Date: Thu, 25 Apr 2013 18:48:37 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andi Kleen <andi@firstfloor.org>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        eranian@google.com, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 1/2] Fix perf LBR filtering
Message-ID: <20130425164837.GD5828@dyad.programming.kicks-ass.net>
References: <1366844694-2770-1-git-send-email-andi@firstfloor.org>
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 <20130425164100.GC16732@two.firstfloor.org>
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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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