Re: vvar, gup && coredump

[Andy Lutomirski]

On Thu, Mar 12, 2015 at 9:54 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 03/12, Andy Lutomirski wrote:
>>
>> On Thu, Mar 12, 2015 at 7:34 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> >
>> > What is not clear: do we really want gup() to fail? Or it is not trivial
>> > to turn __vvar_page into the "normal" page? (to simplify the discussion,
>> > lets ignore hpet mapping for now).
>>
>> We could presumably fiddle with the vma to allow get_user_pages to
>> work on at least the first vvar page.  There are some decently large
>> caveats, though:
>>
>>  - We don't want to COW it.  If someone pokes at that page with
>> ptrace, for example, and it gets COWed, everything will stop working
>> because the offending process will no longer see updates.  That way
>> lies infinite loops.
>
> Of course, but this looks simple... is_cow_mapping() == F so FOLL_FORCE
> won't work anyway?
>
>>  - The implementation could be odd.  The vma is either VM_MIXEDMAP or
>> VM_PFNMAP, and I don't see any practical way to change that.
>>
>>  - The HPET and perhaps pvclock stuff.  The HPET probably doesn't have
>> a struct page at all, so you can't possibly get_user_pages it.
>
> Yes, this is true. OK, lets not dump it. I'll probably send a patch which
> changes vma_dump_size() to check VM_DONTDUMP first...
>
> But this leads to another question: why do we want to expose this
> "vvar" vma at all?
>
> For the moment, forget about compat 32-bit applications running under
> 64-bit kernel.
>
> Can't we simply add FIX_VVAR_PAGE into fixed_addresses{}, map it into
> init_mm via set_fixmap(FIX_VVAR_PAGE, __PAGE_USER) and change __vdso.*
> functions to use fix_to_virt() address?
>
> I don't really understand the low-level details, I'd like to understand
> if this can work or not. And if it can work, why this is undesirable.
>
> As for 32-bit applications. Yes, this can't work because 32-bit simply
> can't access this "high" memory. But you know, it would be very nice to
> have the fixmap-like "global" area in init_mm which is also visible to
> compat applications. If we had it, uprobes could work without xol vma's.
>

It could work for 32-bit native, but not for 32-bit compat.  Also, I
have grand plans to add per-task vvar overrides for seccomp and such.
And RIP-relative addressing is a bit nicer than absolute :)

It used to work that way, but we changed it in 3.15 IIRC.

On a related note, I'm hoping to rework the mm part pretty heavily:

http://lkml.kernel.org/r/cover.1414629045.git.luto@xxxxxxxxxxxxxx

--Andy

> Oleg.
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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