[PATCH 5.15 084/171] nfsd: fix use-after-free due to delegation race

Greg Kroah-Hartman
2021-12-13T10:21:38Z

From: J. Bruce Fields <bfields@redhat.com>

commit 548ec0805c399c65ed66c6641be467f717833ab5 upstream.

A delegation break could arrive as soon as we've called vfs_setlease. A
delegation break runs a callback which immediately (in
nfsd4_cb_recall_prepare) adds the delegation to del_recall_lru. If we
then exit nfs4_set_delegation without hashing the delegation, it will be
freed as soon as the callback is done with it, without ever being
removed from del_recall_lru.

Symptoms show up later as use-after-free or list corruption warnings,
usually in the laundromat thread.

I suspect aba2072f4523 "nfsd: grant read delegations to clients holding
writes" made this bug easier to hit, but I looked as far back as v3.0
and it looks to me it already had the same problem. So I'm not sure
where the bug was introduced; it may have been there from the beginning.

Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/nfsd/nfs4state.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1207,6 +1207,11 @@ hash_delegation_locked(struct nfs4_deleg
return 0;
}

+static bool delegation_hashed(struct nfs4_delegation *dp)
+{
+ return !(list_empty(&dp->dl_perfile));
+}
+
static bool
unhash_delegation_locked(struct nfs4_delegation *dp)
{
@@ -1214,7 +1219,7 @@ unhash_delegation_locked(struct nfs4_del

lockdep_assert_held(&state_lock);

- if (list_empty(&dp->dl_perfile))
+ if (!delegation_hashed(dp))
return false;

dp->dl_stid.sc_type = NFS4_CLOSED_DELEG_STID;
@@ -4598,7 +4603,7 @@ static void nfsd4_cb_recall_prepare(stru
* queued for a lease break. Don't queue it again.
*/
spin_lock(&state_lock);
- if (dp->dl_time == 0) {
+ if (delegation_hashed(dp) && dp->dl_time == 0) {
dp->dl_time = ktime_get_boottime_seconds();
list_add_tail(&dp->dl_recall_lru, &nn->del_recall_lru);
}