Re:A Kernel Hacker Meets Fuchsia OS
by throwaway894345 on Thursday May 26, 2022 @10:31AM
(#31518772)
Semantics aside, is it accurate to say that schemes where the capability is represented as a signed token require care and diligence to mitigate leaks, and schemes where the capability is represented as a table entry (mapping a user/process to a permission) don't require any meaningful care/diligence?
Yes, that's the idea.
Though, it is still worth thinking about how someone might accidentally leak a table-based capability via API misuse. For example, if table entries are reused over time you might get a sort of use-after-free situation where you mean to share an old capability but accidentally share the new one that replaced it.
Crypto backed capabilities that merge the token with the capability itself are one morph of capability based systems, but that's not the scheme being described here. Leaking the name of a capability (the offset in the cap table) does not leak the capability itself in these TCB schemes. Capabilities are still communicable in these TCB schemes because they're explicitly supported in the IPC mechanisms like SCM_RIGHTS or seL4_Send along side normal data. The token naming a capability then is a per process concept and you can clone an identical capability into several tokens in the same cap table, unlike the crypto enforced scheme which are basically content addressed storage for a secret representing a capability.