tree 03990fbc384cb7fc86c9a2057fe8c19a1d424fe7
parent 762475e1f957936144e48c51a6befdef3f5fe1db
author Willy Tarreau <w@1wt.eu> 1547231905 +0100
committer Willy Tarreau <w@1wt.eu> 1547458291 +0100
encoding latin1

BUG/MAJOR: cache: fix confusion between zero and uninitialized cache key

The cache uses the first 32 bits of the uri's hash as the key to reference
the object in the cache. It makes a special case of the value zero to mean
that the object is not in the cache anymore. The problem is that when an
object hashes as zero, it's still inserted but the eb32_delete() call is
skipped, resulting in the object still being chained in the memory area
while the block has been reclaimed and used for something else. Then when
objects which were chained below it (techically any object since zero is
at the root) are deleted, the walk through the upper object may encounter
corrupted values where valid pointers were expected.

But while this should only happen statically once on 4 billion, the problem
gets worse when the cache-use conditions don't match the cache-store ones,
because cache-store runs with an uninitialized key, which can create objects
that will never be found by the lookup code, or worse, entries with a zero
key preventing eviction of the tree node and resulting in a crash. It's easy
to accidently end up on such a config because the request rules generally
can't be used to decide on the response :

  http-request  cache-use cache   if { path_beg /images }
  http-response cache-store cache

In this test, mixing traffic with /images/$RANDOM and /foo/$RANDOM will
result in random keys being inserted, some of them possibly being zero,
and crashes will quickly happen.

The fix consists in 1) always initializing the transaction's cache_hash
to zero, and 2) never storing a response for which the hash has not been
calculated, as indicated by the value zero.

It is worth noting that objects hashing as value zero will never be cached,
but given that there's only one chance among 4 billion that this happens,
this is totally harmless.

This fix must be backported to 1.9 and 1.8.