Re: [PATCH 7/7] smack: Don't use security labels for user namespace mounts

[Casey Schaufler <casey@schaufler-ca.com>]

On 7/15/2015 12:46 PM, Seth Forshee wrote:
> Avoid use of untrusted security labels when s_user_ns !=
> init_user_ns:
>  - smk_fetch: refuse to read labels from disk
>  - smack_inode_init_security: return -ENOTSUPP
>  - smack_d_instantiate: don't use security xattrs from disk
>
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

I do not like this at all at all. Pretending that Smack
doesn't exist in a user namespace can lead to all sorts
of blatant security violations, both while the filesystem
is mounted in the namespace and in the init namespace.

> ---
>  security/smack/smack_lsm.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index a143328f75eb..6a849da94f47 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -255,6 +255,9 @@ static struct smack_known *smk_fetch(const char *name, struct inode *ip,
>  	char *buffer;
>  	struct smack_known *skp = NULL;
>  
> +	if (ip->i_sb->s_user_ns != &init_user_ns)
> +		return NULL;
> +
>  	if (ip->i_op->getxattr == NULL)
>  		return ERR_PTR(-EOPNOTSUPP);
>  
> @@ -833,6 +836,9 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>  	struct smack_known *dsp = smk_of_inode(dir);
>  	int may;
>  
> +	if (inode->i_sb->s_user_ns != &init_user_ns)
> +		return -ENOTSUPP;
> +
>  	if (name)
>  		*name = XATTR_SMACK_SUFFIX;
>  
> @@ -3176,11 +3182,13 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
>  		}
>  		/*
>  		 * No xattr support means, alas, no SMACK label.
> -		 * Use the aforeapplied default.
> +		 * Use the aforeapplied default. Also don't use
> +		 * xattrs from userns mounts.
>  		 * It would be curious if the label of the task
>  		 * does not match that assigned.
>  		 */
> -		if (inode->i_op->getxattr == NULL)
> +		if (inode->i_sb->s_user_ns != &init_user_ns ||
> +		    inode->i_op->getxattr == NULL)
>  			break;
>  		/*
>  		 * Get the dentry for xattr.