[PATCH 1/3] NFSD: Prevent post-shutdown use-after-free in unlock_filesystem

Writing a filesystem path to /proc/fs/nfsd/unlock_filesystem runs
nfsd4_cancel_copy_by_sb() before nfsd_mutex is held and before the
handler confirms that nn->nfsd_serv is set. Once nfsd has shut down,
nfs4_state_destroy_net() has freed nn->conf_id_hashtbl but left the
pointer intact, so the cancel helper iterates freed slab memory as an
array of struct list_head and then dereferences a bogus nfs4_client
when it takes clp->async_lock. A local administrator holding
CAP_SYS_ADMIN can reach this use-after-free by stopping the server and
then writing to unlock_filesystem; KASAN reports a slab-use-after-free
read in nfsd4_cancel_copy_by_sb().

nfsd4_revoke_states() walks the same state tables and for that reason
already runs only under nfsd_mutex with nn->nfsd_serv confirmed
present. Move the async COPY cancel into that protected section so
every NFSv4 state-table walker on this path observes a running server.
Async copies exist only while the server runs, so gating the cancel on
nn->nfsd_serv loses nothing.

Reported-by: Musaab Khan <musaab.khan-g/[email protected]>
Fixes: 3daab3112f03 ("nfsd: cancel async COPY operations when admin revokes filesystem state")
Signed-off-by: Chuck Lever <[email protected]>
---
 fs/nfsd/nfsctl.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 11bbc7e8210c..29d68abfa5c8 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -296,14 +296,15 @@ static ssize_t write_unlock_fs(struct file *file, char *buf, size_t size)
 	 * 2.  Is that directory a mount point, or
 	 * 3.  Is that directory the root of an exported file system?
 	 */
-	nfsd4_cancel_copy_by_sb(netns(file), path.dentry->d_sb);
 	error = nlmsvc_unlock_all_by_sb(path.dentry->d_sb);
 	mutex_lock(&nfsd_mutex);
 	nn = net_generic(netns(file), nfsd_net_id);
-	if (nn->nfsd_serv)
+	if (nn->nfsd_serv) {
+		nfsd4_cancel_copy_by_sb(netns(file), path.dentry->d_sb);
 		nfsd4_revoke_states(nn, path.dentry->d_sb);
-	else
+	} else {
 		error = -EINVAL;
+	}
 	mutex_unlock(&nfsd_mutex);
 
 	path_put(&path);

-- 
2.54.0