From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f177.google.com (mail-ig0-f177.google.com [209.85.213.177]) by kanga.kvack.org (Postfix) with ESMTP id D67BA6B0009 for ; Mon, 25 Jan 2016 17:06:00 -0500 (EST) Received: by mail-ig0-f177.google.com with SMTP id mw1so40486274igb.1 for ; Mon, 25 Jan 2016 14:06:00 -0800 (PST) Received: from mail-io0-x22f.google.com (mail-io0-x22f.google.com. [2607:f8b0:4001:c06::22f]) by mx.google.com with ESMTPS id m3si1124995igj.63.2016.01.25.14.06.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 25 Jan 2016 14:06:00 -0800 (PST) Received: by mail-io0-x22f.google.com with SMTP id 1so166847890ion.1 for ; Mon, 25 Jan 2016 14:06:00 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <56A682B5.8000603@intel.com> References: <1453740953-18109-1-git-send-email-labbott@fedoraproject.org> <1453740953-18109-4-git-send-email-labbott@fedoraproject.org> <56A682B5.8000603@intel.com> Date: Mon, 25 Jan 2016 14:05:59 -0800 Message-ID: Subject: Re: [kernel-hardening] [RFC][PATCH 3/3] mm/page_poisoning.c: Allow for zero poisoning From: Kees Cook Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: "kernel-hardening@lists.openwall.com" , Andrew Morton , "Kirill A. Shutemov" , Vlastimil Babka , Michal Hocko , Laura Abbott , Linux-MM , LKML On Mon, Jan 25, 2016 at 12:16 PM, Dave Hansen wrote: > Thanks for doing this! It all looks pretty straightforward. > > On 01/25/2016 08:55 AM, Laura Abbott wrote: >> By default, page poisoning uses a poison value (0xaa) on free. If this >> is changed to 0, the page is not only sanitized but zeroing on alloc >> with __GFP_ZERO can be skipped as well. The tradeoff is that detecting >> corruption from the poisoning is harder to detect. This feature also >> cannot be used with hibernation since pages are not guaranteed to be >> zeroed after hibernation. > > Ugh, that's a good point about hibernation. I'm not sure how widely it > gets used but it does look pretty widely enabled in distribution kernels. > > Is this something that's fixable? It seems like we could have the > hibernation code run through and zero all the free lists. Or, we could > just disable the optimization at runtime when a hibernation is done. We can also make hibernation run-time disabled when poisoning is used (similar to how kASLR disables it). > Not that we _have_ to do any of this now, but if a runtime knob (like a > sysctl) could be fun too. I would be nice for folks to turn it on and > off if they wanted the added security of "real" poisoning vs. the > potential performance boost from this optimization. > >> +static inline bool should_zero(void) >> +{ >> + return !IS_ENABLED(CONFIG_PAGE_POISONING_ZERO) || >> + !page_poisoning_enabled(); >> +} > > I wonder if calling this "free_pages_prezeroed()" would make things a > bit more clear when we use it in prep_new_page(). > >> static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags, >> int alloc_flags) >> { >> @@ -1401,7 +1407,7 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags, >> kernel_map_pages(page, 1 << order, 1); >> kasan_alloc_pages(page, order); >> >> - if (gfp_flags & __GFP_ZERO) >> + if (should_zero() && gfp_flags & __GFP_ZERO) >> for (i = 0; i < (1 << order); i++) >> clear_highpage(page + i); > > It's probably also worth pointing out that this can be a really nice > feature to have in virtual machines where memory is being deduplicated. > As it stands now, the free lists end up with gunk in them and tend not > to be easy to deduplicate. This patch would fix that. Oh, good point! -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org