Catch AES failure in mbedtls_ctr_drbg_random

The functions mbedtls_ctr_drbg_random() and
mbedtls_ctr_drbg_random_with_add() could return 0 if an AES function
failed. This could only happen with alternative AES
implementations (the built-in implementation of the AES functions
involved never fail), typically due to a failure in a hardware
accelerator.

Bug reported and fix proposed by Johan Uppman Bruce and Christoffer
Lauri, Sectra.
diff --git a/ChangeLog b/ChangeLog
index 8a9e6d6..edd89f6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -79,6 +79,14 @@
      Issue reported by Tuba Yavuz, Farhaan Fowze, Ken (Yihang) Bai,
      Grant Hernandez, and Kevin Butler (University of Florida) and
      Dave Tian (Purdue University).
+   * Fix side channel vulnerability in ECDSA key generation. Obtaining precise
+     timings on the comparison in the key generation enabled the attacker to
+     learn leading bits of the ephemeral key used during ECDSA signatures and to
+     recover the private key. Reported by Jeremy Dubeuf.
+   * Catch failure of AES functions in mbedtls_ctr_drbg_random(). Uncaught
+     failures could happen with alternative implementations of AES. Bug
+     reported and fix proposed by Johan Uppman Bruce and Christoffer Lauri,
+     Sectra.
 
 Bugfix
    * Remove redundant line for getting the bitlen of a bignum, since the variable
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 4e47058..1c71288 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -512,7 +512,7 @@
 exit:
     mbedtls_platform_zeroize( add_input, sizeof( add_input ) );
     mbedtls_platform_zeroize( tmp, sizeof( tmp ) );
-    return( 0 );
+    return( ret );
 }
 
 int mbedtls_ctr_drbg_random( void *p_rng, unsigned char *output, size_t output_len )