random() allocates values in the range [0, RAND_MAX], with RAND_MAX usually
equaling INT_MAX = 2^31-1. Previously, values between 0 and 31 were added
directly to that offset before applying`% CACHE_SIZE` to get an index into
the cache array. If the random value was very high, this resulted in an
integer overflow and a negative index value and, therefore, an out-of-bounds
access of the array and in turn dereferencing invalid pointers when trying
to acquire the read lock. This most likely results in a segmentation fault.
Fixes: 764e8b2211 ("reimplemented certificate cache")
Fixes: CVE-2021-41991
Previously we got no reference to the cached issuer certificate
before releasing the lock of the cache line, this allowed other
threads, or even the same thread if it replaces a cache line, to
destroy that issuer certificate in cache() (or flush()) before
get_ref() for the issuer certificate is finally called.
Andreas Steffen