malloc: Improved double free detection in the tcache

malloc: Improved double free detection in the tcache

The previous double free detection did not account for an attacker to
use a terminating null byte overflowing from the previous
chunk to change the size of a memory chunk is being sorted into.
So that the check in 'tcache_double_free_verify' would pass
even though it is a double free.

Solution:
Let 'tcache_double_free_verify' iterate over all tcache entries to
detect double frees.

This patch only protects from buffer overflows by one byte.
But I would argue that off by one errors are the most common
errors to be made.

Alternatives Considered:
  Store the size of a memory chunk in big endian and thus
  the chunk size would not get overwritten because entries in the
  tcache are not that big.

  Move the tcache_key before the actual memory chunk so that it
  does not have to be checked at all, this would work better in general
  but also it would increase the memory usage.

Signed-off-by: David Lau  <david.lau@fau.de>
Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>