[PATCH v1] SUNRPC: Reject a socket that already has an svc_sock attached

Writing the same socket descriptor to /proc/fs/nfsd/portlist twice
attaches a second svc_sock to one socket. svc_setup_socket() saves
the socket's callbacks before installing its own, so the second
attach records svc_write_space() as the old write_space callback.
svc_udp_init() invokes that callback by way of svc_sock_setbufsize(),
and svc_write_space() then calls itself until the kernel stack is
exhausted:

  BUG: TASK stack guard page was hit at ffffc900037d7ff8
   svc_write_space+0x90/0x2b0 net/sunrpc/svcsock.c:429
   svc_write_space+0xe6/0x2b0 net/sunrpc/svcsock.c:430
   ... 700 more ...
   svc_sock_setbufsize+0x18d/0x220 net/sunrpc/svcsock.c:386
   svc_udp_init net/sunrpc/svcsock.c:854 [inline]
   svc_setup_socket+0xb2f/0x1090 net/sunrpc/svcsock.c:1498
   svc_addsock+0x2fd/0x760 net/sunrpc/svcsock.c:1547
   __write_ports_addfd fs/nfsd/nfsctl.c:742 [inline]
   write_ports+0xa5b/0xcc0 fs/nfsd/nfsctl.c:861
   nfsctl_transaction_write+0x106/0x1a0 fs/nfsd/nfsctl.c:112

svc_data_ready() and svc_tcp_state_change() chain through their saved
callbacks the same way, so a TCP descriptor added twice recurses on
the next incoming segment instead. Reaching any of this takes a
writer on portlist, and the nfsd filesystem sets no FS_USERNS_MOUNT,
so the reproducer needs CAP_SYS_ADMIN in the initial user namespace.

Reject a socket that already carries sk_user_data. svc_setup_socket()
overwrites that field unconditionally, so a socket some other
consumer has claimed is one NFSD would corrupt whether or not the
callbacks recurse.

Fixes: b41b66d63c73 ("[PATCH] knfsd: allow sockets to be passed to nfsd via 'portlist'")
Reported-by: syzbot+54cdc566f64abf51b7f1@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=54cdc566f64abf51b7f1
Signed-off-by: Chuck Lever <cel@kernel.org>
---
 net/sunrpc/svcsock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 7a423e9ee74d..5a2d52284d75 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c

@@ -1614,6 +1614,9 @@ 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;

-- 
2.54.0