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Date:	Fri, 12 Dec 2014 16:11:23 -0800
From:	Andy Lutomirski <>
To:	Dave Hansen <>
Cc:	"" <>,
	Thomas Gleixner <>, X86 ML <>
Subject: Re: [RFC][PATCH 0/8] x86, mpx: Support 32-bit binaries on 64-bit kernels

On Fri, Dec 12, 2014 at 3:16 PM, Dave Hansen <> wrote:
> On 12/12/2014 03:04 PM, Andy Lutomirski wrote:
>> Anyway, do your patches handle the case where a 32-bit app maliciously
>> executes a 64-bit mpx insn with a very large address?  I think it's
>> okay, but I might have missed something.
> You mean in the instruction decoder?  I haven't tried that case
> explicitly, but I did do a substantial amount of testing throwing random
> instruction streams at the decoder to make sure it never fell over.
> (Well, mostly random, I made sure to throw the MPX opcodes in there a
> bunch so it would get much deeper in to the decoder).
> It's not about the instruction size, it's about the mode the CPU is in.
> If a 32-bit app manages to switch over to 64-bit mode and doesn't tell
> the kernel (TIF_IA32 remains set), then we'll treat it as a 32-bit
> instruction.

The insn decoder should probably use user_64bit_mode, not TIF_IA32.
It's actually quite easy to far jump/call/ret or sigreturn to a
different bitness.

> The kernel might end up going and looking for the bounds tables in some
> funky places if the kernel and the hardware disagree about 32 vs. 64-bit
> modes, but it's not going to do any harm since we treat all of the data
> we get from MPX (instruction decoding, register contents, bounds table
> contents, etc...) as completely untrusted.
> It's a nice, paranoid thing to ask and I'm glad you brought it up
> because I hadn't thought about it, but I don't think any harm can come
> of it.

Paranoia is fun!

The only thing I'd really be worried about is if the code that turns
va into bounds table offset generates some absurdly large offset as a
result and causes a problem.


Andy Lutomirski
AMA Capital Management, LLC
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