Re: [PATCH 4/4] dma-debug: Allow poisoning nonzero allocations

From: Robin Murphy
Date: Fri Sep 25 2015 - 13:36:03 EST

Hi Russell,

On 25/09/15 13:44, Russell King - ARM Linux wrote:
On Fri, Sep 25, 2015 at 01:15:46PM +0100, Robin Murphy wrote:
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

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.

[ I still don't see that as an excuse for callers not to be fixed, though - anyone allocating something that may be exposed to userspace has a responsibility to initialise it appropriately. After all, the DMA API is just one source, what do we do if such a careless subsystem got some uninitialised pages of leftover sensitive data from, say, alloc_pages() instead? ]

That's a bit of a separate issue though. If a driver itself _needs_ a zeroed buffer but doesn't specifically request one, or doesn't get one even if it did, then that's just a regular bug, and it's what this patch is intended to help weed out. We've no need for a special poison value for data protection in the general case; zero is just fine for that.

diff --git a/lib/dma-debug.c b/lib/dma-debug.c
index 908fb35..40514ed 100644
--- a/lib/dma-debug.c
+++ b/lib/dma-debug.c
@@ -30,6 +30,7 @@
#include <linux/sched.h>
#include <linux/ctype.h>
#include <linux/list.h>
+#include <linux/poison.h>
#include <linux/slab.h>

#include <asm/sections.h>
@@ -1447,7 +1448,7 @@ void debug_dma_unmap_sg(struct device *dev, struct scatterlist *sglist,

void debug_dma_alloc_coherent(struct device *dev, size_t size,
- dma_addr_t dma_addr, void *virt)
+ dma_addr_t dma_addr, void *virt, gfp_t flags)
struct dma_debug_entry *entry;

@@ -1457,6 +1458,9 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
if (unlikely(virt == NULL))

+ 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?

If I was feeling particularly antagonistic, I'd also point out that as discussed above you've already taken the hit of a memset(0) and cache flush that you _didn't_ ask for, and there was no warning on that ;)

The intent is a specific troubleshooting tool - realistically it's probably only usable at all when restricting DMA debug to a per-driver basis. My hunch is that nobody's too fussed about the performance of a driver that doesn't work properly, especially once they've reached the point of dumping buffers in an attempt to figure out why, when seeing the presence or not of uniform poison values could be helpful.

(Of course, sometimes you end up debugging the allocator itself - see commit 7132813c3845 - which was one of the motivating factors for this patch).


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at