When the TIS, SPAPR, or CRB frontends negotiate a buffer size with the TPM backend, then the tpm_emulator (swtpm) could still adjust this size of the buffer to within bounds supported by swtpm+libtpms if the chosen size was outside the acceptable range. This could theoretically lead to the TPM 2 using a bigger buffer than what was requested and memory allocated for. In practice this would not happend since the requested size of 4096 bytes for TIS and SPAPR and 3968 bytes for CRB are in the (currently) supported range of ~2.5kb to 4096 bytes. With PQC support the range will have an upper bound of 8kb and a lower bound that will support the (pre-PQC) CRB with 3968 bytes.
Signed-off-by: Stefan Berger <[email protected]> --- backends/tpm/tpm_emulator.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/backends/tpm/tpm_emulator.c b/backends/tpm/tpm_emulator.c index 75c33d290e..653989ac0e 100644 --- a/backends/tpm/tpm_emulator.c +++ b/backends/tpm/tpm_emulator.c @@ -364,6 +364,7 @@ static int tpm_emulator_set_buffer_size(TPMBackend *tb, { TPMEmulator *tpm_emu = TPM_EMULATOR(tb); ptm_setbuffersize psbs; + size_t tpm_buffersize; if (tpm_emulator_stop_tpm(tb, errp) < 0) { return -1; @@ -387,8 +388,18 @@ static int tpm_emulator_set_buffer_size(TPMBackend *tb, return -1; } + tpm_buffersize = be32_to_cpu(psbs.u.resp.buffersize); + /* Reject different buffer size used by the TPM than what was requested. */ + if (wanted_size != 0 && wanted_size != tpm_buffersize) { + error_setg(errp, + "tpm-emulator: TPM did not accept the requested buffer size " + "of %zu bytes but adjusted it to %zu bytes\n", + wanted_size, tpm_buffersize); + return -1; + } + if (actual_size) { - *actual_size = be32_to_cpu(psbs.u.resp.buffersize); + *actual_size = tpm_buffersize; } trace_tpm_emulator_set_buffer_size( -- 2.43.0
