Re: [PATCH 0/2] backend/tpm: Resolve issue with TPM 2 DA lockout

[Stefan Berger]

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.



> > Regards,
> >    Stefan
> >
> > Stefan Berger (2):
> >    backends/tpm: Record the last command sent to the TPM
> >    backends/tpm: Send TPM2_Shutdown upon VM reset
> >
> >   backends/tpm/tpm_emulator.c | 44 +++++++++++++++++++++++++++++++++++++
> >   backends/tpm/tpm_int.h      |  3 +++
> >   backends/tpm/tpm_util.c     |  9 ++++++++
> >   backends/tpm/trace-events   |  1 +
> >   include/sysemu/tpm_util.h   |  3 +++
> >   5 files changed, 60 insertions(+)
> >
> > --
> > 2.35.3