[PATCH 2/2][Xenial SRU] fs: Call d_automount with the filesystems creds

[Seth Forshee]

From: "Eric W. Biederman" <ebiederm at xmission.com>

BugLink: http://bugs.launchpad.net/bugs/1612135

Seth Forshee reported a mount regression in nfs autmounts
with "fs: Add user namespace member to struct super_block".

It turns out that the assumption that current->cred is something
reasonable during mount while necessary to improve support of
unprivileged mounts is wrong in the automount path.

To fix the existing filesystems override current->cred with the
init_cred before calling d_automount and restore current->cred after
d_automount completes.

To support unprivileged mounts would require a more nuanced cred
selection, so fail on unprivileged mounts for the time being.  As none
of the filesystems that currently set FS_USERNS_MOUNT implement
d_automount this check is only good for preventing future problems.

Fixes: 6e4eab577a0c ("fs: Add user namespace member to struct super_block")
Tested-by: Seth Forshee <seth.forshee at canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
(backported from commit aeaa4a79ff6a5ed912b7362f206cf8576fca538b)
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
---
 fs/namei.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 3132aab..ec06605 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -35,6 +35,7 @@
 #include <linux/fs_struct.h>
 #include <linux/posix_acl.h>
 #include <linux/hash.h>
+#include <linux/init_task.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -1084,6 +1085,7 @@ static int follow_automount(struct path *path, struct nameidata *nd,
 			    bool *need_mntput)
 {
 	struct vfsmount *mnt;
+	const struct cred *old_cred;
 	int err;
 
 	if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
@@ -1105,11 +1107,16 @@ 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;
 
+	old_cred = override_creds(&init_cred);
 	mnt = path->dentry->d_op->d_automount(path);
+	revert_creds(old_cred);
 	if (IS_ERR(mnt)) {
 		/*
 		 * The filesystem is allowed to return -EISDIR here to indicate
-- 
2.7.4