[PATCH 4/8] nfsd: guard nfsd_serv deref in nfsd_file_net_dispose

[Jeff Layton <jlayton@kernel.org>]

From: Chris Mason <clm@meta.com>

nfsd_file_net_dispose() is the consumer side of l->freeme: the nfsd
service thread loop calls it to drain entries that the filecache
garbage collector and shrinker append via
nfsd_file_dispose_list_delayed().  During per-net teardown,
nn->nfsd_serv is cleared before the filecache laundrette is shut
down, so the service thread can still run a dispose pass that finds
more than eight entries on l->freeme and dereferences a NULL
svc_serv:

    nfsd service thread loop
      nfsd_file_net_dispose(nn)
        if (!list_empty(&l->freeme)) {
            ...
            svc_wake_up(nn->nfsd_serv);   /* nn->nfsd_serv == NULL */
        }

The sibling helper nfsd_file_dispose_list_delayed() already documents
this ordering and caches nn->nfsd_serv into a local before testing it
for NULL.  nfsd_file_net_dispose() was introduced with the same raw
svc_wake_up(nn->nfsd_serv) call and never picked up the guard.

Fix by loading nn->nfsd_serv into a local svc_serv pointer and only
calling svc_wake_up() when it is non-NULL, matching the pattern in
nfsd_file_dispose_list_delayed().

Fixes: ffb402596147 ("nfsd: Don't leave work of closing files to a work queue")
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
---
 fs/nfsd/filecache.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 2f0d4de779af..1e2e1f89216e 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -474,11 +474,20 @@ void nfsd_file_net_dispose(struct nfsd_net *nn)
 		for (i = 0; i < 8 && !list_empty(&l->freeme); i++)
 			list_move(l->freeme.next, &dispose);
 		spin_unlock(&l->lock);
-		if (!list_empty(&l->freeme))
-			/* Wake up another thread to share the work
+		if (!list_empty(&l->freeme)) {
+			/*
+			 * Wake up another thread to share the work
 			 * *before* doing any actual disposing.
+			 *
+			 * The filecache laundrette is shut down after
+			 * the nn->nfsd_serv pointer is cleared, but
+			 * before the svc_serv is freed.
 			 */
-			svc_wake_up(nn->nfsd_serv);
+			struct svc_serv *serv = nn->nfsd_serv;
+
+			if (serv)
+				svc_wake_up(serv);
+		}
 		nfsd_file_dispose_list(&dispose);
 	}
 }

-- 
2.54.0