userns: Don't fail follow_automount based on s_user_ns

[ Upstream commit bbc3e471011417598e598707486f5d8814ec9c01 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/namei.c b/fs/namei.c
index 6cfb45f..917e799 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1121,9 +1121,6 @@ static int follow_automount(struct path *path, struct nameidata *nd,
            path->dentry->d_inode)
                return -EISDIR;
 
-       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;