[twoodfin]

Yeah, would love to see the CPU vendors invent some primitives to let user code pull those kinds of privilege isolation tricks within a single process and address space.

Something like: “From now on, code on these pages can only access data on these pages, and only return to/call into other code through these gates…”

[MarkSweep]

That would be pretty cool. Something like the Win32 function GetWriteWatch, but implemented in hardware instead of the page fault handler (I assume).

https://learn.microsoft.com/en-us/windows/win32/api/memoryap...

Or some sort of special write barrier store op-code, idk.

[ahartmetz]

I've had some ideas about avoiding format validation in IPC receivers if the data is encoded by trusted code, which is also the only code that has rights to send the IPC data / to connect to the receiver. I can't really think of an important problem that it would solve, though. DBus always validates received data, but it's not really meant or very suitable for large amounts of data anyway.

[twoodfin]

What I’m looking for is a way for a process to de/re-escalate its privileges to access memory, without an expensive context switch being required at the transition. The CPU would simply enforce different rules based on (say) the high-order bits of the instruction pointer.

Imagine a server process that wants to run some elaborate third-party content parser. It’d be great to be sure that no matter how buggy or malicious that code, it can’t leak the TLS keys.

Today, high-security architectures must use process isolation to achieve this kind of architectural guarantee, but even finely tuned IPC like L4’s is an order of magnitude slower than a predictable jump.

[gpderetta]

For a brief moment Intel supported MPX which did something similar.

You can also play tricks with the virtualization hardware, bit it need kernel support.

Eventually we will get segments back again.

[hinkley]

Thread based seems like it at least should be possible.

[mike_hearn]

It exists, that's called MPKs.

[twoodfin]

I don’t think MPK’s will fit the need I have. Simply: Run some arbitrary, untrusted, non-sandboxed code in the same thread with assurance it can’t read page X. When that code completes and I’m back in code I trust, X is readable again.

Is that something MPK makes possible? The doc I’ve read suggests either your process can flip permission bits or it can’t. Great for avoiding out-of-sandbox reads. But if there’s arbitrary execution happening, why can’t that code flip the access to secrets back on?

[mike_hearn]

Oracle Labs has tech that does that:

https://youtu.be/T05FI93MBI8?si=EieFgujaGiW2gbO8&t=958

The trick is to do a cascading disassembly of all untrusted code you'll execute to prove it can't change the MPK register.

[twoodfin]

Wow. Neat trick and exactly the kind of thing I was looking for.

Thanks!

EDIT: Looks like this is the relevant paper from the Graal team: https://www.graalvm.org/resources/articles/binsweep.pdf