Re: [PATCH v2 0/2] x86: Relocate the compat vdso per process

[Andy Lutomirski]

On Wed, Mar 12, 2014 at 10:32 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> In taking about 1 here.

I'm still unconvinced that this is worth the effort.  I'm okay with
killing the compat vdso entirely, and I'm okay with adding a minimal,
fully functional fix (like this patch set), but I don't really want to
add funny interactions with ASLR to support such a narrow use case.

>
> On March 12, 2014 9:20:50 AM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>On Tue, Mar 11, 2014 at 11:14 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>>> There is one sensible address: end of address space perhaps minus
>>some small offset.  Unlikely to be used by anything specific.
>>
>>Meh.  The compat vdso logic will end up being either:
>>
>>1) Try to map at the preferred address.  If it fails, don't map it.
>>Force ASLR off for the vdso
>>
>>2) Try to map at the preferred address.  If it fails, map somewhere
>>else and relocate.
>>
>>3) Try to map at the preferred address unless ASLR is on.  If it fails
>>or ASLR is on, map somewhere else and relocate.
>>
>>since the whole point is to simplify this stuff, I don't really like
>>any of these choices.  That's a lot of code to save 4k per process,
>>and when it breaks (which I suspect it will the next time anyone
>>touches the address space layout) so one will notice that 4k gets
>>wasted again.
>>
>>>
>>> On March 11, 2014 11:09:11 PM PDT, Andy Lutomirski
>><luto@xxxxxxxxxxxxxx> wrote:
>>>>On Mar 11, 2014 10:02 PM, "H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
>>>>>
>>>>> On 03/11/2014 03:15 PM, Andy Lutomirski wrote:
>>>>>>
>>>>>> The meat of this patch series is in patch 1.  Patch 2 is split out
>>>>for
>>>>>> improved bisectability.
>>>>>>
>>>>>> Changes from v1: Split into two patches and fixed a comment.
>>>>>>
>>>>>> Andy Lutomirski (2):
>>>>>>    x86: Dynamically relocate the compat vdso
>>>>>>    x86_32: Remove user bit from identity map PDE
>>>>>>
>>>>>>   Documentation/kernel-parameters.txt  |  18 +++-
>>>>>>   arch/x86/Kconfig                     |  24 +++--
>>>>>>   arch/x86/include/asm/elf.h           |   4 -
>>>>>>   arch/x86/include/asm/fixmap.h        |   8 --
>>>>>>   arch/x86/include/asm/pgtable_types.h |   7 +-
>>>>>>   arch/x86/include/asm/vdso.h          |   5 +-
>>>>>>   arch/x86/vdso/vdso-layout.lds.S      |   2 +-
>>>>>>   arch/x86/vdso/vdso32-setup.c         | 173
>>>>++++++++++++++++-------------------
>>>>>>   arch/x86/vdso/vdso32/vdso32.lds.S    |   2 -
>>>>>>   9 files changed, 111 insertions(+), 132 deletions(-)
>>>>>>
>>>>>
>>>>> Why per process?  *If* we have compat vdso turned on, can't we just
>>>>put it in one place system-wide?
>>>>
>>>>My machine has ASLR turned on, and the vma really does end up
>>>>somewhere different each time.  I suspect that most (?) ASLR users
>>>>would happily accept the 4k hit to keep ASLR working fully.
>>>>
>>>>Another downside of trying to have a common address for all
>>processes:
>>>>someone needs to figure out what that address is.  What happens if an
>>>>ELF binary (or interpreter) wants to load something at the chosen
>>>>address?
>>>>
>>>>--Andy
>>>
>>> --
>>> Sent from my mobile phone.  Please pardon brevity and lack of
>>formatting.
>
> --
> Sent from my mobile phone.  Please pardon brevity and lack of formatting.



-- 
Andy Lutomirski
AMA Capital Management, LLC
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