commit 8877ec23a30cce26dd50fb4727f6c212c989ba32
author Gilles Peskine <Gilles.Peskine@normalesup.org> Thu Mar 23 14:37:37 2017 +0100
committer Gilles Peskine <Gilles.Peskine@normalesup.org> Thu Mar 23 15:29:45 2017 +0100
tree 6f51e187a3ae51bf5f63f68f78e4600b5aca7d14
parent 63a48d10e94557cb1e226c37face768c5d149f76

RSA: wipe stack buffers

The RSA private key functions rsa_rsaes_pkcs1_v15_decrypt and
rsa_rsaes_oaep_decrypt put sensitive data (decryption results) on the
stack. Wipe it before returning.

Thanks to Laurent Simon for reporting this issue.

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index fe5ce65..534dd4f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.1.x branch released xxxx-xx-xx
+
+Security
+   * Wipe stack buffers in RSA private key operations
+     (rsa_rsaes_pkcs1_v15_decrypt(), rsa_rsaes_oaep_decrypt).
+     Found by Laurent Simon.
+
 = mbed TLS 2.1.7 branch released 2017-03-08
 
 Security

library/rsa.c

diff --git a/library/rsa.c b/library/rsa.c
index e831875..250f96f 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -55,6 +55,11 @@
 #define mbedtls_free   free
 #endif
 
+/* Implementation that should never be optimized out by the compiler */
+static void mbedtls_zeroize( void *v, size_t n ) {
+    volatile unsigned char *p = (unsigned char*)v; while( n-- ) *p++ = 0;
+}
+
 /*
  * Initialize an RSA context
  */
@@ -724,7 +729,7 @@
           : mbedtls_rsa_private( ctx, f_rng, p_rng, input, buf );
 
     if( ret != 0 )
-        return( ret );
+        goto cleanup;
 
     /*
      * Unmask data and generate lHash
@@ -733,7 +738,7 @@
     if( ( ret = mbedtls_md_setup( &md_ctx, md_info, 0 ) ) != 0 )
     {
         mbedtls_md_free( &md_ctx );
-        return( ret );
+        goto cleanup;
     }
 
 
@@ -784,15 +789,26 @@
      * the different error conditions.
      */
     if( bad != 0 )
-        return( MBEDTLS_ERR_RSA_INVALID_PADDING );
+    {
+        ret = MBEDTLS_ERR_RSA_INVALID_PADDING;
+        goto cleanup;
+    }
 
     if( ilen - ( p - buf ) > output_max_len )
-        return( MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE );
+    {
+        ret = MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE;
+        goto cleanup;
+    }
 
     *olen = ilen - (p - buf);
     memcpy( output, p, *olen );
+    ret = 0;
 
-    return( 0 );
+cleanup:
+    mbedtls_zeroize( buf, sizeof( buf ) );
+    mbedtls_zeroize( lhash, sizeof( lhash ) );
+
+    return( ret );
 }
 #endif /* MBEDTLS_PKCS1_V21 */
 
@@ -826,7 +842,7 @@
           : mbedtls_rsa_private( ctx, f_rng, p_rng, input, buf );
 
     if( ret != 0 )
-        return( ret );
+        goto cleanup;
 
     p = buf;
     bad = 0;
@@ -871,15 +887,25 @@
     bad |= ( pad_count < 8 );
 
     if( bad )
-        return( MBEDTLS_ERR_RSA_INVALID_PADDING );
+    {
+        ret = MBEDTLS_ERR_RSA_INVALID_PADDING;
+        goto cleanup;
+    }
 
     if( ilen - ( p - buf ) > output_max_len )
-        return( MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE );
+    {
+        ret = MBEDTLS_ERR_RSA_OUTPUT_TOO_LARGE;
+        goto cleanup;
+    }
 
     *olen = ilen - (p - buf);
     memcpy( output, p, *olen );
+    ret = 0;
 
-    return( 0 );
+cleanup:
+    mbedtls_zeroize( buf, sizeof( buf ) );
+
+    return( ret );
 }
 #endif /* MBEDTLS_PKCS1_V15 */