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

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Date:	Wed, 10 Aug 2016 12:03:38 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Kees Cook <keescook@...omium.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 Wed, Aug 10, 2016 at 11:36 AM, Kees Cook <keescook@...omium.org> wrote:
> 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?

I wasn't entierly sure. I didn't think PR_SET_TIMERSLACK has a
security hook, but looking again I now see the top-level
security_task_prctl() check, so maybe not skipping it in this case
would be good?

thanks
-john