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

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Date:	Mon, 14 Apr 2014 08:45:36 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.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

For both of these, though, it is really kind of broken that it is a global switch, whereas typically only one application on the whole system needs it, so it would be much better to have application-specific controls.  How to do that is another matter...

On April 14, 2014 12:27:56 AM PDT, Ingo Molnar <mingo@...nel.org> wrote:
>
>* H. Peter Anvin <hpa@...ux.intel.com> wrote:
>
>> 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.
>
>I think the mmap_min_addr model works pretty well:
>
> - it defaults to secure
>
> - allow a security policy to grant an exception to a known package, 
>   built by the distro
>
> - end user can also grant an exception
>
>This essentially punts any 'makes the system less secure' exceptions 
>to the distro and the end user.
>
>Thanks,
>
>	Ingo

-- 
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