[PATCH RFC 1/6] fs: refuse fspick() on internal superblocks

[Christian Brauner <brauner@kernel.org>]

There's a few instances where a internal superblock is reachable by
fspick(). Block that. graft_tree() and fsmount() already refuse them.
fanotify refuses mount and superblock marks on them for the same reason.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/fsopen.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fsopen.c b/fs/fsopen.c
index ae19e5136598..9d5a7a22b529 100644
--- a/fs/fsopen.c
+++ b/fs/fsopen.c
@@ -190,6 +190,9 @@ SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags
 	ret = -EINVAL;
 	if (target.mnt->mnt_root != target.dentry)
 		goto err_path;
+	/* kernel-internal superblocks are nobody's to reconfigure */
+	if (target.dentry->d_sb->s_flags & SB_NOUSER)
+		goto err_path;
 
 	fc = fs_context_for_reconfigure(target.dentry, 0, 0);
 	if (IS_ERR(fc)) {

-- 
2.53.0