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

[Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>]

On Thu, Jan 5, 2017 at 12:49 PM, Dave Hansen <dave.hansen-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> 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.
>
> Remember, we already have (legacy MPX) binaries in the wild that have no
> knowledge of this stuff.  So, we can implicitly have the kernel bump
> this rlimit around, but we can't expect userspace to do it, ever.

If you s/rlimit/prctl, then I think this all makes sense with one
exception.  It would be a bit sad if the personality-setting tool
didn't work if compiled with MPX.

So what if we had a second prctl field that is the value that kicks in
after execve()?

--Andy