[PATCH RFC] sunrpc: reject socket already in use in svc_addsock()

["syzbot" <syzbot@kernel.org>]

When a socket is added to an RPC service via svc_addsock() (for example, by
writing its file descriptor to /proc/fs/nfsd/portlist), svc_setup_socket()
saves the original socket callbacks (such as sk_state_change) into struct
svc_sock and replaces them with RPC-specific callbacks (such as
svc_tcp_state_change). It also attaches the new svc_sock to
sk->sk_user_data.

If the same socket file descriptor is added to an RPC service again,
svc_addsock() invokes svc_setup_socket() a second time on the same socket.
During the second setup, svsk->sk_ostate is assigned the socket's current
sk_state_change callback, which was already replaced with
svc_tcp_state_change. When a state change event subsequently occurs on the
socket (for example, when connect() is called), svc_tcp_state_change()
invokes svsk->sk_ostate, calling itself in an infinite recursion until the
kernel stack overflows and triggers a stack guard page fault:

BUG: TASK stack guard page was hit at ffffc900030b7ff8 (stack is
ffffc900030b8000..ffffc900030c0000)
Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI
...
Call Trace:
 <TASK>
 svc_tcp_state_change+0x76/0x2e0 net/sunrpc/svcsock.c:917
 svc_tcp_state_change+0x76/0x2e0 net/sunrpc/svcsock.c:917
 svc_tcp_state_change+0x76/0x2e0 net/sunrpc/svcsock.c:917
 ...
 tcp_done_with_error net/ipv4/tcp_input.c:4877 [inline]
 tcp_reset+0x176/0x380 net/ipv4/tcp_input.c:4909
 tcp_rcv_synsent_state_process net/ipv4/tcp_input.c:6903 [inline]
 tcp_rcv_state_process+0x13bc/0x48a0 net/ipv4/tcp_input.c:7197
 tcp_v4_do_rcv+0xafc/0x1530 net/ipv4/tcp_ipv4.c:1876
 sk_backlog_rcv include/net/sock.h:1192 [inline]
 __release_sock+0x25b/0x390 net/core/sock.c:3260
 release_sock+0x190/0x260 net/core/sock.c:3859
 inet_wait_for_connect net/ipv4/af_inet.c:616 [inline]
 __inet_stream_connect+0x863/0xe00 net/ipv4/af_inet.c:710
 inet_stream_connect+0x66/0xa0 net/ipv4/af_inet.c:755
 connect_socket net/socket.c:2141 [inline]
 __sys_connect_file net/socket.c:2166 [inline]
 __sys_connect+0x316/0x450 net/socket.c:2183
 ...
 </TASK>

Fix this by checking if so->sk->sk_user_data is already set in
svc_addsock() before proceeding with svc_setup_socket(). If it is already
non-NULL, return -EBUSY to prevent re-initializing an active socket.

Fixes: fa9251afc33c ("SUNRPC: Call the default socket callbacks instead of open coding")
Assisted-by: Gemini:gemini-3.7-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+c9834a0c0215e6d8697a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c9834a0c0215e6d8697a
Link: https://syzkaller.appspot.com/ai_job?id=30de91d1-1d14-4676-8c38-a3564f7acf48
To: "Anna Schumaker" <anna@kernel.org>
To: "Chuck Lever" <cel@kernel.org>
To: "David S. Miller" <davem@davemloft.net>
To: "Eric Dumazet" <edumazet@google.com>
To: "Jeff Layton" <jlayton@kernel.org>
To: "Jakub Kicinski" <kuba@kernel.org>
To: <linux-nfs@vger.kernel.org>
To: <netdev@vger.kernel.org>
To: "Paolo Abeni" <pabeni@redhat.com>
To: "Trond Myklebust" <trondmy@kernel.org>
To: "Trond Myklebust" <trond.myklebust@primarydata.com>
Cc: "Dai Ngo" <Dai.Ngo@oracle.com>
Cc: "Simon Horman" <horms@kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Cc: "NeilBrown" <neil@brown.name>
Cc: "Olga Kornievskaia" <okorniev@redhat.com>
Cc: "Tom Talpey" <tom@talpey.com>

---
diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 50e5e7f5b..e8cc4329f 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -1541,6 +1541,9 @@ int svc_addsock(struct svc_serv *serv, struct net *net, const int fd,
 	err = -EISCONN;
 	if (so->state > SS_UNCONNECTED)
 		goto out;
+	err = -EBUSY;
+	if (so->sk->sk_user_data)
+		goto out;
 	err = -ENOENT;
 	if (!try_module_get(THIS_MODULE))
 		goto out;


base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
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