nfsd: fix refcount leak in nfsd_file_lru_add on insertion failure

nfsd_file_lru_add() unconditionally increments nf_ref before attempting
to insert the nfsd_file into the LRU via list_lru_add_obj(). If the
insertion fails (the item is already linked), the incremented reference
is never released, permanently inflating the refcount.

The LRU shrinker callback (nfsd_file_lru_cb) uses refcount_dec_if_one()
to reclaim entries, which requires nf_ref == 1. An inflated refcount
therefore blocks eviction of the affected file cache entry for the
lifetime of the nfsd instance.

While this failure path is currently unreachable -- the sole caller in
nfsd_file_do_acquire() operates on freshly-allocated objects that cannot
already be on the LRU -- it represents a latent bug that would become
exploitable if a future change adds another call site or alters the
PENDING protocol.

Fix this by:
 - Adding a compensating refcount_dec() on the failure path. Bare
   refcount_dec (rather than nfsd_file_put) is correct here because
   the caller in nfsd_file_do_acquire still holds its own construction
   reference, so the count goes from 2 back to 1 without risk of
   reaching zero.
 - Changing WARN_ON(1) to WARN_ON_ONCE(1) to prevent log flooding if
   this path is ever hit repeatedly.
 - Returning early on failure to skip the unnecessary call to
   nfsd_file_schedule_laundrette(), since no entry was added to the LRU.

Fixes: 56221b42d7 ("nfsd: filecache: don't repeatedly add/remove files on the lru list")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260602-nfsd-testing-v2-6-e4ea62e3cd5c@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.c

@@ -327,8 +327,11 @@ static void nfsd_file_lru_add(struct nfsd_file *nf)
 	refcount_inc(&nf->nf_ref);
 	if (list_lru_add_obj(&nfsd_file_lru, &nf->nf_lru))
 		trace_nfsd_file_lru_add(nf);
-	else
-		WARN_ON(1);
+	else {
+		refcount_dec(&nf->nf_ref);
+		WARN_ON_ONCE(1);
+		return;
+	}
 	nfsd_file_schedule_laundrette();
 }