ksys_umount was refactored to into split into another function
(path_umount) to enable sharing code. This changed the order that flags and
permissions are validated in, and made it so that user_path_at was called
before validating flags and capabilities.

Unfortunately, libfuse2[1] and libmount[2] rely on the old flag validation
behaviour to determine whether or not the kernel supports UMOUNT_NOFOLLOW.
The other path that this validation is being checked on is
init_umount->path_umount->can_umount. That's all internal to the kernel.

[1]: https://github.com/libfuse/libfuse/blob/9bfbeb576c5901b62a171d35510f0d1a922020b7/util/fusermount.c#L403
[2]: https://github.com/karelzak/util-linux/blob/7ed579523b556b1270f28dbdb7ee07dee310f157/libmount/src/context_umount.c#L813

Signed-off-by: Sargun Dhillon <redacted>
Cc: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Fixes: 41525f56e256 ("fs: refactor ksys_umount")
---
 fs/namespace.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index cebaa3e81794..dc76f1cb89f4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -1710,10 +1710,6 @@ static int can_umount(const struct path *path, int flags)
 {
 	struct mount *mnt = real_mount(path->mnt);
 
-	if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
-		return -EINVAL;
-	if (!may_mount())
-		return -EPERM;
 	if (path->dentry != path->mnt->mnt_root)
 		return -EINVAL;
 	if (!check_mnt(mnt))

@@ -1746,6 +1742,12 @@ static int ksys_umount(char __user *name, int flags)
 	struct path path;
 	int ret;
 
+	if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
+		return -EINVAL;
+
+	if (!may_mount())
+		return -EPERM;
+
 	if (!(flags & UMOUNT_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
 	ret = user_path_at(AT_FDCWD, name, lookup_flags, &path);

-- 
2.25.1