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

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Date:	Mon, 10 Mar 2014 10:12:17 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Anvin <hpa@...ux.intel.com>,
	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 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.

>
> That would actually be preferable in a few ways, notably not having to mark
> page directories user accessible in the kennel space area.

Is that where the rabid pte dogs live?

We can already avoid making fixmap pages user-accessible in the
!compat_vdso case for 32-bit tasks -- the vdso lives in a couple of
more-or-less ordinary vmas.

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.

--Andy
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