At Fri, 22 Dec 2006 12:36:56 -0500, Eric Northup digitale@digitaleric.net wrote:
Marcus Brinkmann wrote:
At Wed, 13 Dec 2006 11:51:52 +0100, voelp@os.inf.tu-dresden.de wrote:
From a security perspective this may really be a problem. Do you know how other systems solve it? Like Eros e.g., implements immediate revocation by destruction of the intermediate objects. However, internally they also have a list in which address spaces a capability is mapped.
Not really. "Removing an object" in EROS/KeyKOS means incrementing its allocation count, which immediately invalidates all keys pointing to the object, as long as the keys are on disk (recall we are talking about persistent systems). The story is a bit different for objects in memory ("prepared"): For this case, there is a table of indirection, and the indirection entry is invalidated. The rest is done at the next checkpoint by the scavenger.
You are both correct here. Prepared capabilities are most often found in the address space definition tree of a process. Objects which have been cached in page tables must also invalidate the PTEs referencing them in addition to the allocation count dance described above. KeyKOS and its successors (EROS, Coyotos, and CapROS) use a hash table to track the mapping from object -> list of PTEs which were produced by that object. It's called the "depends table", and is the list Marcus Voelp mentioned. The depends table is analogous to the mapping database in L4.
The time needed to unmap an object's PTEs is bounded by the number of entries in a hash bucket of the depends table (33 [start... end] ranges in EROS). On overflow during a page fault, an existing mapping must reclaimed, though it can be faulted in again later.
Malicious processes could exploit this overflow to degrade performance, and it formed part of an (overt) communication channel in the KeyKOS and EROS specifications (corrected in CapROS and Coyotos).
Eric, thanks for correcting me and providing this information.
The details are a bit complicated. The costs for keeping track of everything are swallowed up and thus amortized by the object cache magic. I am not sure, but it might be possible for a malicious process to try to screw with the checkpointing mechanism, for example by forcing early checkpoints and thus degrading the performance of the system, but if that is a possible threat scenario or just a paranoid feeling is difficult for me to assess at this point, and may be very difficult to analyse. Eventually you will run out of allocation counts, and then you can only recover object space by a garbage collection on the object store.
Very few processes are given the capability to force a checkpoint.
I realize that, but my concern is not that they have a capability to force a checkpoint, but that they can force a checkpoint simply by exhausting the object table. I am stretching out my neck here, because I definitely don't have all the details. Is it feasible that there is a sequence of create and rescind operations one or multiple cooperating programs could perform to spill the object table that provides the indirection layer? What happens in that case? Also, what happens if it happens at a time where a checkpoint is already in progress.
There were designs for both offline and incremental GC of allocation counts, but none were implemented until a need came up - which never happened. With a 16 bit allocation court / 5 min. checkpoint interval (original KeyKOS), it takes over 7 months to wrap in the worst case. With 24 bit / 1 min, it takes almost 32 years.
I agree that we can ignore the allocation count (for now :)
Thanks, Marcus