linux-kernel - Re: [PATCH] proc: use ns_capable instead of capable for timerslack

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Message-ID: <87in1pn9d0.fsf@xmission.com>
Date:   Thu, 25 Oct 2018 13:56:27 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     bmgordon@...gle.com
Cc:     linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Oren Laadan <orenl@...lrox.com>,
        Ruchi Kandoi <kandoiruchi@...gle.com>,
        Rom Lemarchand <romlem@...roid.com>,
        Todd Kjos <tkjos@...gle.com>, Colin Cross <ccross@...roid.com>,
        Nick Kralevich <nnk@...gle.com>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Elliott Hughes <enh@...gle.com>,
        Android Kernel Team <kernel-team@...roid.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] proc: use ns_capable instead of capable for timerslack_ns

bmgordon@...gle.com writes:

> From: Benjamin Gordon <bmgordon@...gle.com>
>
> Access to timerslack_ns is controlled by a process having CAP_SYS_NICE
> in its effective capability set, but the current check looks in the root
> namespace instead of the process' user namespace.  Since a process is
> allowed to do other activities controlled by CAP_SYS_NICE inside a
> namespace, it should also be able to adjust timerslack_ns.

The goal seems legitimate.  However the permission checks look wrong.

In particular the choice of user namespace should be
"p->cred->user_ns".  This will limit this to tasks that have
CAP_SYS_NICE in the same namespace as the task that is being modified.

Testing file->f_cred->user_ns it is testing whoever opened the file and
that could be anyone.

Eric


> Signed-off-by: Benjamin Gordon <bmgordon@...gle.com>
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Arjan van de Ven <arjan@...ux.intel.com>
> Cc: Oren Laadan <orenl@...lrox.com>
> Cc: Ruchi Kandoi <kandoiruchi@...gle.com>
> Cc: Rom Lemarchand <romlem@...roid.com>
> Cc: Todd Kjos <tkjos@...gle.com>
> Cc: Colin Cross <ccross@...roid.com>
> Cc: Nick Kralevich <nnk@...gle.com>
> Cc: Dmitry Shmidt <dimitrysh@...gle.com>
> Cc: Elliott Hughes <enh@...gle.com>
> Cc: Android Kernel Team <kernel-team@...roid.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  fs/proc/base.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 7e9f07bf260d..4b50937dff80 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2356,7 +2356,7 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
>  		return -ESRCH;
>  
>  	if (p != current) {
> -		if (!capable(CAP_SYS_NICE)) {
> +		if (!ns_capable(file->f_cred->user_ns, CAP_SYS_NICE)) {
>  			count = -EPERM;
>  			goto out;
>  		}
> @@ -2393,7 +2393,7 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>  
>  	if (p != current) {
>  
> -		if (!capable(CAP_SYS_NICE)) {
> +		if (!ns_capable(seq_user_ns(m), CAP_SYS_NICE)) {
>  			err = -EPERM;
>  			goto out;
>  		}