Re: [PATCH 10/17] prmem: documentation

From: Igor Stoppa <igor.stoppa@huawei.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Igor Stoppa <igor.stoppa@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nadav Amit <nadav.amit@gmail.com>,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	Matthew Wilcox <willy@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	James Morris <jmorris@namei.org>,
	"Michal Hocko" <mhocko@kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	linux-integrity <linux-integrity@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Laura Abbott <labbott@redhat.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation
Date: Tue, 13 Nov 2018 21:01:43 +0200	[thread overview]
Message-ID: <12b6232f-ac3d-c74c-5f9e-d84905c39f0a@huawei.com> (raw)
In-Reply-To: <CALCETrVaPTbgycjzSB0fLUeGL=j1srUh-cL0qsKn6fw6UQgj7Q@mail.gmail.com>

On 13/11/2018 20:35, Andy Lutomirski wrote:
> On Tue, Nov 13, 2018 at 10:26 AM Igor Stoppa <igor.stoppa@huawei.com> wrote:

[...]

>> The high level API could be something like:
>>
>> wr_memcpy(void *src, void *dst, uint_t size)

[...]

> If you call a wr_memcpy() function with the signature you suggested,
> then you can overwrite any memory of this type.  Having a different
> mm_struct under the hood makes no difference.  As far as I'm
> concerned, for *dynamically allocated* rare-writable memory, you might
> as well allocate all the paging structures at the same time, so the
> mm_struct will always contain the mappings.  If there are serious bugs
> in wr_memcpy() that cause it to write to the wrong place, we have
> bigger problems.

Beside bugs, I'm also thinking about possible vulnerability.
It might be overthinking, though.

I do not think it's possible to really protect against control flow
attacks, unless there is some support from the HW and/or the compiler.

What is left, are data-based attacks. In this case, it would be an
attacker using one existing wr_ invocation with doctored parameters.

However, there is always the objection that it would be possible to come
up with a "writing kit" for plowing through the page tables and
unprotect anything that might be of value.

Ideally, that should be the only type of data-based attack left.

In practice, it might just be an excess of paranoia from my side.

> I can imagine that we'd want a *typed* wr_memcpy()-like API some day,
> but that can wait for v2.  And it still doesn't obviously need
> multiple mm_structs.

I left that out, for now, but yes, typing would play some role here.

[...]

> I think it's entirely reasonable for the API to internally break up
> very large memcpys.

The criteria for deciding if/how to break it down is not clear to me,
though. The single page was easy, but might be (probably is) too much.

--
igor