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

[Russell King - ARM Linux]

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
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.

> 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,
>  EXPORT_SYMBOL(debug_dma_unmap_sg);
>  
>  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))
>  		return;
>  
> +	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.

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