RE: [PATCH 0/4] MSR: MSR: MSR Whitelist and Batch Introduction

Mcfadden, Marty Jay
2016-02-29T02:55:47Z

> On Sun, Feb 28, 2016, Borislav Petkov <bp@alien8.de> wrote:
>
> Can we have some concrete examples for that please?
>

Our environment allows users to have exclusive access to some
number of compute nodes for a limited time. Bit-level control of
MSRs is required when a user might gain root or, more commonly,
interfere with subsequent jobs run by other users.

The canonical examples for bitwise control are
MSR_PKG_POWER_LIMIT and MSR_DRAM_POWER_LIMIT. We
want to provider user space control over power bounds, but if
the lock bit is set the power bound cannot be changed without
rebooting. As setting very low power bounds can slow
performance by a factor of 4x or worse, leaving the lock bit
writable allows a crude denial-of-service attack.

A second use case for bitwise control is IA32_MISC_ENABLE. This
MSR controls a wide variety of processor functionality, some of
which is benign ("Performance Energy Bias Hint") and some that
might not be ("Automatic Thermal Control Circuit Enable"). Rather
than do a formal security review of the dozen features controlled
by this MSR, we'd like to take the simpler step of allowing writes
to only what we know is safe. Note that bit "Enhanced Intel
SpeedStep Technology Select Lock" is a lock bit.

Thanks,

Marty McFadden