Rendered at 18:23:21 GMT+0000 (Coordinated Universal Time) with Cloudflare Workers.
mike_hearn 1 hours ago [-]
The designers of the firmware anticipate this attack but punt it to the vendor, apparently:
//
// Platform implementor should choose a timeout value appropriately:
[snip]
// - The timeout value must be longer than longest possible IO operation in the system
wtallis 27 minutes ago [-]
I wonder whether a Thunderbolt accessory can cause arbitrarily long IO operations. What's the upper bound before some other layer gives up and causes the instruction to fault?
nazgulsenpai 2 hours ago [-]
I'm amused at the lengths the readme goes to in order to drive home the fact that this needs to be a LOOOOOOOOOOOOOOOOOOOONG instruction, including the unnecessarily long code block illustration. The topic is interesting anyway, but that makes it way more entertaining.
BadBadJellyBean 2 hours ago [-]
Do you think a short instruction is okay or does it need to be long? The instructions were a bit unclear in that regard :D
nazgulsenpai 55 minutes ago [-]
Only if the short instruction is incredibly long.
hyperhello 29 minutes ago [-]
SMM calls for a timeout because it wants everything to be between instructions pro forma. So there’s a very long instruction on a core, but after it completes, the core does stop, right? It seems like to make this into an attack you’d have to a very long instruction that also somehow interacts with the thing the SMM is doing, while it’s doing it.
Liftyee 1 hours ago [-]
I don't know much about the specifics of CPU architecture apart from the existence of assembly and different modes. Either way the explanation was still entertaining and interesting. smiiiiiiii
londons_explore 2 hours ago [-]
Unclear why there is a 1 second timeout at all.
Presumably the patch for that will be to make it an infinity timeout.
xxpor 1 hours ago [-]
Can this be patched? Is there a chance it's a hw watchdog that you can't fix in microcode?
ramses0 1 hours ago [-]
Looks like it's ~4 billion (2^32) crossover counter?
kmeisthax 1 hours ago [-]
...huh, I was wondering why serial machine code prankster xoreaxeaxeax was keeping lists of extremely long-running instructions.
Hopefully this is at least only possible in kernel mode, right?
Right?!
tptacek 52 minutes ago [-]
Is it really a long running instruction? I mean, obviously yes, but what makes it slow is that it's doing an MMIO copy from a slow source. It's like a read(2) system call being "slow" because the fd is associated with a socket to the moon.
tuetuopay 44 minutes ago [-]
It's an instruction in the sense that timing boundaries are x86 instruction boundaries, which is what the security model bases itself on. So yeah, not an instruction in the strict CPU sense (microcode + micro-ops), but in the useful sense.
xxpor 1 hours ago [-]
Maybe with vfio/igb_uio/uio_pci_generic? Still root level access.
Presumably the patch for that will be to make it an infinity timeout.
Hopefully this is at least only possible in kernel mode, right?
Right?!