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

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Message-ID: <alpine.DEB.2.21.1909052310590.1902@nanos.tec.linutronix.de>
Date:   Thu, 5 Sep 2019 23:13:47 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kees Cook <keescook@...omium.org>
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, 5 Sep 2019, Kees Cook wrote:
> 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?

Sigh, yes. /me adds it to that append only todo list thingy :)