linux-kernel - RE: [PATCH v3 02/11] mm: Hardened usercopy

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Message-ID: <87mvl5jgl1.fsf@concordia.ellerman.id.au>
Date:	Tue, 26 Jul 2016 12:09:14 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	David Laight <David.Laight@...LAB.COM>,
	'Josh Poimboeuf' <jpoimboe@...hat.com>,
	Kees Cook <keescook@...omium.org>
Cc:	"linux-ia64\@vger.kernel.org" <linux-ia64@...r.kernel.org>,
	"kernel-hardening\@lists.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Linux-MM <linux-mm@...ck.org>,
	sparclinux <sparclinux@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"x86\@kernel.org" <x86@...nel.org>,
	Russell King <linux@...linux.org.uk>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	PaX Team <pageexec@...email.hu>, Borislav Petkov <bp@...e.de>,
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	Brad Spengler <spender@...ecurity.net>,
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	"linuxppc-dev\@lists.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: RE: [PATCH v3 02/11] mm: Hardened usercopy

David Laight <David.Laight@...LAB.COM> writes:

> From: Josh Poimboeuf
>> Sent: 22 July 2016 18:46
>> >
>> > e.g. then if the pointer was in the thread_info, the second test would
>> > fail, triggering the protection.
>> 
>> FWIW, this won't work right on x86 after Andy's
>> CONFIG_THREAD_INFO_IN_TASK patches get merged.
>
> What ends up in the 'thread_info' area?

It depends on the arch.

> If it contains the fp save area then programs like gdb may end up requesting
> copy_in/out directly from that area.

On the arches I've seen thread_info doesn't usually contain register save areas,
but if it did then it would be up to the arch helper to allow that copy to go
through.

However given thread_info generally contains lots of low level flags that would
be a good target for an attacker, the best way to cope with ptrace wanting to
copy to/from it would be to use a temporary, and prohibit copying directly
to/from thread_info - IMHO.

cheers