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Date: Mon, 10 Mar 2014 10:38:30 -0700
From: "H. Peter Anvin" <hpa@linux.intel.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Stefani Seibold <stefani@seibold.net>,
        Andreas Brief <Andreas.Brief@rohde-schwarz.com>,
        Martin Runge <Martin.Runge@rohde-schwarz.com>
Subject: Re: [x86, vdso] BUG: unable to handle kernel paging request at d34bd000
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On 03/10/2014 10:31 AM, Andy Lutomirski wrote:
>>
>>> 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?!
> 
> This isn't even my idea:
> 
> commit 8fb402bccf203ecca8f9e0202b8fd3c937dece6f
> Author: Erik Bosman <ebn310@few.vu.nl>
> Date:   Fri Apr 11 18:54:17 2008 +0200
> 
>     generic, x86: add prctl commands PR_GET_TSC and PR_SET_TSC
> 
>     This patch adds prctl commands that make it possible
>     to deny the execution of timestamp counters in userspace.
>     If this is not implemented on a specific architecture,
>     prctl will return -EINVAL.
> 
> Currently anything that tries to use the vdso will just crash if you
> do that, and it fails to turn off direct HPET access.  Fixing this
> might be nice, but the current vvar implementation makes it
> impossible.  If you want to stick something in a seccomp sandbox and
> make it very difficult for it to exploit timing side channels, then
> this is important :)
> 

Yes, we'd have to switch the vdso to using syscall access.  Doing that
from inside a system call is... "interesting".

	-hpa