On Thu, Apr 4, 2013 at 1:12 PM, H. Peter Anvin <h...@zytor.com> wrote: > On 04/04/2013 01:07 PM, Kees Cook wrote: >> However, the benefits of >> this feature in certain environments exceed the perceived weaknesses[2]. > > Could you clarify?
I would summarize the discussion of KASLR weaknesses into to two general observations: 1- it depends on address location secrecy and leaks are common/easy. 2- it has low entropy so attack success rates may be high. For "1", as Julien mentions, remote attacks and attacks from a significantly contained process (via seccomp-bpf) minimizes the leak exposure. For local attacks, cache timing attacks and other things also exist, but the ASLR can be improved to defend against that too. So, KASLR is useful on systems that are virtualization hosts, providing remote services, or running locally confined processes. For "2", I think that the comparison to userspace ASLR entropy isn't as direct. For userspace, most systems don't tend to have any kind of watchdog on segfaulting processes, so a remote attacker could just keep trying an attack until they got lucky, in which case low entropy is a serious problem. In the case of KASLR, a single attack failure means the system goes down, which makes mounting an attack much more difficult. I think 8 bits is fine to start with, and I think start with a base offset ASLR is a good first step. We can improve things in the future. -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
