tpm20: Support partial reads

[jhibbits]

Summary

In some cases the TPM utilities may read only a partial block, instead
of a full block. If a new command starts while in the middle of a read
it may cause the TPM to go catatonic and no longer respond to SPI.

Obtained from: Juniper Networks, Inc.

[kd]

sys/dev/tpm/tpm20.c

87

We need to clear the part of the buffer that we've just read here.
Otherwise you could potentially leak the contents to another caller.

[jhibbits]

sys/dev/tpm/tpm20.c

87

I'm fine clearing it out, but I'm confused how that would happen. We're already adjusting the offset for the next read, so nobody should get "stale" data, only the next data.

[kd]

sys/dev/tpm/tpm20.c

87

Oh, good point.
And we can't really get stale data after the current reader is done with reading, since the sc->total_length is going to be either 0, or the response length of the subsequent command.

One last thing then. Could you please also update tpm_crb.c, so that it's not broken with this commit.
Just add sc->total_length = bytes_available; at the end of tpmcrb_transmit.

[jhibbits]

Address @kd's feedback.