linux-kernel - Re: [x86, vdso] BUG: unable to handle kernel paging request at d34bd000

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Date:	Mon, 10 Mar 2014 10:24:25 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stefani Seibold <stefani@...bold.net>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>,
	Martin Runge <Martin.Runge@...de-schwarz.com>
Subject: Re: [x86, vdso] BUG: unable to handle kernel paging request at d34bd000

On 03/10/2014 10:12 AM, Andy Lutomirski wrote:
> On Mon, Mar 10, 2014 at 8:11 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> On Mar 10, 2014 8:01 AM, "H. Peter Anvin" <hpa@...ux.intel.com> wrote:
>>>
>>> I have mentioned in the past wanting to move the fixmap to the low part
>>> of the kernel space, because the top isn't really fixed...
>>
>> How about the high part of the user address space, just above the stack?
>> Leave a unmapped page in between, or something. The stack is already
>> randomized, isn't it?
> 
> For the !compat_vdso case, I don't like it -- this will put the vdso
> (which is executable) at a constant offset from the stack, which will
> make it much easier to use the vdso to defeat ASLR.
> 
> For the compat_vdso case, this only works if the address is *not*
> random, unless we're going to start giving each process its very own
> relocated vdso.
> 

I presumed we were talking about compat_vdso, which thus simply turns
into a "don't randomize the vdso flag."  A significant side benefit is
that this should make the code more similar.

> For 64-bit, this is an entirely different story.  The vsyscall page is
> stuck in the fixmap forever, although I want to add a way for
> userspace to opt out.  The vvar page, hpet, etc could move into vmas,
> though.  I kind of want to do that anyway to allow processes to turn
> off the ability to read the clock.

Wait... you want to do what?!

	-hpa

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