Hi
On Sat, May 28, 2022 at 7:23 PM Stefan Berger <stefanb@linux.ibm.com> wrote:
On 5/27/22 15:31, Stefan Berger wrote:
On 5/27/22 15:24, Marc-André Lureau wrote:
Hi
On Fri, May 27, 2022 at 7:36 PM Stefan Berger <stefanb@linux.ibm.com>
wrote:
This series of patches resolves an issue with a TPM 2's dictionary
attack
lockout logic being triggered upon well-timed VM resets. Normally,
the OS
TPM driver sends a TPM2_Shutdown to the TPM 2 upon reboot and before
a VM
is reset. However, the OS driver cannot do this when the user resets
a VM.
In this case QEMU must send the command because otherwise several well-
timed VM resets will trigger the TPM 2's dictionary attack (DA) logic
and
it will then refuse to do certain key-related operations until the DA
logic has timed out.
How does real hardware deal with that situation? Shouldn't this
"shutdown"/reset logic be implemented on swtpm side instead, when
CMD_INIT is received? (when the VM is restarted)
I don't know what real hardware can actually do when the machine is
reset, presumably via some reset line, or the power is removed. Probably
it has no way to react to this.
Typically the OS driver has to send the command and since it cannot do
this I would defer it to the TPM emulator reset handler code, so the
next layer down.
Also, when this is done in QEMU we don't need to do a data channel
operation (run TPM2_Shutdown) from within the control channel (upon
CMD_INIT) inside of swtpm. This way we can deal with it properly. The
usage model for the TPM 2 prescribes that a TPM2_Shutdown must be sent
before a shutdown or reset of the system, so let's let QEMU do it if the
OS cannot do it.
What if qemu is killed or crashed, and a new instance is connected to
swtpm? Or more subtle, the VM reboots without qemu help (no reset
handler). It feels like it would be more robust to handle the
situation in swtpm. Why not have the same last operation tracking and
shutdown logic there? If CMD_INIT is received and the last operation
is not shutdown, I'd have a warning and do it (if this is compliant
with the spec, I am trying to find relevant text).