Date: Wed, 14 Feb 2018 11:29:21 -0800 From: Kees Cook <keescook@...omium.org> To: Laura Abbott <labbott@...hat.com> Cc: Jann Horn <jannh@...gle.com>, Igor Stoppa <igor.stoppa@...wei.com>, Boris Lukashev <blukashev@...pervictus.com>, Christopher Lameter <cl@...ux.com>, Matthew Wilcox <willy@...radead.org>, Jerome Glisse <jglisse@...hat.com>, Michal Hocko <mhocko@...nel.org>, Christoph Hellwig <hch@...radead.org>, linux-security-module <linux-security-module@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>, kernel list <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-arm-kernel <linux-arm-kernel@...ts.infradead.org> Subject: Re: arm64 physmap (was Re: [PATCH 4/6] Protectable Memory) On Wed, Feb 14, 2018 at 11:06 AM, Laura Abbott <labbott@...hat.com> wrote: > On 02/13/2018 01:43 PM, Kees Cook wrote: >> >> On Tue, Feb 13, 2018 at 8:09 AM, Laura Abbott <labbott@...hat.com> wrote: >>> >>> No, arm64 doesn't fixup the aliases, mostly because arm64 uses larger >>> page sizes which can't be broken down at runtime. CONFIG_PAGE_POISONING >>> does use 4K pages which could be adjusted at runtime. So yes, you are >>> right we would have physmap exposure on arm64 as well. >> >> >> Errr, so that means even modules and kernel code are writable via the >> arm64 physmap? That seems extraordinarily bad. :( >> >> -Kees >> > > (adding linux-arm-kernel and changing the subject) > > Kernel code should be fine, if it isn't that is a bug that should be > fixed. Modules yes are not fully protected. The conclusion from past I think that's a pretty serious problem: we can't have aliases with mismatched permissions; this degrades a deterministic protection (read-only) to a probabilistic protection (knowing where the alias of a target is mapped). Having an attack be "needs some info leaks" instead of "need execution control to change perms" is a much lower bar, IMO. > experience has been that we cannot safely break down larger page sizes > at runtime like x86 does. We could theoretically > add support for fixing up the alias if PAGE_POISONING is enabled but > I don't know who would actually use that in production. Performance > is very poor at that point. Why does using finer granularity on the physmap degrade performance? I assume TLB pressure, but what is heavily using that area? (I must not be understanding what physmap actually gets used for -- I thought it was just a convenience to have a 1:1 virt/phys map for some lookups?) -Kees -- Kees Cook Pixel Security
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