commit 430a4f39683d25e4291afa61e367b081240cfe23
author Sarvesh Bodakhe <sarvesh.bodakhe@espressif.com> Thu Jul 27 14:51:25 2023 +0530
committer Sarvesh Bodakhe <sarvesh.bodakhe@espressif.com> Thu Jul 27 14:51:25 2023 +0530
tree 745c5f4b847f6cdefa09612208c072200c49c1f4
parent 51ed3139d1f833c0698c834e1869747f97bcd432

rsa_signature: Use heap memory to allocate DER encoded RSA private key

'mbedtls_pk_psa_rsa_sign_ext' function allocates a buffer of maximum
size 5679 bytes (MBEDTLS_PK_RSA_PRV_DER_MAX_BYTES) on the stack to store
DER encoded private key. This increased stack usage significantly for
RSA signature operations when MBEDTLS_PSA_CRYPTO_C is defined.

This issue was discovered when adding support for EAP-TLS 1.3 (rfc9190).

Signed-off-by: Sarvesh Bodakhe <sarvesh.bodakhe@espressif.com>

ChangeLog.d/use_heap_rsa_signature.txt

diff --git a/ChangeLog.d/use_heap_rsa_signature.txt b/ChangeLog.d/use_heap_rsa_signature.txt
new file mode 100644
index 0000000..e6d7b12
--- /dev/null
+++ b/ChangeLog.d/use_heap_rsa_signature.txt
@@ -0,0 +1,4 @@
+Changes
+   * Use heap memory to allocate DER encoded RSA private key.
+     This reduces stack usage significantly for RSA signature
+     operations when MBEDTLS_PSA_CRYPTO_C is defined.

library/pk_wrap.c

diff --git a/library/pk_wrap.c b/library/pk_wrap.c
index 4781151..a92c754 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -302,11 +302,16 @@
     psa_status_t status;
     mbedtls_pk_context key;
     int key_len;
-    unsigned char buf[MBEDTLS_PK_RSA_PRV_DER_MAX_BYTES];
+    unsigned char *buf = NULL;
+    buf = mbedtls_calloc(1, MBEDTLS_PK_RSA_PRV_DER_MAX_BYTES);
+    if (buf == NULL) {
+        return MBEDTLS_ERR_PK_ALLOC_FAILED;
+    }
     mbedtls_pk_info_t pk_info = mbedtls_rsa_info;
 
     *sig_len = mbedtls_rsa_get_len(rsa_ctx);
     if (sig_size < *sig_len) {
+        mbedtls_free(buf);
         return MBEDTLS_ERR_PK_BUFFER_TOO_SMALL;
     }
 
@@ -314,8 +319,9 @@
      * re-construct one to make it happy */
     key.pk_info = &pk_info;
     key.pk_ctx = rsa_ctx;
-    key_len = mbedtls_pk_write_key_der(&key, buf, sizeof(buf));
+    key_len = mbedtls_pk_write_key_der(&key, buf, MBEDTLS_PK_RSA_PRV_DER_MAX_BYTES);
     if (key_len <= 0) {
+        mbedtls_free(buf);
         return MBEDTLS_ERR_PK_BAD_INPUT_DATA;
     }
     psa_set_key_usage_flags(&attributes, PSA_KEY_USAGE_SIGN_HASH);
@@ -323,7 +329,7 @@
     psa_set_key_type(&attributes, PSA_KEY_TYPE_RSA_KEY_PAIR);
 
     status = psa_import_key(&attributes,
-                            buf + sizeof(buf) - key_len, key_len,
+                            buf + MBEDTLS_PK_RSA_PRV_DER_MAX_BYTES - key_len, key_len,
                             &key_id);
     if (status != PSA_SUCCESS) {
         ret = PSA_PK_TO_MBEDTLS_ERR(status);
@@ -339,6 +345,7 @@
     ret = 0;
 
 cleanup:
+    mbedtls_free(buf);
     status = psa_destroy_key(key_id);
     if (ret == 0 && status != PSA_SUCCESS) {
         ret = PSA_PK_TO_MBEDTLS_ERR(status);