Re: [PATCH 4/8] ucounts: Only except the root user in init_user_ns from RLIMIT_NPROC

["Michal Koutný" <mkoutny@suse.com>]

On Thu, Feb 10, 2022 at 08:13:20PM -0600, "Eric W. Biederman" <ebiederm@xmission.com> wrote:
> @@ -1881,7 +1881,7 @@ static int do_execveat_common(int fd, struct filename *filename,
[...]
> -	    (current_user() != INIT_USER) &&
> +	    (current_ucounts() != &init_ucounts) &&
[...]
> @@ -2027,7 +2027,7 @@ static __latent_entropy struct task_struct *copy_process(
[...]
> -		if (p->real_cred->user != INIT_USER &&
> +		if ((task_ucounts(p) != &init_ucounts) &&

These substitutions make sense to me.

>  		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>  			goto bad_fork_cleanup_count;
>  	}
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 6b2e3ca7ee99..f0c04073403d 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -123,6 +123,8 @@ int create_user_ns(struct cred *new)
>  		ns->ucount_max[i] = INT_MAX;
>  	}
>  	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC));
> +	if (new->ucounts == &init_ucounts)
> +		set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_NPROC, RLIMIT_INFINITY);
>  	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_MSGQUEUE, rlimit(RLIMIT_MSGQUEUE));
>  	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_SIGPENDING, rlimit(RLIMIT_SIGPENDING));
>  	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_MEMLOCK, rlimit(RLIMIT_MEMLOCK));

First, I wanted to object this double fork_init() but I realized it's
relevant for newly created user_ns.

Second, I think new->ucounts would be correct at this point and the
check should be

> if (ucounts == &init_ucounts)

i.e. before set_cred_ucounts() new->ucounts may not be correct.

I'd suggest also a comment in the create_user_ns() explaining the
reason is to exempt global root from RLIMINT_NRPOC also indirectly via
descendant user_nss.

Thanks,
Michal