linux-kernel - RE: [PATCH v3 00/11] mm: Hardened usercopy

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Date:	Wed, 20 Jul 2016 09:52:25 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Kees Cook' <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Jan Kara <jack@...e.cz>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Will Deacon <will.deacon@....com>,
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	"sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
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	Christoph Lameter <cl@...ux.com>,
	"Andrea Arcangeli" <aarcange@...hat.com>,
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	"x86@...nel.org" <x86@...nel.org>,
	Russell King <linux@...linux.org.uk>,
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	<linux-arm-kernel@...ts.infradead.org>,
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	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: RE: [PATCH v3 00/11] mm: Hardened usercopy

From: Kees Cook
> Sent: 15 July 2016 22:44
> This is a start of the mainline port of PAX_USERCOPY[1]. 
...
> - if address range is in the current process stack, it must be within the
>   current stack frame (if such checking is possible) or at least entirely
>   within the current process's stack.
...

That description doesn't seem quite right to me.
I presume the check is:
  Within the current process's stack and not crossing the ends of the
  current stack frame.

The 'current' stack frame is likely to be that of copy_to/from_user().
Even if you use the stack of the caller, any problematic buffers
are likely to have been passed in from a calling function.
So unless you are going to walk the stack (good luck on that)
I'm not sure checking the stack frames is worth it.

I'd also guess that a lot of copies are from the middle of structures
so cannot fail the tests you are adding.

	David