Re: Scheduler fixes for hyperthreading

From: Bruce Evans (bde_at_zeta.org.au)
Date: 05/22/05

  • Next message: Marcel Moolenaar: "Re: Scheduler fixes for hyperthreading" 
    Date: Sun, 22 May 2005 16:46:07 +1000 (EST)
To: Colin Percival <cperciva@FreeBSD.org>

    On Sat, 21 May 2005, Colin Percival wrote:

    > Stephan Uphoff wrote:
    >> Would it be enough to disable access to RDTSC for user processes?

    Doing so would be a kernel bug.

    > On single-core systems (single socket, not dual-core), yes. Otherwise, no.
    >
    >> I believe the attack needs a very exact time source.
    >
    > It needs ~100 cycle resolution. If you have two processors, you can get
    > good enough precision for the attack by constructing a "virtual TSC" by
    > using a second thread which repeatedly increments a location in memory.

    clock_gettime() with takes about 600 cycles if kern.timecounter.hardware=TSC.
    It should take fewer cycles, but syscalls are inefficient in FreeBSD.
    Does a factor of 6 make much difference to the attack?

    >> Beside benchmarking - is there any other real use for RDTSC ?
    >
    > Some (broken) software uses the TSC in combination with external events in
    > order to obtain entropy for cryptographic key generation. As a result,
    > disabling RDTSC could lead to non-obvious but very problematic breakage.

    Profiling (a form of benchmarking) should optionally be high resolution and
    high resolution profiling would use the TSC if it is available.

    Bruce
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