On Fri, 2005-05-13 at 19:44 -0400, Dave Jones wrote:
> On Fri, May 13, 2005 at 07:38:08PM -0400, Lee Revell wrote:
> > On Fri, 2005-05-13 at 19:27 -0400, Dave Jones wrote:
> > > On Fri, May 13, 2005 at 07:00:12PM -0400, Lee Revell wrote:
> > > > On Fri, 2005-05-13 at 23:47 +0100, Alan Cox wrote:
> > > > > On Gwe, 2005-05-13 at 22:59, Matt Mackall wrote:
> > > > > > It might not be much of a problem though. If he's a bit off per guess
> > > > > > (really impressive), he'll still be many bits off by the time there's
> > > > > > enough entropy in the primary pool to reseed the secondary pool so he
> > > > > > can check his guesswork.
> > > > >
> > > > > You can also disable the tsc to user space in the intel processors.
> > > > > Thats something they anticipated as being neccessary in secure
> > > > > environments long ago. This makes the attack much harder.
> > > >
> > > > And break the hundreds of apps that depend on rdtsc? Am I missing
> > > > something?
> > >
> > > If those apps depend on rdtsc being a) present, and b) working
> > > without providing fallbacks, they're already broken.
> > >
> > > There's a reason its displayed in /proc/cpuinfo's flags field,
> > > and visible through cpuid. Apps should be testing for presence
> > > before assuming features are present.
> > >
> >
> > Well yes but you would still have to recompile those apps.
>
> Not if the app is written correctly. See above.
The apps that bother to use rdtsc vs. gettimeofday need a cheap high res
timer more than a correct one anyway - it's not guaranteed that rdtsc
provides a reliable time source at all, due to SMP and frequency scaling
issues.
I'll try to benchmark the difference. Maybe it's not that big a deal.
Lee
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