userns: Don't fail follow_automount based on s_user_ns

When vfs_submount was added the test to limit automounts from
filesystems that with s_user_ns != &init_user_ns accidentially left
in follow_automount.  The test was never about any security concerns
and was always about how do we implement this for filesystems whose
s_user_ns != &init_user_ns.

At the moment this check makes no difference as there are no
filesystems that both set FS_USERNS_MOUNT and implement d_automount.

Remove this check now while I am thinking about it so there will not
be odd booby traps for someone who does want to make this combination
work.

vfs_submount still needs improvements to allow this combination to work,
and vfs_submount contains a check that presents a warning.

The autofs4 filesystem could be modified to set FS_USERNS_MOUNT and it would
need not work on this code path, as userspace performs the mounts.

Fixes: 93faccbbfa95 ("fs: Better permission checking for submounts")
Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems creds")
Acked-by:  Ian Kent <raven@themaw.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/fs/namei.c b/fs/namei.c
index f0c7a7b9b6ca7562217746369cd8d5d82d43a99f..f47118ed36e753607f9daf5d7b870f2c82cbdb1f 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1142,9 +1142,6 @@ static int follow_automount(struct path *path, struct nameidata *nd,
                        return -ENOENT;
        }
 
-       if (path->dentry->d_sb->s_user_ns != &init_user_ns)
-               return -EACCES;
-
        nd->total_link_count++;
        if (nd->total_link_count >= 40)
                return -ELOOP;