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

[Dave Hansen]

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.