linux-kernel - Re: [PATCH] prctl: Allow local CAP_SYS_ADMIN changing exe

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Date:   Fri, 12 May 2017 09:56:36 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     <mhocko@...e.com>, <avagin@...nvz.org>,
        <skinsbursky@...tuozzo.com>, <oleg@...hat.com>,
        <linux-kernel@...r.kernel.org>, <gorcunov@...nvz.org>,
        <akpm@...ux-foundation.org>, <ptikhomirov@...tuozzo.com>,
        <serge@...lyn.com>
Subject: Re: [PATCH] prctl: Allow local CAP_SYS_ADMIN changing exe_file

Kirill Tkhai <ktkhai@...tuozzo.com> writes:

> During checkpointing and restore of userspace tasks
> we bumped into the situation, that it's not possible
> to restore the tasks, which user namespace does not
> have uid 0 or gid 0 mapped.
>
> People create user namespace mappings like they want,
> and there is no a limitation on obligatory uid and gid
> "must be mapped". So, if there is no uid 0 or gid 0
> in the mapping, it's impossible to restore mm->exe_file
> of the processes belonging to this user namespace.
>
> Also, there is no a workaround. It's impossible
> to create a temporary uid/gid mapping, because
> only one write to /proc/[pid]/uid_map and gid_map
> is allowed during a namespace lifetime.
> If there is an entry, then no more mapings can't be
> written. If there isn't an entry, we can't write
> there too, otherwise user task won't be able
> to do that in the future.
>
> The patch changes the check, and looks for CAP_SYS_ADMIN
> instead of zero uid and gid. This allows to restore
> a task independently of its user namespace mappings.

Applied thanks.  Testing against 0 in the modern kernel is perhaps the
most bizarre permisssion check I have seen lately.

Eric
>
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: Serge Hallyn <serge@...lyn.com>
> CC: "Eric W. Biederman" <ebiederm@...ssion.com>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Michal Hocko <mhocko@...e.com>
> CC: Andrei Vagin <avagin@...nvz.org>
> CC: Cyrill Gorcunov <gorcunov@...nvz.org>
> CC: Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>
> CC: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
> ---
>  kernel/sys.c |    8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 8a94b4eabcaa..7c6d78148fa0 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1802,15 +1802,11 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
>  
>  	/*
>  	 * Finally, make sure the caller has the rights to
> -	 * change /proc/pid/exe link: only local root should
> +	 * change /proc/pid/exe link: only local sys admin should
>  	 * be allowed to.
>  	 */
>  	if (prctl_map->exe_fd != (u32)-1) {
> -		struct user_namespace *ns = current_user_ns();
> -		const struct cred *cred = current_cred();
> -
> -		if (!uid_eq(cred->uid, make_kuid(ns, 0)) ||
> -		    !gid_eq(cred->gid, make_kgid(ns, 0)))
> +		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
>  			goto out;
>  	}
>