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Date: Thu, 11 Jan 2018 15:21:09 -0800
From: Kees Cook <>
To: Russell King - ARM Linux <>
Cc: LKML <>, Ingo Molnar <>, 
	Christian Borntraeger <>, "Peter Zijlstra (Intel)" <>,, 
	Linus Torvalds <>, David Windsor <>, 
	Alexander Viro <>, Andrew Morton <>, 
	Andy Lutomirski <>, Christoph Hellwig <>, Christoph Lameter <>, 
	"David S. Miller" <>, Laura Abbott <>, 
	Mark Rutland <>, "Martin K. Petersen" <>, 
	Paolo Bonzini <>, Christoffer Dall <>, 
	Dave Kleikamp <>, Jan Kara <>, 
	Luis de Bethencourt <>, Marc Zyngier <>, Rik van Riel <>, 
	Matthew Garrett <>, 
	"" <>, linux-arch <>, 
	Network Development <>, Linux-MM <>,
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
<> 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,

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



Kees Cook
Pixel Security

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