[PATCH v2 1/3] KEYS: trusted_tpm1: Compare HMAC values in constant time

[Eric Biggers]

To prevent timing attacks, HMAC value comparison needs to be constant
time.  Replace the memcmp() with the correct function, crypto_memneq().

[For the Fixes commit I used the commit that introduced the memcmp().
It predates the introduction of crypto_memneq(), but it was still a bug
at the time even though a helper function didn't exist yet.]

Fixes: d00a1c72f7f4 ("keys: add new trusted key-type")
Cc: sta...@vger.kernel.org
Signed-off-by: Eric Biggers <ebigg...@kernel.org>
---
 security/keys/trusted-keys/trusted_tpm1.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/security/keys/trusted-keys/trusted_tpm1.c 
b/security/keys/trusted-keys/trusted_tpm1.c
index 89c9798d18007..e73f2c6c817a0 100644
--- a/security/keys/trusted-keys/trusted_tpm1.c
+++ b/security/keys/trusted-keys/trusted_tpm1.c
@@ -5,10 +5,11 @@
  *
  * See Documentation/security/keys/trusted-encrypted.rst
  */
 
 #include <crypto/hash_info.h>
+#include <crypto/utils.h>
 #include <linux/init.h>
 #include <linux/slab.h>
 #include <linux/parser.h>
 #include <linux/string.h>
 #include <linux/err.h>
@@ -239,11 +240,11 @@ int TSS_checkhmac1(unsigned char *buffer,
                          TPM_NONCE_SIZE, enonce, TPM_NONCE_SIZE, ononce,
                          1, continueflag, 0, 0);
        if (ret < 0)
                goto out;
 
-       if (memcmp(testhmac, authdata, SHA1_DIGEST_SIZE))
+       if (crypto_memneq(testhmac, authdata, SHA1_DIGEST_SIZE))
                ret = -EINVAL;
 out:
        kfree_sensitive(sdesc);
        return ret;
 }
@@ -332,20 +333,20 @@ static int TSS_checkhmac2(unsigned char *buffer,
        ret = TSS_rawhmac(testhmac1, key1, keylen1, SHA1_DIGEST_SIZE,
                          paramdigest, TPM_NONCE_SIZE, enonce1,
                          TPM_NONCE_SIZE, ononce, 1, continueflag1, 0, 0);
        if (ret < 0)
                goto out;
-       if (memcmp(testhmac1, authdata1, SHA1_DIGEST_SIZE)) {
+       if (crypto_memneq(testhmac1, authdata1, SHA1_DIGEST_SIZE)) {
                ret = -EINVAL;
                goto out;
        }
        ret = TSS_rawhmac(testhmac2, key2, keylen2, SHA1_DIGEST_SIZE,
                          paramdigest, TPM_NONCE_SIZE, enonce2,
                          TPM_NONCE_SIZE, ononce, 1, continueflag2, 0, 0);
        if (ret < 0)
                goto out;
-       if (memcmp(testhmac2, authdata2, SHA1_DIGEST_SIZE))
+       if (crypto_memneq(testhmac2, authdata2, SHA1_DIGEST_SIZE))
                ret = -EINVAL;
 out:
        kfree_sensitive(sdesc);
        return ret;
 }
-- 
2.50.1