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

Thomas Gleixner
2019-09-05T14:57:15Z

On Thu, 5 Sep 2019, Frederic Weisbecker 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.
>
> 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.

Well, reading clocks is one part of the issue. Arming timers on any process
is a different story.

Also /proc/$PID access can be restricted nowadays. So that posic clock
stuff should at least have exactly the same restrictions.

Thanks,

tglx