libtpms (CryptHmacSign, vTPM): Out-of-bounds read when signKey and signScheme are mismatched, aborting the vTPM

Impact

Out-of-bounds read when signKey and signScheme are mismatched, aborting the vTPM. libtpms is the TPM behind QEMU/KVM guests, so on a GPU cloud that rents VMs this is a guest reaching out and killing its own virtual TPM - which takes down measured boot and any disk unlock or key sealing that depended on it, and on some stacks takes the guest with it. The libtpms instance of the same defect as the TCG reference implementation issue, which is worth noting because they patch through completely different channels.

Who can reach it

A guest able to issue TPM commands to its vTPM - i.e. any tenant in a VM you provisioned with a virtual TPM. No escape required, no host access.

What to do

libtpms package update on the hypervisor hosts plus a restart of affected guests' swtpm processes - so a rolling VM restart rather than a firmware flash, which is the cheap end of this database. Do not assume patching the host TPM stack covers your physical TPMs or your platform firmware TPM; those are separate code paths with separate fixes.

References