Date: Wed, 21 Feb 2018 14:37:11 -0800 From: Kees Cook <keescook@...omium.org> To: Igor Stoppa <igor.stoppa@...wei.com> Cc: Laura Abbott <labbott@...hat.com>, Boris Lukashev <blukashev@...pervictus.com>, Christopher Lameter <cl@...ux.com>, Matthew Wilcox <willy@...radead.org>, Jann Horn <jannh@...gle.com>, 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> Subject: Re: [PATCH 4/6] Protectable Memory On Tue, Feb 20, 2018 at 9:16 AM, Igor Stoppa <igor.stoppa@...wei.com> wrote: > > > On 13/02/18 20:10, Laura Abbott wrote: >> On 02/13/2018 07:20 AM, Igor Stoppa wrote: >>> Why alterations of page properties are not considered a risk and the physmap is? >>> And how would it be easier (i suppose) to attack the latter? >> >> Alterations are certainly a risk but with the physmap the >> mapping is already there. Find the address and you have >> access vs. needing to actually modify the properties >> then do the access. I could also be complete off base >> on my threat model here so please correct me if I'm >> wrong. > > It's difficult for me to comment on this without knowing *how* the > attack would be performed, in your model. > > Ex: my expectation is that the attacked has R/W access to kernel data > and has knowledge of the location of static variables. > > This is not just a guess, but a real-life scenario, found in attacks > that, among other things, are capable of disabling SELinux, to proceed > toward gaining full root capability. > > At that point, I think that variables which are allocated dynamically, > in vmalloc address space, are harder to locate, because of the virtual > mapping and the randomness of the address chosen (this I have not > confirmed yet, but I suppose there is some randomness in picking the > address to assign to a certain allocation request to vmalloc, otherwise, > it could be added). Machine-to-machine runtime variation certainly affects the mapping location, but for early boot allocations, these become surprisingly deterministic, especially across similar hardware/memory layouts (both the virtmap and physmap locations). However, using CONFIG_RANDOMIZE_MEMORY makes it MUCH more difficult. (Note that RANDOMIZE_BASE on arm64 effectively includes RANDOMIZE_MEMORY, as it uses the entropy for multiple base offsets, including the physmap, IIRC.) >> I think your other summaries are good points though >> and should go in the cover letter. > > Ok, I'm just afraid it risks becoming a lengthy dissertation :-) It's rare to have anyone say "your commit log is too long". :) -Kees -- Kees Cook Pixel Security
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