4 ms·

This is bad in a DoS-type of way. BootGuard only loads correctly signed firmware, the root of trust is an Intel RSA keypair and the hash of the pubkey is burned

This is bad in a DoS-type of way. BootGuard only loads correctly signed firmware, the root of trust is an Intel RSA keypair and the hash of the pubkey is burned into the CPU during Intel's manufacturing. The PCH (on the same chip in mobile cases) has the ME which also has fuses, and as long as the CPU only accepts signed code form the ME, and the ME has BootGuard fuse blown, the only thing you can really do is disable a system by nuking the SPI (or just flipping one random bit to invalidate the signature). To fix it, you'd have to re-flash the SPI chip. As far as I know, manufacturing mode only allows you to add signing keys if those keys are signed by Intel, so adding your own keys won't help. Adding a key from another manufacturer with known exploitable firmware could work, but loading that firmware on incompatible hardware won't let you boot the machine so you still get nothing. All in all; nice find, yes you can disable machines via software, but other than that, not as interesting as I had hoped it to be. (IBB or ACM exploits would be very very very sweet)

jarfil 8y ago
It also means that instead of having to compromise Intel's signing key to gain full control, now it's also possible to use any compromised key that's been signed by Intel, which may not be as well guarded as Intel's one.
- oneplane 8y ago
  Yes, well, it was always possible (either using direct flashing or HDA_SDO), or by forcing a reboot with the upgrade bit set (which was enabled by default on most platforms I've seen up to 2017. (no real data after that since I no longer have to deal with fleets of random systems). I suppose the ME MFG mode here adds for an additional path that can be abused silently, which is a real problem. On the key side of things: I have not yet found a leaked key. That would have made coreboot life so much easier...

jarfil 8y ago

It also means that instead of having to compromise Intel's signing key to gain full control, now it's also possible to use any compromised key that's been signed by Intel, which may not be as well guarded as Intel's one.

oneplane 8y ago
Yes, well, it was always possible (either using direct flashing or HDA_SDO), or by forcing a reboot with the upgrade bit set (which was enabled by default on most platforms I've seen up to 2017. (no real data after that since I no longer have to deal with fleets of random systems). I suppose the ME MFG mode here adds for an additional path that can be abused silently, which is a real problem. On the key side of things: I have not yet found a leaked key. That would have made coreboot life so much easier...