Re: [PATCH] x86,seccomp,prctl: Remove PR_TSC_SIGSEGV and seccomp TSC filtering

[Peter Zijlstra]

On Fri, Oct 03, 2014 at 10:27:47AM -0700, Andy Lutomirski wrote:
> [adding linux-api.  whoops.]
> 
> On Fri, Oct 3, 2014 at 10:18 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > PR_SET_TSC / PR_TSC_SIGSEGV is a security feature to prevent heavily
> > sandboxed programs from learning the time, presumably to avoid
> > disclosing the wall clock and to make timing attacks much harder to
> > exploit.
> >
> > Unfortunately, this feature is very insecure, for multiple reasons,
> > and has probably been insecure since before it was written.
> >
> > Weakness 1: Before Linux 3.16, the vvar page and the HPET (!) were
> > part of the kernel's fixmap, so any user process could read them.
> > The vvar page contains low-resolution timing information (with real
> > wall clock and frequency data), and the HPET can be used for high
> > precision timing.  Even in Linux 3.16, there clean way to disable
> > access to these pages.
> >
> > Weakness 2: On most configurations, most or all userspace processes
> > have unrestricted access to RDPMC, which is even better than RDTSC
> > for exploiting timing attacks.
> >
> > I would like to fix both of these issues.  I want to deny access to
> > RDPMC to processes that haven't asked for access via
> > perf_event_open.  I also want to implement real TSC blocking, which
> > will require some vdso enhancements

So the problem with the default deny is that its:
 1) pointless -- the attacker can do sys_perf_event_open() just fine;
 2) and expensive -- the people trying to measure performance get the
    penalty of the CR4 write.

So I would suggest a default on, but allow a disable for the seccomp
users, which might have also disabled the syscall. Note that is is
possible to disable RDPMC while still allowing the syscall.
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