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 2.x.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 2.4.1 branch released 2016-12-13
 
 Changes

library/cipher.c

@@ -326,9 +326,9 @@ 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 <= block_size ) ||
+                ilen <= block_size - ctx->unprocessed_len ) ||
              ( ctx->operation == MBEDTLS_ENCRYPT &&
-                ilen + ctx->unprocessed_len < block_size ) )
+                ilen < block_size - 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;

tests/suites/test_suite_ctr_drbg.function

@@ -39,6 +39,11 @@ void ctr_drbg_special_behaviours( )
     TEST_ASSERT( mbedtls_ctr_drbg_reseed( &ctx, additional,
                         MBEDTLS_CTR_DRBG_MAX_SEED_INPUT + 1 ) ==
                         MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
+
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, ~0 );
+    TEST_ASSERT( mbedtls_ctr_drbg_reseed( &ctx, additional,
+                        MBEDTLS_CTR_DRBG_MAX_SEED_INPUT ) ==
+                        MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
 exit:
     mbedtls_ctr_drbg_free( &ctx );
 }