[PATCH v5 5/7] tpm: Increase TPM_BUFSIZE to 8KB for chunking support

[Arun Menon <armenon@redhat.com>]

From: Arun Menon <armenon@redhat.com>

TPM_BUFSIZE limits the TPM2 command the kernel builds before handing
it to a chip driver's send() callback. TPM 2.0 Library Part 3:
Commands v185 adds ML-DSA key objects and the ML-KEM
TPM2_Encapsulate()/TPM2_Decapsulate() commands (sections 14.10,
14.11) [1]. NIST FIPS 204, Table 2, gives the largest ML-DSA
parameter set, ML-DSA-87, a 4896-byte private key and a 2592-byte
public key [2].
Loading such a key pair with TPM2_Load, plus header, handle and session,
comes to roughly 7.7KB - more than the current 4096-byte TPM_BUFSIZE.

Raise TPM_BUFSIZE to 8192 bytes. It covers this command with
headroom, and keeps every kzalloc(TPM_BUFSIZE, ...) call in the TPM
stack at an order-1 page allocation rather than a higher order.

This matches the largest parameter set defined today. NIST has already
published PQC families with larger keys and signatures than ML-DSA
(e.g. FIPS 205/SLH-DSA), so a future algorithm could exceed 8192 bytes.
CRB chunking, added later in this series, does not change that; it only
lets an already-built command cross a smaller MMIO window in pieces,
it does not change how large a command the generic stack can build.

[1] https://medias.cdn.trustedcomputinggroup.org/production/Trusted_Platform_Module_2_0_Library_Part_3_Commands_Version_185_pub_87a5c10230.pdf
[2] https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.204.pdf

Signed-off-by: Arun Menon <armenon@redhat.com>
---
 include/linux/tpm_command.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/tpm_command.h b/include/linux/tpm_command.h
index fc446a1282e2..4ad11068b090 100644
--- a/include/linux/tpm_command.h
+++ b/include/linux/tpm_command.h
@@ -434,7 +434,7 @@ struct tpm2_context {
  */
 
 #define TPM_DIGEST_SIZE		20	/* Max TPM v1.2 PCR size */
-#define TPM_BUFSIZE		4096
+#define TPM_BUFSIZE		8192
 
 /*
  * SHA-512 is, as of today, the largest digest in the TCG algorithm repository.
-- 
2.56.0