commit 01902779a3a0232a21afee4c139b495835a10cae
author Bence Szépkúti <bence.szepkuti@arm.com> Fri Oct 13 14:27:18 2023 +0000
committer GitHub <noreply@github.com> Fri Oct 13 14:27:18 2023 +0000
tree 1ee4fff7ef1df3e3583724d2cc951827adfee862
parent 3841af02f59a4a71a937b07ca84eb39abe05c0c6
parent 4b0b97e18bc6bb3ac2e5be0756cc1493d4b9af53

Merge pull request #8162 from yanrayw/2.28-save_stack_usage_pkwrite

Backport 2.28: pkwrite: use heap to save stack usage for writing keys in PEM string

ChangeLog.d/pkwrite-pem-use-heap.txt

diff --git a/ChangeLog.d/pkwrite-pem-use-heap.txt b/ChangeLog.d/pkwrite-pem-use-heap.txt
new file mode 100644
index 0000000..11db7b6
--- /dev/null
+++ b/ChangeLog.d/pkwrite-pem-use-heap.txt
@@ -0,0 +1,4 @@
+Changes
+   * Use heap memory to allocate DER encoded public/private key.
+     This reduces stack usage significantly for writing a public/private
+     key to a PEM string.

library/pkwrite.c

diff --git a/library/pkwrite.c b/library/pkwrite.c
index 88e6855..5e3fcc9 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -571,38 +571,49 @@
 int mbedtls_pk_write_pubkey_pem(mbedtls_pk_context *key, unsigned char *buf, size_t size)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    unsigned char output_buf[PUB_DER_MAX_BYTES];
+    unsigned char *output_buf = NULL;
+    output_buf = mbedtls_calloc(1, PUB_DER_MAX_BYTES);
+    if (output_buf == NULL) {
+        return MBEDTLS_ERR_PK_ALLOC_FAILED;
+    }
     size_t olen = 0;
 
     PK_VALIDATE_RET(key != NULL);
     PK_VALIDATE_RET(buf != NULL || size == 0);
 
     if ((ret = mbedtls_pk_write_pubkey_der(key, output_buf,
-                                           sizeof(output_buf))) < 0) {
-        return ret;
+                                           PUB_DER_MAX_BYTES)) < 0) {
+        goto cleanup;
     }
 
     if ((ret = mbedtls_pem_write_buffer(PEM_BEGIN_PUBLIC_KEY, PEM_END_PUBLIC_KEY,
-                                        output_buf + sizeof(output_buf) - ret,
+                                        output_buf + PUB_DER_MAX_BYTES - ret,
                                         ret, buf, size, &olen)) != 0) {
-        return ret;
+        goto cleanup;
     }
 
-    return 0;
+    ret = 0;
+cleanup:
+    mbedtls_free(output_buf);
+    return ret;
 }
 
 int mbedtls_pk_write_key_pem(mbedtls_pk_context *key, unsigned char *buf, size_t size)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    unsigned char output_buf[PRV_DER_MAX_BYTES];
+    unsigned char *output_buf = NULL;
+    output_buf = mbedtls_calloc(1, PRV_DER_MAX_BYTES);
+    if (output_buf == NULL) {
+        return MBEDTLS_ERR_PK_ALLOC_FAILED;
+    }
     const char *begin, *end;
     size_t olen = 0;
 
     PK_VALIDATE_RET(key != NULL);
     PK_VALIDATE_RET(buf != NULL || size == 0);
 
-    if ((ret = mbedtls_pk_write_key_der(key, output_buf, sizeof(output_buf))) < 0) {
-        return ret;
+    if ((ret = mbedtls_pk_write_key_der(key, output_buf, PRV_DER_MAX_BYTES)) < 0) {
+        goto cleanup;
     }
 
 #if defined(MBEDTLS_RSA_C)
@@ -617,15 +628,22 @@
         end = PEM_END_PRIVATE_KEY_EC;
     } else
 #endif
-    return MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE;
-
-    if ((ret = mbedtls_pem_write_buffer(begin, end,
-                                        output_buf + sizeof(output_buf) - ret,
-                                        ret, buf, size, &olen)) != 0) {
-        return ret;
+    {
+        ret = MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE;
+        goto cleanup;
     }
 
-    return 0;
+    if ((ret = mbedtls_pem_write_buffer(begin, end,
+                                        output_buf + PRV_DER_MAX_BYTES - ret,
+                                        ret, buf, size, &olen)) != 0) {
+        goto cleanup;
+    }
+
+    ret = 0;
+cleanup:
+    mbedtls_platform_zeroize(output_buf, PRV_DER_MAX_BYTES);
+    mbedtls_free(output_buf);
+    return ret;
 }
 #endif /* MBEDTLS_PEM_WRITE_C */