Re: [PATCH v5 13/16] ima: Move some IMA policy and filesystem related variables into ima_namespace

Stefan Berger
2021-12-13T16:27:24Z

On 12/13/21 10:50, Christian Brauner wrote:
> On Mon, Dec 13, 2021 at 10:33:40AM -0500, Stefan Berger wrote:
>> On 12/11/21 04:50, Christian Brauner wrote:
>>> On Fri, Dec 10, 2021 at 08:57:11AM -0500, Stefan Berger wrote:
>>>>
>>>> there anything that would prevent us from setns()'ing to that target user
>>>> namespace so that we would now see that of a user namespace that we are not
>>>> allowed to see?
>>> If you're really worried about someone being able to access a securityfs
>>> instance whose userns doesn't match the userns the securityfs instance
>>> was mounted in there are multiple ways to fix it. The one that I tend to
>>> prefer is:
>>>
>>> From e0ff6a8dcc573763568e685dd70d1547efd68df9 Mon Sep 17 00:00:00 2001
>>> From: Christian Brauner <christian.brauner@ubuntu.com>
>>> Date: Fri, 10 Dec 2021 11:47:37 +0100
>>> Subject: !!!! HERE BE DRAGONS - COMPLETELY UNTESTED !!!!
>>>
>>> securityfs: only allow access to securityfs from within same namespace
>>>
>>> Limit opening of securityfs files to callers located in the same namespace.
>>>
>>> ---
>>> security/inode.c | 33 +++++++++++++++++++++++++++++++--
>>> 1 file changed, 31 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/security/inode.c b/security/inode.c
>>> index eaccba7017d9..9eaf757c08cb 100644
>>> --- a/security/inode.c
>>> +++ b/security/inode.c
>>> @@ -80,6 +80,35 @@ static struct file_system_type fs_type = {
>>> .fs_flags = FS_USERNS_MOUNT,
>>> };
>>> +static int securityfs_permission(struct user_namespace *mnt_userns,
>>> + struct inode *inode, int mask)
>>> +{
>>> + int err;
>>> +
>>> + err = generic_permission(&init_user_ns, inode, mask);
>>> + if (!err) {
>>> + if (inode->i_sb->s_user_ns != current_user_ns())
>>> + err = -EACCES;
>>> + }
>>> +
>>> + return err;
>>> +}
>>> +
>>> +const struct inode_operations securityfs_dir_inode_operations = {
>>> + .permission = securityfs_permission,
>>> + .lookup = simple_lookup,
>>> +};
>>> +
>>> +const struct file_operations securityfs_dir_operations = {
>>> + .permission = securityfs_permission,
>>
>> This interface function on file operations doesn't exist.
> It's almost as if the subject line of this patch warned about its draft
> character. That was supposed for regular files.
>
>> I'll use the inode_operations and also hook it to the root dentry of the
>> super_block. Then there's no need to have this check on symlinks and
>> files...
> Don't special case the inode_operations for the root inode!

I modified the inode_operations *also* for the root node, since that one
is initialized with &simple_dir_inode_operationsby simple_fill_super, so
I didn't want to miss it...

> If a privileged process opens an fd refering to a struct file for the
> root inode and leaks it to an unprivileged process by accident the
> unprivileged process can open any file or directory beneath via openat()
> and friends.
>
> Symlinks don't need a .permission handler anyway because they just
> contain the name of another file and that is checked for .permission
> unless you have a separate .getlink handler where you want to restrict
> link contents further.
>
> But regular files need to have a .permission method see openat().

So here's what I have now for the hardening.

diff --git a/security/inode.c b/security/inode.c
index fee01ff4d831..a0d9f086e3d5 100644
--- a/security/inode.c
+++ b/security/inode.c
@@ -26,6 +26,29 @@
static struct vfsmount *init_securityfs_mount;
static int init_securityfs_mount_count;

+static int securityfs_permission(struct user_namespace *mnt_userns,
+                                struct inode *inode, int mask)
+{
+       int err;
+
+       err = generic_permission(&init_user_ns, inode, mask);
+       if (!err) {
+               if (inode->i_sb->s_user_ns != current_user_ns())
+                       err = -EACCES;
+       }
+
+       return err;
+}
+
+const struct inode_operations securityfs_dir_inode_operations = {
+       .permission     = securityfs_permission,
+       .lookup         = simple_lookup,
+};
+
+const struct inode_operations securityfs_file_inode_operations = {
+       .permission     = securityfs_permission,
+};
+
static void securityfs_free_inode(struct inode *inode)
{
if (S_ISLNK(inode->i_mode))
@@ -41,20 +64,25 @@ static const struct super_operations
securityfs_super_operations = {
static int securityfs_fill_super(struct super_block *sb, struct
fs_context *fc)
{
static const struct tree_descr files[] = {{""}};
+       struct user_namespace *ns = fc->user_ns;
int error;

+       if (WARN_ON(ns != current_user_ns()))
+               return -EINVAL;
+
error = simple_fill_super(sb, SECURITYFS_MAGIC, files);
if (error)
return error;

sb->s_op = &securityfs_super_operations;
+       sb->s_root->d_inode->i_op = &securityfs_dir_inode_operations;

return 0;
}
[...]
@@ -157,7 +186,7 @@ static struct dentry *securityfs_create_dentry(const
char *name, umode_t mode,
inode->i_atime = inode->i_mtime = inode->i_ctime =
current_time(inode);
inode->i_private = data;
if (S_ISDIR(mode)) {
-               inode->i_op = &simple_dir_inode_operations;
+               inode->i_op = &securityfs_dir_inode_operations;
inode->i_fop = &simple_dir_operations;
inc_nlink(inode);
inc_nlink(dir);
@@ -165,10 +194,10 @@ static struct dentry
*securityfs_create_dentry(const char *name, umode_t mode,
inode->i_op = iops ? iops :
&simple_symlink_inode_operations;
inode->i_link = data;
} else {
+               inode->i_op = &securityfs_file_inode_operations;
inode->i_fop = fops;
}
d_instantiate(dentry, inode);