>> @@ -1999,13 +1999,20 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg, >>> unix_state_lock(sk); >>> >>> err = 0; >>> - if (unix_peer(sk) == other) { >>> + if (sk->sk_type == SOCK_SEQPACKET) { >>> + /* We are here only when racing with unix_release_sock() >>> + * is clearing @other. Never change state to TCP_CLOSE >>> + * unlike SOCK_DGRAM wants. >>> + */ >>> + unix_state_unlock(sk); >>> + err = -EPIPE; >>> + } else if (unix_peer(sk) == other) { >>> unix_peer(sk) = NULL; >>> unix_dgram_peer_wake_disconnect_wakeup(sk, other); >>> >>> + sk->sk_state = TCP_CLOSE; >>> unix_state_unlock(sk); >>> >>> - sk->sk_state = TCP_CLOSE; >>> unix_dgram_disconnected(sk, other); >>> sock_put(other); >>> err = -ECONNREFUSED; >
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