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

Stefan Berger
2021-12-13T15:34:01Z

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.

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...

> + .open = dcache_dir_open,
> + .release = dcache_dir_close,
> + .llseek = dcache_dir_lseek,
> + .read = generic_read_dir,
> + .iterate_shared = dcache_readdir,
> + .fsync = noop_fsync,
> +};
> +
> /**
> * securityfs_create_dentry - create a dentry in the securityfs filesystem
> *
> @@ -167,8 +196,8 @@ 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_fop = &simple_dir_operations;
> + inode->i_op = &securityfs_dir_inode_operations;
> + inode->i_fop = &securityfs_dir_operations;
> inc_nlink(inode);
> inc_nlink(dir);
> } else if (S_ISLNK(mode)) {