Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Jan 11, 2017 at 6:29 AM, Kirill A. Shutemov
<kirill@shutemov.name> wrote:
> On Thu, Jan 05, 2017 at 12:49:44PM -0800, Dave Hansen wrote:
>> On 01/05/2017 12:14 PM, Andy Lutomirski wrote:
>> >> I'm not sure I'm comfortable with this.  Do other rlimit changes cause
>> >> silent data corruption?  I'm pretty sure doing this to MPX would.
>> >>
>> > What actually goes wrong in this case?  That is, what combination of
>> > MPX setup of subsequent allocations will cause a problem, and is the
>> > problem worse than just a segfault?  IMO it would be really nice to
>> > keep the messy case confined to MPX.
>>
>> The MPX bounds tables are indexed by virtual address.  They need to grow
>> if the virtual address space grows.   There's an MSR that controls
>> whether we use the 48-bit or 57-bit layout.  It basically decides
>> whether we need a 2GB (48-bit) or 1TB (57-bit) bounds directory.
>>
>> The question is what we do with legacy MPX applications.  We obviously
>> can't let them just allocate a 2GB table and then go let the hardware
>> pretend it's 1TB in size.  We also can't hand the hardware using a 2GB
>> table an address >48-bits.
>>
>> Ideally, I'd like to make sure that legacy MPX can't be enabled if this
>> RLIMIT is set over 48-bits (really 47).  I'd also like to make sure that
>> legacy MPX is active, that the RLIMIT can't be raised because all hell
>> will break loose when the new addresses show up.
>
> I think we can do this. See the patch below.
>
> Basically, we refuse to enable MPX and issue warning in dmesg if there's
> anything mapped above 47-bits. Once MPX is enabled, mmap_max_addr() cannot
> be higher than 47-bits too.
>
> Function call from mmap_max_addr() is unfortunate, but I don't see a
> way around.

How about preventing the max addr from being changed to too high a
value while MPX is on instead of overriding the set value?  This would
have the added benefit that it would prevent silent failures where you
think you've enabled large addresses but MPX is also on and mmap
refuses to return large addresses.