[PATCH] NFS: fix delegation_hash_table leak when nfs4_server_common_setup() fails

[PATCH] NFS: fix delegation_hash_table leak when nfs4_server_common_setup() fails

[Nate Prodromou]

nfs4_server_common_setup() allocates server->delegation_hash_table
first, but server->destroy - the only path that frees the table via
nfs4_destroy_server() - is not assigned until the very end of the
function. If any intermediate step fails (the is_ds_only_client()
check, nfs4_init_session(), nfs4_get_rootfh(), or nfs_probe_server()),
the function returns with server->destroy still NULL, so the caller's
nfs_free_server() skips the destroy callback and the hash table is
leaked (4 KiB per attempt with the default delegation watermark).

This is trivially reachable from userspace: every failed NFSv4 mount
leaks one allocation. A client that persistently retries a mount that
cannot succeed leaks kernel memory without bound. Observed in
production where a Longhorn backup poller retried mount.nfs4 against
an NFSv3-only server roughly 10 times per second, leaking ~3.4 GiB of
unreclaimable slab (kmalloc-rnd-13-4k) per day; the node accumulated
12 GiB of leaked slab before the source was identified via the
kmem:kmalloc tracepoint (call_site=nfs4_delegation_hash_alloc).

Reproducer:

  # server exports NFSv3 only (or export path absent for v4)
  while :; do mount -t nfs4 <server>:/missing /mnt; done
  # watch SUnreclaim in /proc/meminfo grow 4 KiB per iteration

Free the table on the error paths between the allocation and the
assignment of server->destroy.

Fixes: f5b3108e6a14 ("NFS: use a hash table for delegation lookup")
Cc: stable@vger.kernel.org
Signed-off-by: Nate Prodromou <nate@prodromou.com>
---
diff --git a/fs/nfs/nfs4client.c b/fs/nfs/nfs4client.c
--- a/fs/nfs/nfs4client.c
+++ b/fs/nfs/nfs4client.c
@@ -915,20 +915,22 @@ static int nfs4_server_common_setup(struct nfs_server 
 		return error;
 
 	/* data servers support only a subset of NFSv4.1 */
-	if (is_ds_only_client(server->nfs_client))
-		return -EPROTONOSUPPORT;
+	if (is_ds_only_client(server->nfs_client)) {
+		error = -EPROTONOSUPPORT;
+		goto out_free_delegation_hash;
+	}
 
 	/* We must ensure the session is initialised first */
 	error = nfs4_init_session(server->nfs_client);
 	if (error < 0)
-		return error;
+		goto out_free_delegation_hash;
 
 	nfs_server_set_init_caps(server);
 
 	/* Probe the root fh to retrieve its FSID and filehandle */
 	error = nfs4_get_rootfh(server, mntfh, auth_probe);
 	if (error < 0)
-		return error;
+		goto out_free_delegation_hash;
 
 	dprintk("Server FSID: %llx:%llx\n",
 			(unsigned long long) server->fsid.major,
@@ -937,7 +939,7 @@ static int nfs4_server_common_setup(struct nfs_server 
 
 	error = nfs_probe_server(server, mntfh);
 	if (error < 0)
-		return error;
+		goto out_free_delegation_hash;
 
 	nfs4_session_limit_rwsize(server);
 	nfs4_session_limit_xasize(server);
@@ -949,6 +951,11 @@ static int nfs4_server_common_setup(struct nfs_server 
 	server->mount_time = jiffies;
 	server->destroy = nfs4_destroy_server;
 	return 0;
+
+out_free_delegation_hash:
+	kfree(server->delegation_hash_table);
+	server->delegation_hash_table = NULL;
+	return error;
 }
 
 /*
-- 
2.47.0

Re: [PATCH] NFS: fix delegation_hash_table leak when nfs4_server_common_setup() fails

[hch]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>