linux-kernel - Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

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Date:	Fri, 11 Apr 2014 11:46:46 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>
CC:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	stable <stable@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit
 kernels

On 04/11/2014 11:41 AM, Linus Torvalds wrote:
> 
> Ok, so you actually do this on x86-64, and it currently works? For
> some reason I thought that 16-bit windows apps already didn't work.
> 

Some will work, because not all 16-bit software care about the upper
half of ESP getting randomly corrupted.

That is the "functionality bit" of the problem.  The other bit, of
course, is that that random corruption is the address of the kernel stack.

> Because if we have working users of this, then I don't think we can do
> the "we don't support 16-bit segments", or at least we need to make it
> runtime configurable.

I'll let you pick what the policy should be here.  I personally think
that we have to be able to draw a line somewhere sometimes (Microsoft
themselves haven't supported running 16-bit binaries for several Windows
generations now), but it is your policy, not mine.

	-hpa

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