On Fri, 25 Sep 2015 18:35:39 +0100 Robin Murphy [off-list ref] wrote:

Hi Russell,

On 25/09/15 13:44, Russell King - ARM Linux wrote:

quoted

On Fri, Sep 25, 2015 at 01:15:46PM +0100, Robin Murphy wrote:

quoted

Since some dma_alloc_coherent implementations return a zeroed buffer
regardless of whether __GFP_ZERO is passed, there exist drivers which
are implicitly dependent on this and pass otherwise uninitialised
buffers to hardware. This can lead to subtle and awkward-to-debug issues
using those drivers on different platforms, where nonzero uninitialised
junk may for instance occasionally look like a valid command which
causes the hardware to start misbehaving. To help with debugging such
issues, add the option to make uninitialised buffers much more obvious.

The reason people started to do this is to stop a security leak in the
ALSA code: ALSA allocates the ring buffer with dma_alloc_coherent()
which used to grab pages and return them uninitialised.  These pages
could contain anything - including the contents of /etc/shadow, or
your bank details.

ALSA then lets userspace mmap() that memory, which means any user process
which has access to the sound devices can read data leaked from kernel
memory.

I think I did bring it up at the time I found it, and decided that the
safest thing to do was to always return an initialised buffer - short of
constantly auditing every dma_alloc_coherent() user which also mmap()s
the buffer into userspace, I couldn't convince myself that it was safe
to avoid initialising the buffer.

I don't know whether the original problem still exists in ALSA or not,
but I do know that there are dma_alloc_coherent() implementations out
there which do not initialise prior to returning memory.

Indeed, I think we've discussed this before, and I don't imagine we'll 
be changing the actual behaviour of the existing allocators any time soon.

If I'm understanding things correctly, some allocators zero the memory
by default and others do not.  And we have an unknown number of drivers
which are assuming that the memory is zeroed.

Correct?

If so, our options are

a) audit all callers, find the ones which expect zeroed memory but
   aren't passing __GFP_ZERO and fix them.

b) convert all allocators to zero the memory by default.

Obviously, a) is better.  How big a job is it?

This patch will help the process, if people use it.

quoted

quoted

+	if (IS_ENABLED(CONFIG_DMA_API_DEBUG_POISON) && !(flags & __GFP_ZERO))
+		memset(virt, DMA_ALLOC_POISON, size);
+

This is likely to be slow in the case of non-cached memory and large
allocations.  The config option should come with a warning.

It depends on DMA_API_DEBUG, which already has a stern performance 
warning, is additionally hidden behind EXPERT, and carries a slightly 
flippant yet largely truthful warning that actually using it could break 
pretty much every driver in your system; is that not enough?

It might be helpful to provide a runtime knob as well - having to
rebuild&reinstall just to enable/disable this feature is a bit painful.