From bbc3e471011417598e598707486f5d8814ec9c01 Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm@xmission.com>
Date: Wed, 29 Nov 2017 17:29:20 -0600
Subject: [PATCH] userns: Don't fail follow_automount based on s_user_ns
Git-commit: bbc3e471011417598e598707486f5d8814ec9c01
Patch-mainline: v4.16-rc1
References: bsc#1099918

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: Coly Li <colyli@suse.de>

---
 fs/namei.c |    3 ---
 1 file changed, 3 deletions(-)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1133,9 +1133,6 @@ static int follow_automount(struct path
 	    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;