fs: Call d_automount with the filesystems creds

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@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 629823f19a6a..ef573df3297f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -36,6 +36,7 @@
 #include <linux/posix_acl.h>
 #include <linux/hash.h>
 #include <linux/bitops.h>
+#include <linux/init_task.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -1099,6 +1100,7 @@ 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)
@@ -1120,11 +1122,16 @@ 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