[PATCH v3] statmount: Fix the null-ptr-deref in do_statmount()

[Qing Wang]

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() and return.

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

diff --git a/fs/namespace.c b/fs/namespace.c
index a67cbe42746d..90700df65f0d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5678,6 +5678,8 @@ 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 (IS_ERR(ns))
+			return PTR_ERR(ns);
 		if (!ns)
 			/*
 			 * We can't set mount point and mnt_ns_id since we don't have a
-- 
2.34.1