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.
diff --git a/lib/dma-debug.c b/lib/dma-debug.c
index 908fb35..40514ed 100644
@@ -30,6 +30,7 @@
@@ -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))
+ 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.