[elfenpiff]

> This smells like they are using shared memory, which is almost certainly a security nightmare.

Yes, we are using shared memory, and I agree that shared memory is a challenge but there are some mechanisms that can make it secure.

The main problem with shared memory is, that one process can corrupt the data structure while another process is consuming it. Even verifying the contents of the data structure is insufficient since it can always be corrupted afterwards. We have named the problem "modify-after-delivery problem" - a sender modifies the data after it has been delivered to a receiver.

This can be handled with:

1. memfd: The sender acquires it, writes its payload, seals it so that it is read-only and then transfers the file descriptor to all receivers. The receiver can verify the read-only seal with fcntl. Since linux guarantees us that it cannot be reverted the receiver can now safely consume the data. This allows it to be used even in a zero-trust environment. [1] provides a good introduction (see the File-Sealing IPC subsection). 2. Memory protection keys [2]: I do not have too much experience with them, but as far as I understand, they solve the problem with mprotect, meaning, the sender can call mprotect and make the segment read only for it self, but the receiver has no way of verifying it or to prevent the sender from calling mprotect again and granting it read/write access again to corrupt the data.

So, the approach is that a sender acquires shared memory, writes its payload into it, makes it read-only, and then transfers it to the receivers.

> Shared memory works as a transport if you either assume that all parties are trusted (in which case why do IPC in the first place?

Robustness is another use case. In mission-critical systems you trust each process but a crash caused by a bug in one sub-system shall not bring down the whole system. So you split up the monolith in many processes and the overall system survives if one process goes down or deadlocks, assuming you have a shared memory library that itself is safe. If you detect a process crash, you can restart it and continue operations.

[1] https://dvdhrm.wordpress.com/2014/06/10/memfd_create2/ [2] https://www.kernel.org/doc/html/latest/core-api/protection-k...