[PATCH v2 8/9] nfsd: hold net namespace reference for delayed-dispose nfsd_files

[Jeff Layton <jlayton@kernel.org>]

Take a net-namespace reference in nfsd_file_dispose_list_delayed()
(get_net) and release it in nfsd_file_free() (put_net), so that
nf_net is always valid for files that the GC or shrinker has isolated
from the hash table and LRU -- which __nfsd_file_cache_purge() cannot
see.

Without this, nf_net can dangle for in-flight files whose net namespace
is torn down concurrently, causing a use-after-free when
nfsd_file_dispose_list_delayed() calls net_generic(nf->nf_net, ...).

Only files entering the delayed-dispose path need the net reference;
files freed synchronously (non-GC stateful opens, purge, direct put)
are always freed while the net namespace is still alive, so they skip
get_net()/put_net() entirely.  A new NFSD_FILE_NET_HELD flag tracks
whether a given nfsd_file holds a net reference.

Because nfsd_file_free() may now call put_net(nf->nf_net), the old
nfsd_file_put_local() pattern of returning nf->nf_net after
nfsd_file_put() is unsafe -- put_net() could theoretically drop the
last net namespace reference, leaving the returned pointer stale.
Fix this by moving the nfsd_net_put() call into nfsd_file_put_local()
itself, before the nfsd_file_put() that may trigger nfsd_file_free().
The function now returns void and the caller no longer needs to handle
the net reference.

Fixes: 43fd953fa7e2 ("nfsd: simplify the delayed disposal list code")
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/filecache.c        | 34 ++++++++++++++++++++++++++--------
 fs/nfsd/filecache.h        |  3 ++-
 fs/nfsd/localio.c          |  4 ++--
 include/linux/nfslocalio.h |  9 ++-------
 4 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 03f01a0beced..957fe57be063 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -295,6 +295,9 @@ nfsd_file_free(struct nfsd_file *nf)
 	if (WARN_ON_ONCE(!list_empty(&nf->nf_lru)))
 		return;
 
+	if (test_bit(NFSD_FILE_NET_HELD, &nf->nf_flags))
+		put_net(nf->nf_net);
+
 	call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
 }
 
@@ -375,24 +378,28 @@ nfsd_file_put(struct nfsd_file *nf)
 }
 
 /**
- * nfsd_file_put_local - put nfsd_file reference and arm nfsd_net_put in caller
+ * nfsd_file_put_local - put nfsd_file reference and release nfsd_net ref
  * @pnf: nfsd_file of which to put the reference
  *
- * First save the associated net to return to caller, then put
- * the reference of the nfsd_file.
+ * Drops both the nfsd_file reference and the associated nfsd_net
+ * reference.  The nfsd_net ref is released before the file ref so
+ * that put_net() inside nfsd_file_free() cannot drop the last net
+ * namespace reference while the caller still needs it.
  */
-struct net *
+void
 nfsd_file_put_local(struct nfsd_file __rcu **pnf)
 {
 	struct nfsd_file *nf;
-	struct net *net = NULL;
 
 	nf = unrcu_pointer(xchg(pnf, NULL));
 	if (nf) {
-		net = nf->nf_net;
+		struct net *net = nf->nf_net;
+
+		rcu_read_lock();
+		nfsd_net_put(net);
+		rcu_read_unlock();
 		nfsd_file_put(nf);
 	}
-	return net;
 }
 
 /**
@@ -433,9 +440,20 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
 	while (!list_empty(dispose)) {
 		struct nfsd_file *nf = list_first_entry(dispose,
 						struct nfsd_file, nf_gc);
-		struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
+		struct nfsd_net *nn;
 		struct svc_serv *serv;
 
+		/*
+		 * Pin the net namespace so nf_net stays valid while the
+		 * file sits on the per-net dispose list.  Callers (the GC
+		 * worker, shrinker, and fsnotify callbacks) always run
+		 * before nfsd_net_exit(), so nf_net is still live here.
+		 * The matching put_net() is in nfsd_file_free().
+		 */
+		get_net(nf->nf_net);
+		set_bit(NFSD_FILE_NET_HELD, &nf->nf_flags);
+
+		nn = net_generic(nf->nf_net, nfsd_net_id);
 		spin_lock(&nn->fcache_dispose_lock);
 		list_move_tail(&nf->nf_gc, &nn->fcache_dispose_list);
 		spin_unlock(&nn->fcache_dispose_lock);
diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
index 683b6437cacc..7ae3c0ea0a2a 100644
--- a/fs/nfsd/filecache.h
+++ b/fs/nfsd/filecache.h
@@ -45,6 +45,7 @@ struct nfsd_file {
 #define NFSD_FILE_REFERENCED	(2)
 #define NFSD_FILE_GC		(3)
 #define NFSD_FILE_RECENT	(4)
+#define NFSD_FILE_NET_HELD	(5)
 	unsigned long		nf_flags;
 	refcount_t		nf_ref;
 	unsigned char		nf_may;
@@ -66,7 +67,7 @@ void nfsd_file_cache_shutdown(void);
 int nfsd_file_cache_start_net(struct net *net);
 void nfsd_file_cache_shutdown_net(struct net *net);
 void nfsd_file_put(struct nfsd_file *nf);
-struct net *nfsd_file_put_local(struct nfsd_file __rcu **nf);
+void nfsd_file_put_local(struct nfsd_file __rcu **nf);
 struct nfsd_file *nfsd_file_get(struct nfsd_file *nf);
 struct file *nfsd_file_file(struct nfsd_file *nf);
 void nfsd_file_close_inode_sync(struct inode *inode);
diff --git a/fs/nfsd/localio.c b/fs/nfsd/localio.c
index c3eb0557b3e1..e3295bae75a4 100644
--- a/fs/nfsd/localio.c
+++ b/fs/nfsd/localio.c
@@ -40,8 +40,8 @@
  * avoid all the NFS overhead with reads, writes and commits.
  *
  * On successful return, returned nfsd_file will have its nf_net member
- * set. Caller (NFS client) is responsible for calling nfsd_net_put and
- * nfsd_file_put (via nfs_to_nfsd_file_put_local).
+ * set. Caller (NFS client) is responsible for calling nfsd_file_put
+ * (via nfs_to_nfsd_file_put_local), which also releases the nfsd_net ref.
  */
 static struct nfsd_file *
 nfsd_open_local_fh(struct net *net, struct auth_domain *dom,
diff --git a/include/linux/nfslocalio.h b/include/linux/nfslocalio.h
index 3d91043254e6..7267a69092d1 100644
--- a/include/linux/nfslocalio.h
+++ b/include/linux/nfslocalio.h
@@ -62,7 +62,7 @@ struct nfsd_localio_operations {
 						const struct nfs_fh *,
 						struct nfsd_file __rcu **pnf,
 						const fmode_t);
-	struct net *(*nfsd_file_put_local)(struct nfsd_file __rcu **);
+	void (*nfsd_file_put_local)(struct nfsd_file __rcu **);
 	struct file *(*nfsd_file_file)(struct nfsd_file *);
 	void (*nfsd_file_dio_alignment)(struct nfsd_file *,
 					u32 *, u32 *, u32 *);
@@ -96,12 +96,7 @@ static inline void nfs_to_nfsd_file_put_local(struct nfsd_file __rcu **localio)
 	 * must prevent nfsd shutdown from completing as nfs_close_local_fh()
 	 * does by blocking the nfs_uuid from being finally put.
 	 */
-	struct net *net;
-
-	net = nfs_to->nfsd_file_put_local(localio);
-
-	if (net)
-		nfs_to_nfsd_net_put(net);
+	nfs_to->nfsd_file_put_local(localio);
 }
 
 #else   /* CONFIG_NFS_LOCALIO */

-- 
2.54.0