Re: [PATCH 0/4] Add 32 bit VDSO time function support

[Andy Lutomirski]

On Thu, Jan 30, 2014 at 10:03 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 01/30/2014 09:57 AM, Andy Lutomirski wrote:
>>
>> By definition there aren't any broken users of the new functions,
>> because there aren't any users at all.  So... should we start
>> randomizing this thing from the beginning?
>>
>
> The vdso already exists.  It isn't new.  Randomizing it might be a good
> idea, though; it already is randomized on 64 bits.
>
>> Also, since the VVAR page has a real vma, should something be done to
>> prevent mprotect or ptrace from COWing it?  Users will be rather
>> surprised if it suddenly stops updating.
>
> What happens currently on 64 bits?  I think we just take the attitude
> that "don't do that, then", and it hasn't seemed to be a problem.

On 64 bits, the vvar page is not part of the vdso vma -- it lives in
the fixmap.  So mprotect will return -EINVAL, I think.

With IA32 emulation, the vvar page has to live in the user address
space range (since there is no user-addressable kernel space), so it
seems more likely that user code will at least try to poke at it.

>
>> Finally, this might be the time to kill off the userspace mapping of
>> the HPET.  I suspect that there are few if any machines for which the
>> HPET is fast enough that avoiding a syscall matters at all.  (On my
>> box at work, reading the HPET takes ~500 nanoseconds.  I can do a lot
>> of syscalls in that amount of time.)
>
> I think this can be independent of extending the current 64-bit
> functionality to 32 bits.  It is a valid question, though.

My thinking is that if we do it now then there will never be a caller
of vclock_gettime that can't deal with it being randomized.
Admittedly, this isn't too likely regardless.

--Andy

>
>         -hpa
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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