3 ms·

SMM is merely a macguffin in this situation, it’s a tool to get the system into a privileged mode whereupon you buffer overflow it with a mailbox message or som

by paulmd 2y ago

SMM is merely a macguffin in this situation, it’s a tool to get the system into a privileged mode whereupon you buffer overflow it with a mailbox message or something. I’d really think that based on your username you have to already be aware of this? Similar mechanisms were demoed in the ryzenfall exploit series like 5+ years ago. https://youtu.be/QuqefIZrRWc?t=1195 https://youtu.be/QuqefIZrRWc?t=1195

mjg59 2y ago
You don't need to do that if you're able to just shove arbitrary code into SMM in the first place, which is what's supposedly happening here. But having SMM doesn't let you change any persistent CPU state - the set of additional privileges that SMM has is known, and basically comes down to "Can read and write SMRAM, trap certain hardware accesses, and write to protected areas of firmware flash". Nothing in SMM is going to allow you to backdoor the CPU itself so badly that you have to discard it.
- paulmd 2y ago
  SMM is implemented as part of the PSP (again, see the link in the previous comment) and yes, writing to protected areas of firmware flash is the point. using SMM calls to jump to arbitrary code execution/control of the PSP is a big deal, that’s what happened before and probably what’s happened here.
  - mjg59 2y ago
    You've got that the wrong way around - the PSP is able to access arbitrary areas of physical memory that can only otherwise be accessed via SMM, and so compromising the PSP gives you the ability to tamper with SMM code (as discussed in https://blog.trailofbits.com/2018/03/15/amd-flaws-technical-summary/ https://blog.trailofbits.com/2018/03/15/amd-flaws-technical-...). Having access to SMM doesn't give you access to the PSP, and being able to modify flash doesn't circumvent Platform Secure Boot. Power off the board and the CPU will return to exactly the state it left the factory in.

by paulmd 2y ago

mjg59 2y ago

You don't need to do that if you're able to just shove arbitrary code into SMM in the first place, which is what's supposedly happening here. But having SMM doesn't let you change any persistent CPU state - the set of additional privileges that SMM has is known, and basically comes down to "Can read and write SMRAM, trap certain hardware accesses, and write to protected areas of firmware flash". Nothing in SMM is going to allow you to backdoor the CPU itself so badly that you have to discard it.

paulmd 2y ago
SMM is implemented as part of the PSP (again, see the link in the previous comment) and yes, writing to protected areas of firmware flash is the point. using SMM calls to jump to arbitrary code execution/control of the PSP is a big deal, that’s what happened before and probably what’s happened here.
- mjg59 2y ago
  You've got that the wrong way around - the PSP is able to access arbitrary areas of physical memory that can only otherwise be accessed via SMM, and so compromising the PSP gives you the ability to tamper with SMM code (as discussed in https://blog.trailofbits.com/2018/03/15/amd-flaws-technical-summary/ https://blog.trailofbits.com/2018/03/15/amd-flaws-technical-...). Having access to SMM doesn't give you access to the PSP, and being able to modify flash doesn't circumvent Platform Secure Boot. Power off the board and the CPU will return to exactly the state it left the factory in.