Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752422AbaJYW0M (ORCPT ); Sat, 25 Oct 2014 18:26:12 -0400 Received: from mail-wi0-f170.google.com ([209.85.212.170]:44403 "EHLO mail-wi0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752396AbaJYW0I (ORCPT ); Sat, 25 Oct 2014 18:26:08 -0400 MIME-Version: 1.0 In-Reply-To: References: Date: Sun, 26 Oct 2014 00:26:07 +0200 Message-ID: Subject: Re: vmalloced stacks on x86_64? From: Richard Weinberger To: Andy Lutomirski Cc: "H. Peter Anvin" , X86 ML , "linux-kernel@vger.kernel.org" , Linus Torvalds Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Oct 25, 2014 at 2:22 AM, Andy Lutomirski wrote: > Is there any good reason not to use vmalloc for x86_64 stacks? > > The tricky bits I've thought of are: > > - On any context switch, we probably need to probe the new stack > before switching to it. That way, if it's going to fault due to an > out-of-sync pgd, we still have a stack available to handle the fault. > > - Any time we change cr3, we may need to check that the pgd > corresponding to rsp is there. If now, we need to sync it over. > > - For simplicity, we probably want all stack ptes to be present all > the time. This is fine; vmalloc already works that way. > > - If we overrun the stack, we double-fault. This should be easy to > detect: any double-fault where rsp is less than 20 bytes from the > bottom of the stack is a failure to deliver a non-IST exception due to > a stack overflow. The question is: what do we do if this happens? > We could just panic (guaranteed to work). We could also try to > recover by killing the offending task, but that might be a bit > challenging, since we're in IST context. We could do something truly > awful: increment RSP by a few hundred bytes, point RIP at do_exit, and > return from the double fault. > > Thoughts? This shouldn't be all that much code. FWIW, grsecurity has this already. Maybe we can reuse their GRKERNSEC_KSTACKOVERFLOW feature. It allocates the kernel stack using vmalloc() and installs guard pages. -- Thanks, //richard -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/