Exploiting System Management Mode with a very long interrupt

hyperhello•2 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.

mike_hearn•43 minutes 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

nazgulsenpai•about 1 hour 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•about 1 hour 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•28 minutes ago

Only if the short instruction is incredibly long.

Liftyee•33 minutes 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•about 1 hour 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•about 1 hour ago

Can this be patched? Is there a chance it's a hw watchdog that you can't fix in microcode?

ramses0•about 1 hour ago

Looks like it's ~4 billion (2^32) crossover counter?

kmeisthax•about 1 hour 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•25 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•17 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•about 1 hour ago

Maybe with vfio/igb_uio/uio_pci_generic? Still root level access.

Exploiting System Management Mode with a very long interrupt

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