From: Slawomir Stepien 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: 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 ... 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 Signed-off-by: Slawomir Stepien --- 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 -- See https://goo.gle/syzbot-ai-patches for information about AI-generated patches. The person who has signed off on the patch is responsible for addressing comments. syzbot engineers can be reached at syzkaller@googlegroups.com.