From 213aec86885befa99a2db3bdd2091438a9013706 Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@normalesup.org>
Date: Thu, 23 Mar 2017 14:37:37 +0100
Subject: [PATCH] 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     |  7 +++++++
 library/rsa.c | 42 ++++++++++++++++++++++++++++++++++--------
 2 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index b46c72879..49b02f01a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 1.3.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 1.3.19 branch released 2017-03-08
 
 Security
diff --git a/library/rsa.c b/library/rsa.c
index 79726c1b5..09477cbc4 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -56,6 +56,11 @@
 #define polarssl_free   free
 #endif
 
+/* Implementation that should never be optimized out by the compiler */
+static void polarssl_zeroize( void *v, size_t n ) {
+    volatile unsigned char *p = (unsigned char*)v; while( n-- ) *p++ = 0;
+}
+
 /*
  * Initialize an RSA context
  */
@@ -720,7 +725,7 @@ int rsa_rsaes_oaep_decrypt( rsa_context *ctx,
           : rsa_private( ctx, f_rng, p_rng, input, buf );
 
     if( ret != 0 )
-        return( ret );
+        goto cleanup;
 
     /*
      * Unmask data and generate lHash
@@ -785,15 +790,26 @@ int rsa_rsaes_oaep_decrypt( rsa_context *ctx,
      * the different error conditions.
      */
     if( bad != 0 )
-        return( POLARSSL_ERR_RSA_INVALID_PADDING );
+    {
+        ret = POLARSSL_ERR_RSA_INVALID_PADDING;
+        goto cleanup;
+    }
 
     if( ilen - ( p - buf ) > output_max_len )
-        return( POLARSSL_ERR_RSA_OUTPUT_TOO_LARGE );
+    {
+        ret = POLARSSL_ERR_RSA_OUTPUT_TOO_LARGE;
+        goto cleanup;
+    }
 
     *olen = ilen - (p - buf);
     memcpy( output, p, *olen );
+    ret = 0;
 
-    return( 0 );
+cleanup:
+    polarssl_zeroize( buf, sizeof( buf ) );
+    polarssl_zeroize( lhash, sizeof( lhash ) );
+
+    return( ret );
 }
 #endif /* POLARSSL_PKCS1_V21 */
 
@@ -827,7 +843,7 @@ int rsa_rsaes_pkcs1_v15_decrypt( rsa_context *ctx,
           : rsa_private( ctx, f_rng, p_rng, input, buf );
 
     if( ret != 0 )
-        return( ret );
+        goto cleanup;
 
     p = buf;
     bad = 0;
@@ -872,15 +888,25 @@ int rsa_rsaes_pkcs1_v15_decrypt( rsa_context *ctx,
     bad |= ( pad_count < 8 );
 
     if( bad )
-        return( POLARSSL_ERR_RSA_INVALID_PADDING );
+    {
+        ret = POLARSSL_ERR_RSA_INVALID_PADDING;
+        goto cleanup;
+    }
 
     if( ilen - ( p - buf ) > output_max_len )
-        return( POLARSSL_ERR_RSA_OUTPUT_TOO_LARGE );
+    {
+        ret = POLARSSL_ERR_RSA_OUTPUT_TOO_LARGE;
+        goto cleanup;
+    }
 
     *olen = ilen - (p - buf);
     memcpy( output, p, *olen );
+    ret = 0;
 
-    return( 0 );
+cleanup:
+    polarssl_zeroize( buf, sizeof( buf ) );
+
+    return( ret );
 }
 #endif /* POLARSSL_PKCS1_V15 */