bootutil: Fix AES and SHA-256 contexts not zeroized with mbedTLS

For some reason, the calls to mbedtls_aes_free, mbedtls_nist_kw_free and
mbedtls_sha256_free_drop were commented out which means the AES and
SHA-256 contexts were not properly de-initialized after usage when
mbedTLS is used. In the case of AES-KW it seems that might lead to a
memory leak depending on the mbedTLS configuration, but in any case and
independently of the mbedTLS configuration, this leads to the contexts
not be zeroized after usage.

Not zeroizing a context means it stays in RAM an undefined amount of
time, which might enable an attacker to access it and to dump the
sensitive data it contains.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>

boot/bootutil/include/bootutil/crypto/aes_ctr.h

@@ -53,9 +53,7 @@ static inline void bootutil_aes_ctr_init(bootutil_aes_ctr_context *ctx)
 
 static inline void bootutil_aes_ctr_drop(bootutil_aes_ctr_context *ctx)
 {
-    /* XXX: config defines MBEDTLS_PLATFORM_NO_STD_FUNCTIONS so no need to free */
-    /* (void)mbedtls_aes_free(ctx); */
-    (void)ctx;
+    mbedtls_aes_free(ctx);
 }
 
 static inline int bootutil_aes_ctr_set_key(bootutil_aes_ctr_context *ctx, const uint8_t *k)

boot/bootutil/include/bootutil/crypto/aes_kw.h

@@ -45,9 +45,7 @@ static inline void bootutil_aes_kw_init(bootutil_aes_kw_context *ctx)
 
 static inline void bootutil_aes_kw_drop(bootutil_aes_kw_context *ctx)
 {
-    /* XXX: config defines MBEDTLS_PLATFORM_NO_STD_FUNCTIONS so no need to free */
-    /* (void)mbedtls_aes_free(ctx); */
-    (void)ctx;
+    mbedtls_nist_kw_free(ctx);
 }
 
 static inline int bootutil_aes_kw_set_unwrap_key(bootutil_aes_kw_context *ctx, const uint8_t *k, uint32_t klen)

boot/bootutil/include/bootutil/crypto/sha.h

@@ -126,9 +126,7 @@ static inline int bootutil_sha_init(bootutil_sha_context *ctx)
 
 static inline int bootutil_sha_drop(bootutil_sha_context *ctx)
 {
-    /* XXX: config defines MBEDTLS_PLATFORM_NO_STD_FUNCTIONS so no need to free */
-    /* (void)mbedtls_sha256_free(ctx); */
-    (void)ctx;
+    mbedtls_sha256_free(ctx);
     return 0;
 }