Fix integer overflows in buffer bound checks

Fix potential integer overflows in the following functions:
  * mbedtls_md2_update() to be bypassed and cause
  * mbedtls_cipher_update()
  * mbedtls_ctr_drbg_reseed()
This overflows would mainly be exploitable in 32-bit systems and could
cause buffer bound checks to be bypassed.

ChangeLog

@@ -1,5 +1,15 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 1.3.x branch released xxxx-xx-xx
+
+Bugfix
+   * Fixed potential arithmetic overflow in mbedtls_ctr_drbg_reseed() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+   * Fixed potential arithmetic overflows in mbedtls_cipher_update() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+   * Fixed potential arithmetic overflow in mbedtls_md2_update() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+
 = mbed TLS 1.3.18 branch 2016-10-17
 
 Security

library/cipher.c

@@ -315,9 +315,9 @@ int cipher_update( cipher_context_t *ctx, const unsigned char *input,
          * If there is not enough data for a full block, cache it.
          */
         if( ( ctx->operation == POLARSSL_DECRYPT &&
-                ilen + ctx->unprocessed_len <= cipher_get_block_size( ctx ) ) ||
+                ilen <= cipher_get_block_size( ctx ) - ctx->unprocessed_len ) ||
              ( ctx->operation == POLARSSL_ENCRYPT &&
-                ilen + ctx->unprocessed_len < cipher_get_block_size( ctx ) ) )
+                ilen < cipher_get_block_size( ctx ) - ctx->unprocessed_len ) )
         {
             memcpy( &( ctx->unprocessed_data[ctx->unprocessed_len] ), input,
                     ilen );

library/ctr_drbg.c

@@ -277,7 +277,8 @@ int ctr_drbg_reseed( ctr_drbg_context *ctx,
     unsigned char seed[CTR_DRBG_MAX_SEED_INPUT];
     size_t seedlen = 0;
 
-    if( ctx->entropy_len + len > CTR_DRBG_MAX_SEED_INPUT )
+    if( ctx->entropy_len > CTR_DRBG_MAX_SEED_INPUT ||
+        len > CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
         return( POLARSSL_ERR_CTR_DRBG_INPUT_TOO_BIG );
 
     memset( seed, 0, CTR_DRBG_MAX_SEED_INPUT );

library/md2.c

@@ -155,7 +155,7 @@ void md2_update( md2_context *ctx, const unsigned char *input, size_t ilen )
 
     while( ilen > 0 )
     {
-        if( ctx->left + ilen > 16 )
+        if( ilen > 16 - ctx->left )
             fill = 16 - ctx->left;
         else
             fill = ilen;