linux-kernel - Re: [PATCH 34/38] arm: Implement thread_struct whitelist for hardened usercopy

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Date:   Thu, 11 Jan 2018 15:21:09 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        linux-arm-kernel@...ts.infradead.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Windsor <dave@...lcore.net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        "David S. Miller" <davem@...emloft.net>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        Jan Kara <jack@...e.cz>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Rik van Riel <riel@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 34/38] arm: Implement thread_struct whitelist for hardened usercopy

On Thu, Jan 11, 2018 at 2:24 AM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> On Wed, Jan 10, 2018 at 06:03:06PM -0800, Kees Cook wrote:
>> ARM does not carry FPU state in the thread structure, so it can declare
>> no usercopy whitelist at all.
>
> This comment seems to be misleading.  We have stored FP state in the
> thread structure for a long time - for example, VFP state is stored
> in thread->vfpstate.hard, so we _do_ have floating point state in
> the thread structure.
>
> What I think this commit message needs to describe is why we don't
> need a whitelist _despite_ having FP state in the thread structure.
>
> At the moment, the commit message is making me think that this patch
> is wrong and will introduce a regression.

Yeah, I will improve this comment; it's not clear enough. The places
where I see state copied to/from userspace are all either static sizes
or already use bounce buffers (or both). e.g.:

        err |= __copy_from_user(&hwstate->fpregs, &ufp->fpregs,
                                sizeof(hwstate->fpregs));

I will adjust the commit log and comment to more clearly describe the
lack of whitelisting due to all-static sized copies.

Thanks!

-Kees

-- 
Kees Cook
Pixel Security