Mill Computing, Inc. Forums The Mill Architecture Security Reply To: Security

Ivan Godard
Keymaster
Post count: 629

Could a thread manipulate its own spReg to allow it to make calls without overflowing the stack, BUT overflowing the state in the spiller?

Not unless it had rights to the MMIO register that shadows spReg, and if it has rights like that then it’s part of the OS and better know what it is doing. spReg (and nearly all the specRegs) are not user-assignable except via MMIO. Ofg course, they change all the time as a side effect of executed operations like call, but not directly.

Could a thread do a grant in a loop, generating regions until something breaks? It sounds like the OS won’t get a chance to stop it until PLB eviction occurs. Also, if it’s on a family member which does it in hardware, that removes the OSs ability to regulate a rogue process.

The grant quantum, while set by policy, is managed by hardware and will throw a fault if exceeded. We didn’t want to track it at evict time in case we have members that do evicts (with PLB entry creation) in hardware.

If sounds like if a thread fills the region table with regions overlapping one address, the augmented interval tree will handle it happily (ignoring the previous question), however what if the PLB is (mostly) full of regions that all overlap an address, which a thread then accesses? Would it need to hit an excessive amount of regions in the cache to resolve the final permission set?

The region table is searched until the first entry that would give permission is found. Entries are not concatenated during the search; permission for the whole access must be provided by a single entry. That keeps the search cost manageable; if you really need to have an access that is part permitted in different entries then you must use a service that will push a new grant covering the union of your existing entries.

On the topic of overlapping regions, I assume it’s an OR of the permissions, not an AND?

Any single entry is satisfied if all of the region, thread, turf and rights match.

Does the iPLB ignore read permissions? (ie, can code be execute, non-read)

The iPLB is used only for execute access, and its entries need only distinguish eXecute from Portal. Similarly the dPLB is used only for load and store, and need only have Read and Write permission bits. Table entries have all the bits so as to reduce the number of entries.

  • This reply was modified 8 years, 6 months ago by  Ivan Godard.