Re: [PATCH v4 3/5] fuse: Restrict allow_other to uids already controlled by the user

[Andy Lutomirski <luto@amacapital.net>]

On 10/14/2014 07:25 AM, Seth Forshee wrote:
> Unprivileged users are normally restricted from mounting with the
> allow_other option by system policy, but this could be bypassed
> for a mount done with user namespace root permissions. In such
> cases allow_other should not allow users outside the user
> namespace to access the mount as doing so would give the
> unprivileged user the ability to manipulate processes it would
> otherwise be unable to manipulate.

What threat is this intended to protect against?  I think that, if this
is needed, tasks outside the userns or its descendents should be
blocked, even if the user ids match.  That is, I think you should check
the namespace, not the uid and gid.

--Andy

> 
> Cc: Eric W. Biederman <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
> Cc: Serge H. Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
> Signed-off-by: Seth Forshee <seth.forshee-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>
> ---
>  fs/fuse/dir.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 123db1e06c78..e3123bfbc711 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -1089,12 +1089,20 @@ int fuse_reverse_inval_entry(struct super_block *sb, u64 parent_nodeid,
>   */
>  int fuse_allow_current_process(struct fuse_conn *fc)
>  {
> -	const struct cred *cred;
> +	const struct cred *cred = current_cred();
>  
> -	if (fc->flags & FUSE_ALLOW_OTHER)
> -		return 1;
> +	if (fc->flags & FUSE_ALLOW_OTHER) {
> +		if (kuid_has_mapping(fc->user_ns, cred->euid) &&
> +		    kuid_has_mapping(fc->user_ns, cred->suid) &&
> +		    kuid_has_mapping(fc->user_ns, cred->uid) &&
> +		    kgid_has_mapping(fc->user_ns, cred->egid) &&
> +		    kgid_has_mapping(fc->user_ns, cred->sgid) &&
> +		    kgid_has_mapping(fc->user_ns, cred->gid))
> +			return 1;
> +
> +		return 0;
> +	}
>  
> -	cred = current_cred();
>  	if (uid_eq(cred->euid, fc->user_id) &&
>  	    uid_eq(cred->suid, fc->user_id) &&
>  	    uid_eq(cred->uid,  fc->user_id) &&
>