linux-kernel - Re: [PATCH] keys/encrypted: Fix two crypto-on-the-stack bugs

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Message-ID: <CALCETrWsTKq0NOpwiJtB50OU7w99-m82NhPG_Uxs2Fqbpz0LLA@mail.gmail.com>
Date:   Tue, 13 Dec 2016 08:40:00 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     David Laight <David.Laight@...lab.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        Eric Biggers <ebiggers3@...il.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Stephan Mueller <smueller@...onox.de>
Subject: Re: [PATCH] keys/encrypted: Fix two crypto-on-the-stack bugs

[add some people who might know]

On Tue, Dec 13, 2016 at 4:20 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Andy Lutomirski
>> Sent: 12 December 2016 20:53
>> The driver put a constant buffer of all zeros on the stack and
>> pointed a scatterlist entry at it in two places.  This doesn't work
>> with virtual stacks.  Use a static 16-byte buffer of zeros instead.
> ...
>
> I didn't think you could dma from static data either.

According to lib/dma-debug.c, you can't dma to or from kernel text or
rodata, but you can dma to or from kernel bss or data.  So
empty_zero_page should be okay, because it's not rodata right now.

But I think this is rather silly.  Joerg, Linus, etc: would it be okay
to change lib/dma-debug.c to allow DMA *from* rodata?  After all,
rodata is ordinary memory, is backed by struct page, etc.  And DMA
from the zero page had better be okay because I think it happens if
you mmap some zeros, don't write to them, and then direct I/O them to
a device.  Then I could also move empty_zero_page to rodata.

--Andy