Date: Sun, 11 Mar 2012 10:56:58 +0100 From: Ingo Molnar <mingo@...e.hu> To: Solar Designer <solar@...nwall.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Djalal Harouni <tixxdz@...ndz.org>, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com, Andrew Morton <akpm@...ux-foundation.org>, Al Viro <viro@...iv.linux.org.uk>, Alexey Dobriyan <adobriyan@...il.com>, "Eric W. Biederman" <ebiederm@...ssion.com>, Vasiliy Kulikov <segoon@...nwall.com>, Kees Cook <keescook@...omium.org>, WANG Cong <xiyou.wangcong@...il.com>, James Morris <james.l.morris@...cle.com>, Oleg Nesterov <oleg@...hat.com>, linux-security-module@...r.kernel.org, linux-fsdevel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <gregkh@...uxfoundation.org>, Stephen Wilson <wilsons@...rt.ca>, "Jason A. Donenfeld" <Jason@...c4.com> Subject: Re: [PATCH 1/9] exec: add a global execve counter * Solar Designer <solar@...nwall.com> wrote: > > Actually, using a pure counter is horrible, because even if > > it takes four days to wrap, it *will* wrap, and the attacker > > can just count his own execve's. > > Four days (for a 32-bit counter) is just not enough, so the > counter needs to be e.g. 64-bit as proposed. A 64-bit counter > won't wrap during lifetime of a system. A 64-bit counter is OK on 32-bit platforms as well as long as it's not *atomic*. Linus's scheme of using the CPU ID for the high bits would solve that particular problem IMO. Each CPU would have its own count set apart in a percpu area, accessible via __this_cpu_inc() or so. 16 bits for the CPU ID and 48 bits for the actual count should be enough for everyone! ;-) It wraps in about 700 years, with current CPU speeds and assuming that exec() will be relatively slow in the future as well. Some other kernel code might make use of such a fast, global generation count as well, so if this is reasonably abstracted out and named properly it would not just be a single-purpose security facility. DEFINE_GENCOUNT() or so? [the count itself would not be reused, of course.] > The CPUs' timestamp counters were not designed for security. > I would not be too surprised if some implementation of a CPU > architecture (maybe emulated, maybe under a hypervisor) has > such timestamp counter granularity that we may see the same > value across a second execve(). Not using the TSC would certainly make this logic simpler and faster - which is a big plus for any security measure. Thanks, Ingo
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