[PATCH] statmount: Fix the null-ptr-deref in do_statmount()

[Qing Wang <wangqing7171@gmail.com>]

If the mount is internal, it's mnt_ns will be MNT_NS_INTERNAL, which is
defined as ERR_PTR(-EINVAL). So, in the do_statmount(), need to check ns
of mount by IS_ERR_OR_NULL().

Fixes: 0e5032237ee5 ("statmount: accept fd as a parameter")
Reported-by: syzbot+9e03a9535ea65f687a44@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/698e287a.a70a0220.2c38d7.009e.GAE@google.com/
Tested-by: syzbot+9e03a9535ea65f687a44@syzkaller.appspotmail.com
Signed-off-by: Qing Wang <wangqing7171@gmail.com>
---
 fs/namespace.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index a67cbe42746d..d769d50de5d6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5678,13 +5678,15 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 
 		s->mnt = mnt_file->f_path.mnt;
 		ns = real_mount(s->mnt)->mnt_ns;
-		if (!ns)
+		if (IS_ERR_OR_NULL(ns)) {
 			/*
 			 * We can't set mount point and mnt_ns_id since we don't have a
 			 * ns for the mount. This can happen if the mount is unmounted
-			 * with MNT_DETACH.
+			 * with MNT_DETACH or if it's an internal mount.
 			 */
 			s->mask &= ~(STATMOUNT_MNT_POINT | STATMOUNT_MNT_NS_ID);
+			ns = NULL;
+		}
 	} else {
 		/* Has the namespace already been emptied? */
 		if (mnt_ns_id && mnt_ns_empty(ns))
-- 
2.34.1