[PATCH 8/8] nfsd: hold net namespace reference in nfsd_file

[Jeff Layton <jlayton@kernel.org>]

Take a net-namespace reference in nfsd_file_alloc() (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, ...).

Because nfsd_file_free() now calls 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        | 22 ++++++++++++++--------
 fs/nfsd/filecache.h        |  2 +-
 fs/nfsd/localio.c          |  4 ++--
 include/linux/nfslocalio.h |  9 ++-------
 4 files changed, 19 insertions(+), 18 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 03f01a0beced..6f75df344091 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -221,7 +221,7 @@ nfsd_file_alloc(struct net *net, struct inode *inode, unsigned char need,
 	nf->nf_birthtime = ktime_get();
 	nf->nf_file = NULL;
 	nf->nf_cred = get_current_cred();
-	nf->nf_net = net;
+	nf->nf_net = get_net(net);
 	nf->nf_flags = want_gc ?
 		BIT(NFSD_FILE_HASHED) | BIT(NFSD_FILE_PENDING) | BIT(NFSD_FILE_GC) :
 		BIT(NFSD_FILE_HASHED) | BIT(NFSD_FILE_PENDING);
@@ -295,6 +295,8 @@ nfsd_file_free(struct nfsd_file *nf)
 	if (WARN_ON_ONCE(!list_empty(&nf->nf_lru)))
 		return;
 
+	put_net(nf->nf_net);
+
 	call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
 }
 
@@ -375,24 +377,28 @@ 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;
 }
 
 /**
diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
index 683b6437cacc..88e397176c48 100644
--- a/fs/nfsd/filecache.h
+++ b/fs/nfsd/filecache.h
@@ -66,7 +66,7 @@ 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