tpm: Add function to load keys via their parent's SHA1 hash

If we want to load a key into a TPM, we need to know the designated parent
key's handle, so that the TPM is able to insert the key at the correct place in
the key hierarchy.

However, if we want to load a key whose designated parent key we also
previously loaded ourselves, we first need to memorize this parent key's handle
(since the handles for the key are chosen at random when they are inserted into
the TPM). If we are, however, unable to do so, for example if the parent key is
loaded into the TPM during production, and its child key during the actual
boot, we must find a different mechanism to identify the parent key.

To solve this problem, we add a function that allows U-Boot to load a key into
the TPM using their designated parent key's SHA1 hash, and the corresponding
auth data.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>

1 files changed, 40 insertions, 0 deletions

diff --git a/lib/tpm.c b/lib/tpm.c
index fb12214..cd7f88f 100644
--- a/lib/tpm.c
+++ b/lib/tpm.c
@@ -996,4 +996,44 @@ uint32_t tpm_get_pub_key_oiap(uint32_t key_handle, const void *usage_auth,
 	return 0;
 }
 
+#ifdef CONFIG_TPM_LOAD_KEY_BY_SHA1
+uint32_t tpm_find_key_sha1(const uint8_t auth[20], const uint8_t
+			   pubkey_digest[20], uint32_t *handle)
+{
+	uint16_t key_count;
+	uint32_t key_handles[10];
+	uint8_t buf[288];
+	uint8_t *ptr;
+	uint32_t err;
+	uint8_t digest[20];
+	size_t buf_len;
+	unsigned int i;
+
+	/* fetch list of already loaded keys in the TPM */
+	err = tpm_get_capability(TPM_CAP_HANDLE, TPM_RT_KEY, buf, sizeof(buf));
+	if (err)
+		return -1;
+	key_count = get_unaligned_be16(buf);
+	ptr = buf + 2;
+	for (i = 0; i < key_count; ++i, ptr += 4)
+		key_handles[i] = get_unaligned_be32(ptr);
+
+	/* now search a(/ the) key which we can access with the given auth */
+	for (i = 0; i < key_count; ++i) {
+		buf_len = sizeof(buf);
+		err = tpm_get_pub_key_oiap(key_handles[i], auth, buf, &buf_len);
+		if (err && err != TPM_AUTHFAIL)
+			return -1;
+		if (err)
+			continue;
+		sha1_csum(buf, buf_len, digest);
+		if (!memcmp(digest, pubkey_digest, 20)) {
+			*handle = key_handles[i];
+			return 0;
+		}
+	}
+	return 1;
+}
+#endif /* CONFIG_TPM_LOAD_KEY_BY_SHA1 */
+
 #endif /* CONFIG_TPM_AUTH_SESSIONS */