linux-kernel - Re: [RFC][PATCH 0/8] x86, mpx: Support 32-bit binaries on 64-bit kernels

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Date:	Fri, 12 Dec 2014 15:04:38 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave@...1.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>
Subject: Re: [RFC][PATCH 0/8] x86, mpx: Support 32-bit binaries on 64-bit kernels

n Fri, Dec 12, 2014 at 1:41 PM, Dave Hansen <dave@...1.net> wrote:
> On 12/12/2014 12:48 PM, Andy Lutomirski wrote:
>> On Fri, Dec 12, 2014 at 12:27 PM, Dave Hansen <dave@...1.net> wrote:
>>> You want the same size structures with the same format for 32-bit and
>>> 64-bit modes?
>>
>> Yes.  Especially because programs can switch between 32-bit and 64-bit
>> mode entirely in userspace.  I don't know whether any do in practice,
>> but programs *can*.
>
> So, you want a 2GB of the 32-bit address space dedicated to a bounds
> directory, and half of the space for the bounds tables to be simply
> zero-filled unused address bits?  That seems, um, a bit unreasonable.

Fair enough.

>
>> Or better yet: Intel could have skipped supporting it at all in 32-bit
>> mode.
>
> So, we should not have this security feature for 32-bit apps... because
> it costs us 50 lines of code in the kernel to support?  Did you look at
> the diffstat?
>
>> Isn't mpx somewhat of an address space hog anyway?
>
> Yes, it will be troublesome for 32-bit apps that are already bumping up
> against the virtual address space size to support it.  But, really, how
> many of those *are* there these days?

I wonder how many 32-bit apps will end up using mpx regardless.

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.

--Andy
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