linux-kernel - Re: [RFC PATCH v3] arm DMA: Fix allocation from CMA for coherent DMA

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Date:	Mon, 29 Jun 2015 11:46:04 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Lorenzo Nava <lorenx4@...il.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH v3] arm DMA: Fix allocation from CMA for coherent DMA

On Mon, Jun 29, 2015 at 11:37:37AM +0100, Catalin Marinas wrote:
> I forgot about the arm_dma_mmap fix here:
> 
> https://lkml.org/lkml/2015/5/7/512
> 
> So we either fix both cases by changing __get_dma_pgprot() or just go
> for Mike's and your patches as above. It's up to Russell.

I'd prefer the "least code" option. :)

> At some point, we could do with some more clean-up in the dma-mapping.c.
> For example, both __alloc_simple_buffer() and __alloc_from_contiguous()
> end up calling __dma_clear_buffer() even when not necessary (cacheable
> mapping). Not too bad though as this is only done when setting up the
> buffer.

The reason we always clear the buffer is that we can't be sure that a
driver will not map a buffer allocated by dma_alloc_coherent() into
userspace without it first being initialised.  There have been drivers
which do this (ALSA in particular.)  I haven't checked whether this
instance still does this, but it used to - and the problem is once
one instance exists, it gets copied...

If it isn't initialised, userspace gets access to whatever data was in
that page prior to the allocation, which could be something like your
root password, or bank details, some other sensitive data.

So, removing the clearing probably opens a rather large security hole.

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