tpm_emulator: Reject a buffer size different than what was requested

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 happen 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.

Fixes: 9375c44fdfc0 ("tpm: tpm_emulator: get and set buffer size of device")
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20260511142219.797048-2-stefanb@linux.ibm.com
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>

diff --git a/backends/tpm/tpm_emulator.c b/backends/tpm/tpm_emulator.c
index 75c33d290e7149170e2a5b5e97e70bea01288f0e..ac5427b84e279874f1b9384eb2de7ac7a7ba1792 100644 (file)
--- 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",
+                   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(