[PATCH 2/2] keys: trusted: tpm1: reject unauthenticated response tag in TSS_checkhmac1/2

[sn0x-sharma <sanketsharmacsec@gmail.com>]

TSS_checkhmac1() and TSS_checkhmac2() silently return success when the
response tag is TPM_TAG_RSP_COMMAND (0x00c4), bypassing HMAC
verification entirely. A forged TPM response with this unauthenticated
tag allows an attacker to skip integrity checks on TPM responses.

Reject TPM_TAG_RSP_COMMAND with -EINVAL in both functions, requiring
all responses to carry a proper authenticated tag.

Reported-by: sn0x-sharma <sanketsharmacsec@gmail.com>
Link: https://lore.kernel.org/linux-integrity/
Signed-off-by: sn0x-sharma <sanketsharmacsec@gmail.com>
---
 security/keys/trusted-keys/trusted_tpm1.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/security/keys/trusted-keys/trusted_tpm1.c b/security/keys/trusted-keys/trusted_tpm1.c
index d47cb7108..c89fc6cfe 100644
--- a/security/keys/trusted-keys/trusted_tpm1.c
+++ b/security/keys/trusted-keys/trusted_tpm1.c
@@ -202,7 +202,7 @@ static int TSS_checkhmac1(unsigned char *buffer,
 	ordinal = command;
 	result = LOAD32N(buffer, TPM_RETURN_OFFSET);
 	if (tag == TPM_TAG_RSP_COMMAND)
-		return 0;
+		return -EINVAL;
 	if (tag != TPM_TAG_RSP_AUTH1_COMMAND)
 		return -EINVAL;
 	authdata = buffer + bufsize - SHA1_DIGEST_SIZE;
@@ -270,7 +270,7 @@ static int TSS_checkhmac2(unsigned char *buffer,
 	result = LOAD32N(buffer, TPM_RETURN_OFFSET);
 
 	if (tag == TPM_TAG_RSP_COMMAND)
-		return 0;
+		return -EINVAL;
 	if (tag != TPM_TAG_RSP_AUTH2_COMMAND)
 		return -EINVAL;
 	authdata1 = buffer + bufsize - (SHA1_DIGEST_SIZE + 1
-- 
2.53.0