Stack overflow in nscd due to unbounded alloca use

The nscd service in the GNU C Library 2.3.4 onwards may crash due to a
stack overflow when a malicious DNS server returns too large a response
for a DNS query, resulting in degraded DNS resolution for the system.

Exploitation of this bug needs a system that has nscd enabled and using
an untrusted DNS server for name resolution, with the compromised DNS
server being capable of processing records large enough to result in a
stack overflow in an nscd thread stack.  During experimentation, bind 9
was unable to handle large records, but that could change in future or
with a different name server.  In typical installations, nscd is
executed in an isolated context as its own user without a shell, due to
which any compromise of that service is isolated.

There is a remote possibility of nscd cache corruption if an attacker
manages to get the stack pointer into a desired point in the heap,
potentially resulting in other caches in nscd being overwritten with
corrupt data through the stack overflow, until the buggy code path
eventually results in a crash.

Finally, a crash in nscd may result in performance degradation when
resolving names, but it does not result in a denial of service.