linux-kernel - Re: [patch 0/6] posix-cpu-timers: Fallout fixes and permission tightening

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Message-ID: <201909051329.A630E97F@keescook>
Date:   Thu, 5 Sep 2019 13:32:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [patch 0/6] posix-cpu-timers: Fallout fixes and permission
 tightening

On Thu, Sep 05, 2019 at 02:03:39PM +0200, Thomas Gleixner wrote:
> Sysbot triggered an issue in the posix timer rework which was trivial to
> fix, but after running another test case I discovered that the rework broke
> the permission checks subtly. That's also a straightforward fix.
> 
> Though when staring at it I discovered that the permission checks for
> process clocks and process timers are completely bonkers. The only
> requirement is that the target PID is a group leader. Which means that any
> process can read the clocks and attach timers to any other process without
> priviledge restrictions.
> 
> That's just wrong because the clocks and timers can be used to observe
> behaviour and both reading the clocks and arming timers adds overhead and
> influences runtime performance of the target process.
> 
> The last 4 patches deal with that and introduce ptrace based permission
> checks and also make the behaviour consistent between thread and process
> timers/clocks.

I like these changes! Thanks for working on it. :)

Since this is a subtle bit of checking and there are concerns about ABI
breaks, can you also write some selftests for this area just to nail
down what should work and what should be blocked, etc?

-- 
Kees Cook