Re: [PATCH 1/2] Fix perf LBR filtering

[Peter Zijlstra]

On Thu, Apr 25, 2013 at 06:41:00PM +0200, Andi Kleen wrote:
> > So why not do the same as we do for userspace? Copy MAX_INSN_SIZE bytes
> > and trap -EFAULT.
> 
> Read the whole description, then you'll know why that is insecure.

You didn't actually explicitly mention it; you just said unconditional reading
of random addresses was bad.

You list:

> But that is dangerous if can be controlled by the user:
> - It can be used to crash the kernel
> - It allows to probe any physical address for a small set of values
> (valid call op codes) which is an information leak.
> - It may point to a side effect on read MMIO region

Traping the read deals with the first. The second shouldn't be a problem since
we generally only allow kernel info for CAP_ADMIN; if we don't already for LBR
that needs to be fixed separately.

That only leaves the third.. can we descern MMIO maps from the kernel page tables?

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