nfsd_nl_listener_set_doit() holds nfsd_mutex across the whole listener
teardown/rebuild. NFSD_A_SOCK_TRANSPORT_NAME is only checked for
presence, not content, so an arbitrary name reaches
svc_xprt_create_from_sa(), where a name matching no registered class
triggers request_module("svc%s", name) -- a TASK_KILLABLE usermode-helper
upcall run under nfsd_mutex.

Vet the name against the classes NFSD can instantiate (tcp, udp, rdma) in
nfsd_nl_validate_listeners(), which runs before nfsd_mutex is taken.

This narrows the upcall rather than removing it. "rdma" is accepted
unconditionally, so on a kernel where svcrdma is not built it still
reaches request_module("svcrdma") under nfsd_mutex -- as it must for the
modular case, where autoloading is legitimate.

Assisted-by: LLM
Link: https://syzkaller.appspot.com/bug?extid=c7eae0eb80858a2dba0f
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/nfsctl.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 4e5e083d8477..e5844d8454b8 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -1975,14 +1975,31 @@ int nfsd_nl_version_get_doit(struct sk_buff *skb, struct genl_info *info)
 	return err;
 }
 
+/*
+ * Transport classes NFSD knows how to instantiate. Vetting the name here
+ * keeps a bogus string from reaching svc_xprt_create_from_sa(), where an
+ * unknown name triggers a request_module("svc%s", name) upcall under
+ * nfsd_mutex.
+ */
+static bool nfsd_nl_transport_supported(const char *name)
+{
+	static const char * const supported[] = { "tcp", "udp", "rdma" };
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(supported); i++)
+		if (!strcmp(name, supported[i]))
+			return true;
+	return false;
+}
+
 /**
  * nfsd_nl_validate_listeners - sanity-check the listener list from userland
  * @info: netlink metadata and command arguments
  *
  * Walk every NFSD_A_SERVER_SOCK_ADDR attribute and confirm that each entry
  * is well-formed: it parses against the policy, carries both an address and
- * a transport name, and the address is long enough for its family. Doing
- * this up front lets the callers below assume every entry is valid and
+ * a supported transport name, and the address is long enough for its family.
+ * Doing this up front lets the callers below assume every entry is valid and
  * guarantees we make no changes when the request is malformed.
  *
  * Return: 0 if every entry is valid, or a negative errno otherwise.
@@ -2006,6 +2023,9 @@ static int nfsd_nl_validate_listeners(struct genl_info *info)
 		if (!tb[NFSD_A_SOCK_ADDR] || !tb[NFSD_A_SOCK_TRANSPORT_NAME])
 			return -EINVAL;
 
+		if (!nfsd_nl_transport_supported(nla_data(tb[NFSD_A_SOCK_TRANSPORT_NAME])))
+			return -EPROTONOSUPPORT;
+
 		sa = nla_data(tb[NFSD_A_SOCK_ADDR]);
 		if (nla_len(tb[NFSD_A_SOCK_ADDR]) < sizeof(sa->sa_family))
 			return -EINVAL;

-- 
2.55.0