linux-kernel - [PATCH 1/2] userns: Don't fail follow_automount based on s_user

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Date:   Wed, 29 Nov 2017 18:04:54 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linux Containers <containers@...ts.linux-foundation.org>
Cc:     <linux-kernel@...r.kernel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Ian Kent <raven@...maw.net>, <linux-fsdevel@...r.kernel.org>,
        Seth Forshee <seth.forshee@...onical.com>
Subject: [PATCH 1/2] 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")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/namei.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index f0c7a7b9b6ca..f47118ed36e7 100644
--- 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;
-- 
2.14.1