[PATCH] nfsd: prevent hung task in nfsd_nl_listener_set_doit()

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

From: Slawomir Stepien <sst@poczta.fm>

In nfsd_nl_listener_set_doit(), the kernel iterates over all
NFSD_A_SERVER_SOCK_ADDR attributes provided in a netlink message to
configure NFS server listeners. There is currently no limit on the number
of attributes a user can send.

For each attribute, svc_xprt_create_from_sa() is called, which may
synchronously invoke request_module() to load the corresponding transport
module. If a user provides a large number of invalid transport names,
request_module() is called sequentially for each, taking a massive amount
of time. Since this entire process occurs while holding the global
nfsd_mutex, it blocks other tasks attempting to acquire the mutex and
triggers a hung task timeout:

  INFO: task blocked for more than 10 seconds.
  ...
  Call Trace:
   <TASK>
   __schedule+0x17e7/0x5630 kernel/sched/core.c:7234
   schedule+0x164/0x2b0 kernel/sched/core.c:7326
   schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7383
   __mutex_lock_common kernel/locking/mutex.c:726 [inline]
   __mutex_lock+0x7bf/0x1550 kernel/locking/mutex.c:821
   nfsd_nl_version_get_doit+0x17c/0xd20 fs/nfsd/nfsctl.c:1889
   genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114
   genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
   genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
  ...
  2 locks held by task/5864:
   #0: ffffffff90114168 (cb_lock){++++}-{4:4}, at: genl_rcv+0x19/0x40
   net/netlink/genetlink.c:1217
   #1: ffffffff8eeb1a60 (nfsd_mutex){+.+.}-{4:4}, at:
   nfsd_nl_listener_set_doit+0x135/0x1750 fs/nfsd/nfsctl.c:1964

Furthermore, the function does not break out of the loop if
svc_xprt_create_from_sa() fails, and it suffers from an O(N^2) complexity
issue because svc_find_listener() iterates over the serv->sv_permsocks list
for each attribute.

Address this by introducing a hard limit of 128 NFSD_A_SERVER_SOCK_ADDR
attributes per netlink message. The limit is enforced before acquiring the
nfsd_mutex to prevent lock contention. Additionally, modify the loop to
break immediately if listener creation fails, avoiding needless sequential
request_module() calls for invalid configurations.

Fixes: 16a471177496 ("NFSD: add listener-{set,get} netlink command")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+41bc60511c2884783c27@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=41bc60511c2884783c27
Link: https://syzkaller.appspot.com/ai_job?id=30871f87-7a83-46b8-ab83-919894b0787b
Signed-off-by: Slawomir Stepien <sst@poczta.fm>

---
diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index fa92e31d1..516c04c14 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -1943,6 +1943,8 @@ int nfsd_nl_version_get_doit(struct sk_buff *skb, struct genl_info *info)
 	return err;
 }
 
+#define NFSD_MAX_LISTENERS 128
+
 /**
  * nfsd_nl_listener_set_doit - set the nfs running sockets
  * @skb: reply buffer
@@ -1955,12 +1957,19 @@ int nfsd_nl_listener_set_doit(struct sk_buff *skb, struct genl_info *info)
 	struct net *net = genl_info_net(info);
 	struct svc_xprt *xprt, *tmp;
 	const struct nlattr *attr;
+	unsigned int count = 0;
 	struct svc_serv *serv;
 	LIST_HEAD(permsocks);
 	struct nfsd_net *nn;
 	bool delete = false;
 	int err, rem;
 
+	nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_SOCK_ADDR, info->nlhdr,
+				 GENL_HDRLEN, rem) {
+		if (++count > NFSD_MAX_LISTENERS)
+			return -EINVAL;
+	}
+
 	mutex_lock(&nfsd_mutex);
 
 	err = nfsd_create_serv(net);
@@ -2073,8 +2082,10 @@ int nfsd_nl_listener_set_doit(struct sk_buff *skb, struct genl_info *info)
 		ret = svc_xprt_create_from_sa(serv, xcl_name, net, sa, 0,
 					      current_cred());
 		/* always save the latest error */
-		if (ret < 0)
+		if (ret < 0) {
 			err = ret;
+			break;
+		}
 	}
 
 	if (!serv->sv_nrthreads && list_empty(&nn->nfsd_serv->sv_permsocks))


base-commit: 075b74841bd0065a3bda3440873c747938e69b68
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