[PATCH 18/19] fuse: Restrict allow_other to the superblock's namespace or a descendant

[Serge E. Hallyn]

Quoting Seth Forshee (seth.forshee at canonical.com):
> 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 userns
> to access the mount as doing so would give the unprivileged user
> the ability to manipulate processes it would otherwise be unable
> to manipulate. Restrict allow_other to apply to users in the same
> userns used at mount or a descendant of that namespace.
> 
> Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> ---
>  fs/fuse/dir.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index f67f4dd86b36..5b8edb1203b8 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -1018,8 +1018,14 @@ int fuse_allow_current_process(struct fuse_conn *fc)
>  {
>  	const struct cred *cred;
>  
> -	if (fc->flags & FUSE_ALLOW_OTHER)
> -		return 1;
> +	if (fc->flags & FUSE_ALLOW_OTHER) {
> +		struct user_namespace *ns;
> +		for (ns = current_user_ns(); ns; ns = ns->parent) {
> +			if (ns == fc->user_ns)
> +				return 1;
> +		}

	use current_in_userns() ?

> +		return 0;
> +	}
>  
>  	cred = current_cred();
>  	if (uid_eq(cred->euid, fc->user_id) &&
> -- 
> 1.9.1
> 
> --
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