[PATCH] ceph: quota: fix null pointer dereference in quota check

[Luis Henriques <lhenriques@xxxxxxxx>]

This patch fixes a possible null pointer dereference in
check_quota_exceeded, detected by the static checker smatch, with the
following warning:

    fs/ceph/quota.c:240 check_quota_exceeded()
     error: we previously assumed 'realm' could be null (see line 188)

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Link: https://lkml.kernel.org/n/20181101065318.2cylxol6d444nzeu@kili.mountain
Fixes: b7a2921765cf ("ceph: quota: support for ceph.quota.max_files")
Signed-off-by: Luis Henriques <lhenriques@xxxxxxxx>
---
 fs/ceph/quota.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ceph/quota.c b/fs/ceph/quota.c
index 32d4f13784ba..03f4d24db8fe 100644
--- a/fs/ceph/quota.c
+++ b/fs/ceph/quota.c
@@ -237,7 +237,8 @@ static bool check_quota_exceeded(struct inode *inode, enum quota_check_op op,
 		ceph_put_snap_realm(mdsc, realm);
 		realm = next;
 	}
-	ceph_put_snap_realm(mdsc, realm);
+	if (realm)
+		ceph_put_snap_realm(mdsc, realm);
 	up_read(&mdsc->snap_rwsem);
 
 	return exceeded;