linux-kernel - Re: [PATCH] proc: Fix timerslack_ns CAP_SYS

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Date:	Wed, 10 Aug 2016 11:36:48 -0700
From:	Kees Cook <keescook@...omium.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>
Subject: Re: [PATCH] proc: Fix timerslack_ns CAP_SYS_NICE check when adjusting self

On Tue, Aug 9, 2016 at 4:54 PM, John Stultz <john.stultz@...aro.org> wrote:
> In changing from checking ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)
> to capable(CAP_SYS_NICE), I missed that ptrace_my_access succeeds
> when p == current, but the CAP_SYS_NICE doesn't.
>
> Thus while the previous commit was intended to loosen the needed
> privledges to modify a processes timerslack, it needlessly restricted
> a task modifying its own timerslack via the proc/<tid>/timerslack_ns
> (which is permitted also via the PR_SET_TIMERSLACK method).
>
> This patch corrects this by checking if p == current before checking
> the CAP_SYS_NICE value.
>
> This patch applies on top of my two previous patches currently in -mm
>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> 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>
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  fs/proc/base.c | 34 +++++++++++++++++++---------------
>  1 file changed, 19 insertions(+), 15 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 02f8389..01c3c2d 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2281,15 +2281,17 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
>         if (!p)
>                 return -ESRCH;
>
> -       if (!capable(CAP_SYS_NICE)) {
> -               count = -EPERM;
> -               goto out;
> -       }
> +       if (p != current) {
> +               if (!capable(CAP_SYS_NICE)) {
> +                       count = -EPERM;
> +                       goto out;
> +               }
>
> -       err = security_task_setscheduler(p);
> -       if (err) {
> -               count = err;
> -               goto out;
> +               err = security_task_setscheduler(p);
> +               if (err) {
> +                       count = err;
> +                       goto out;
> +               }
>         }

This entirely bypasses LSM when p == current. Is that intended?

-Kees

>
>         task_lock(p);
> @@ -2315,14 +2317,16 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>         if (!p)
>                 return -ESRCH;
>
> -       if (!capable(CAP_SYS_NICE)) {
> -               err = -EPERM;
> -               goto out;
> -       }
> +       if (p != current) {
>
> -       err = security_task_getscheduler(p);
> -       if (err)
> -               goto out;
> +               if (!capable(CAP_SYS_NICE)) {
> +                       err = -EPERM;
> +                       goto out;
> +               }
> +               err = security_task_getscheduler(p);
> +               if (err)
> +                       goto out;
> +       }
>
>         task_lock(p);
>         seq_printf(m, "%llu\n", p->timer_slack_ns);
> --
> 1.9.1
>



-- 
Kees Cook
Nexus Security