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

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Date:	Mon, 10 Mar 2014 10:46:48 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 10:38 AM, H. Peter Anvin <hpa@...ux.intel.com> wrote:
> 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@....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".

It's a little less interesting if it just involves changing a vma.
It's still tricky, though -- would each struct mm have its own struct
file for the vvar page?  Can this be done with some
vm_operations_struct magic?  There are possible races, too, though --
another thread could access the thing concurrently with a syscall.

It might be nice in general for there to be a /dev/vdso and for the
vdso to literally be a mapping of that device node.  I bet that CRIU
would appreciate this.  (The mmap flags would be a little odd, since
different pages have different protections.)

Anyway, this is totally off topic for the current issue :)

--Andy
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