improve heap canary generation

[thestinger]

The canaries are created with secret_key ^ canary_address so an attacker with a read overflow could use them to leak information about addresses. Ideally, they would be implemented as mac(secret_key, canary_address) but it needs to be fast. Using secret_key ^ hash(canary_address) with the existing hash function would be less bad but it wouldn't provide clear security properties.

There's also the option of generating a canary for each chunk_info struct with the same one reused for all chunks within that page. That's a pretty good compromise between a global value and one that may leak pointer data.

[thestinger]

The canaries are now generated with secret_key ^ hash(canary_address) so it's not quite as bad. This still needs improvement.

[thestinger]

The canaries are now generated with secret_key ^ hash(canary_address) so it's not quite as bad. This still needs improvement.

[thestinger]

Need to revisit this based on upstream changes.

[thestinger]

Need to revisit this based on upstream changes.