Date: Thu, 10 Nov 2016 11:11:34 -0800 From: Kees Cook <keescook@...omium.org> To: Juerg Haefliger <juerg.haefliger@....com> Cc: LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, linux-x86_64@...r.kernel.org, vpk@...columbia.edu Subject: Re: [RFC PATCH v3 1/2] Add support for eXclusive Page Frame Ownership (XPFO) On Fri, Nov 4, 2016 at 7:45 AM, Juerg Haefliger <juerg.haefliger@....com> wrote: > This patch adds support for XPFO which protects against 'ret2dir' kernel > attacks. The basic idea is to enforce exclusive ownership of page frames > by either the kernel or userspace, unless explicitly requested by the > kernel. Whenever a page destined for userspace is allocated, it is > unmapped from physmap (the kernel's page table). When such a page is > reclaimed from userspace, it is mapped back to physmap. > > Additional fields in the page_ext struct are used for XPFO housekeeping. > Specifically two flags to distinguish user vs. kernel pages and to tag > unmapped pages and a reference counter to balance kmap/kunmap operations > and a lock to serialize access to the XPFO fields. Thanks for keeping on this! I'd really like to see it land and then get more architectures to support it. > Known issues/limitations: > - Only supports x86-64 (for now) > - Only supports 4k pages (for now) > - There are most likely some legitimate uses cases where the kernel needs > to access userspace which need to be made XPFO-aware > - Performance penalty In the Kconfig you say "slight", but I'm curious what kinds of benchmarks you've done and if there's a more specific cost we can declare, just to give people more of an idea what the hit looks like? (What workloads would trigger a lot of XPFO unmapping, for example?) Thanks! -Kees -- Kees Cook Nexus Security
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