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

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Message-ID: <20190905144829.GA18251@lenoir>
Date:   Thu, 5 Sep 2019 16:48:30 +0200
From:   Frederic Weisbecker <frederic@...nel.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>,
        Kees Cook <keescook@...omium.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.

Yeah I stumbled upon that by the past and found out the explanation behind
in old history: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/kernel/posix-cpu-timers.c?id=a78331f2168ef1e67b53a0f8218c70a19f0b2a4c

"This makes no constraint on who can see whose per-process clocks.  This
information is already available for the VIRT and PROF (i.e.  utime and stime)
information via /proc.  I am open to suggestions on if/how security
constraints on who can see whose clocks should be imposed."

I'm all for mitigating that, let's just hope that won't break some ABIs.