From 292d915d3ba6fd15eeb88351fa10581683073109 Mon Sep 17 00:00:00 2001 From: Chuck Lever Date: Sat, 13 Jun 2026 18:16:32 -0400 Subject: [PATCH] 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 Fixes: 3daab3112f03 ("nfsd: cancel async COPY operations when admin revokes filesystem state") Cc: stable@vger.kernel.org Reviewed-by: Jeff Layton Link: https://patch.msgid.link/20260613-unlock-filesystem-uaf-v1-1-462b9bec8c84@kernel.org Signed-off-by: Chuck Lever --- 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 8428b4edeb80..f1ecbb13f642 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);