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

@@ -6,6 +6,12 @@ Bugfix
    * Fixed multiple buffer overreads in mbedtls_pem_read_buffer() when parsing
      the input string in PEM format to extract the different components. Found
      by Eyal Itkin.
+   * 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 2.1.6 branch released 2016-10-17
 

library/cipher.c

@@ -300,9 +300,9 @@ int mbedtls_cipher_update( mbedtls_cipher_context_t *ctx, const unsigned char *i
          * If there is not enough data for a full block, cache it.
          */
         if( ( ctx->operation == MBEDTLS_DECRYPT &&
-                ilen + ctx->unprocessed_len <= mbedtls_cipher_get_block_size( ctx ) ) ||
+                ilen <= mbedtls_cipher_get_block_size( ctx ) - ctx->unprocessed_len ) ||
              ( ctx->operation == MBEDTLS_ENCRYPT &&
-                ilen + ctx->unprocessed_len < mbedtls_cipher_get_block_size( ctx ) ) )
+                ilen < mbedtls_cipher_get_block_size( ctx ) - ctx->unprocessed_len ) )
         {
             memcpy( &( ctx->unprocessed_data[ctx->unprocessed_len] ), input,
                     ilen );

library/ctr_drbg.c

@@ -290,7 +290,8 @@ int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
     unsigned char seed[MBEDTLS_CTR_DRBG_MAX_SEED_INPUT];
     size_t seedlen = 0;
 
-    if( ctx->entropy_len + len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
+    if( ctx->entropy_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT ||
+        len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
         return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
 
     memset( seed, 0, MBEDTLS_CTR_DRBG_MAX_SEED_INPUT );

library/md2.c

@@ -158,7 +158,7 @@ void mbedtls_md2_update( mbedtls_md2_context *ctx, const unsigned char *input, s
 
     while( ilen > 0 )
     {
-        if( ctx->left + ilen > 16 )
+        if( ilen > 16 - ctx->left )
             fill = 16 - ctx->left;
         else
             fill = ilen;