linux-kernel - [PATCH AUTOSEL for 4.15 038/102] userns: Don't fail follow_automount based on s_user

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Message-ID: <20180303222318.26006-38-alexander.levin@microsoft.com>
Date:   Sat, 3 Mar 2018 22:24:31 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.15 038/102] userns: Don't fail follow_automount
 based on s_user_ns

From: "Eric W. Biederman" <ebiederm@...ssion.com>

[ 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@...maw.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 fs/namei.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 9cc91fb7f156..4e3fc58dae72 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1133,9 +1133,6 @@ 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;
-- 
2.14.1