Date: Wed, 25 Nov 2015 09:31:05 -0800 From: "H. Peter Anvin" <hpa@...or.com> To: Mathias Krause <minipli@...glemail.com>, kernel-hardening@...ts.openwall.com Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Kees Cook <keescook@...omium.org>, Andy Lutomirski <luto@...capital.net>, Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>, Michael Ellerman <mpe@...erman.id.au>, linux-arch@...r.kernel.org, PaX Team <pageexec@...email.hu>, Emese Revfy <re.emese@...il.com> Subject: Re: [PATCH 0/2] introduce post-init read-only memory On 11/25/15 01:13, Mathias Krause wrote: > > While having that annotation makes perfect sense, not only from a > security perspective but also from a micro-optimization point of view > (much like the already existing __read_mostly annotation), it has its > drawbacks. Violating the "r/o after init" rule by writing to such > annotated variables from non-init code goes unnoticed as far as it > concerns the toolchain. Neither the compiler nor the linker will flag > that incorrect use. It'll just trap at runtime and that's bad. > > I myself had some educating experience seeing my machine triple fault > when resuming from a S3 sleep. The root cause was a variable that was > annotated __read_only but that was (unnecessarily) modified during CPU > bring-up phase. Debugging that kind of problems is sort of a PITA, you > could imagine. > > So, prior extending the usage of the __read_only annotation some > toolchain support is needed. Maybe a gcc plugin that'll warn/error on > code that writes to such a variable but is not __init itself. The > initify and checker plugins from the PaX patch might be worth to look > at for that purpose, as they're doing similar things already. Adding > such a check to sparse might be worth it, too. > A modpost check probably won't work as it's unable to tell if it's a > legitimate access (r/o) or a violation (/w access). So the gcc plugin > is the way to go, IMHO. > We should not wait for compile-time support, that doesn't make any sense. What would be useful would be a way to override this on the command line -- that way, if disabling RO or RO-after-init memory makes something work, we have an instant diagnosis. -hpa
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