Re: [Intel-gfx] [Linaro-mm-sig] [PATCH] drm-buf: Add debug option

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 15.01.21 um 14:02 schrieb Daniel Vetter:
> We have too many people abusing the struct page they can get at but
> really shouldn't in importers. Aside from that the backing page might
> simply not exist (for dynamic p2p mappings) looking at it and using it
> e.g. for mmap can also wreak the page handling of the exporter
> completely. Importers really must go through the proper interface like
> dma_buf_mmap for everything.
>
> I'm semi-tempted to enforce this for dynamic importers since those
> really have no excuse at all to break the rules.
>
> Unfortuantely we can't store the right pointers somewhere safe to make
> sure we oops on something recognizable, so best is to just wrangle
> them a bit by flipping all the bits. At least on x86 kernel addresses
> have all their high bits sets and the struct page array is fairly low
> in the kernel mapping, so flipping all the bits gives us a very high
> pointer in userspace and hence excellent chances for an invalid
> dereference.
>
> v2: Add a note to the @map_dma_buf hook that exporters shouldn't do
> fancy caching tricks, which would blow up with this address scrambling
> trick here (Chris)
>
> Enable by default when CONFIG_DMA_API_DEBUG is enabled.
>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: David Stevens <stevensd@chromium.org>
> Cc: linux-media@vger.kernel.org
> Cc: linaro-mm-sig@lists.linaro.org
> ---
>   drivers/dma-buf/Kconfig   |  8 +++++++
>   drivers/dma-buf/dma-buf.c | 49 +++++++++++++++++++++++++++++++++++----
>   include/linux/dma-buf.h   |  6 +++++
>   3 files changed, 59 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> index 4f8224a6ac95..4e16c71c24b7 100644
> --- a/drivers/dma-buf/Kconfig
> +++ b/drivers/dma-buf/Kconfig
> @@ -50,6 +50,14 @@ config DMABUF_MOVE_NOTIFY
>   	  This is marked experimental because we don't yet have a consistent
>   	  execution context and memory management between drivers.
>   
> +config DMABUF_DEBUG
> +	bool "DMA-BUF debug checks"
> +	default y if DMA_API_DEBUG
> +	help
> +	  This option enables additional checks for DMA-BUF importers and
> +	  exporters. Specifically it validates that importers do not peek at the
> +	  underlying struct page when they import a buffer.
> +
>   config DMABUF_SELFTESTS
>   	tristate "Selftests for the dma-buf interfaces"
>   	default n
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 1c9bd51db110..6e4725f7dfde 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -666,6 +666,30 @@ void dma_buf_put(struct dma_buf *dmabuf)
>   }
>   EXPORT_SYMBOL_GPL(dma_buf_put);
>   
> +static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
> +				       enum dma_data_direction direction)
> +{
> +	struct sg_table *sg_table;
> +
> +	sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
> +
> +#if CONFIG_DMABUF_DEBUG
> +	if (sg_table) {
> +		int i;
> +		struct scatterlist *sg;
> +
> +		/* To catch abuse of the underlying struct page by importers mix
> +		 * up the bits, but take care to preserve the low SG_ bits to
> +		 * not corrupt the sgt. The mixing is undone in __unmap_dma_buf
> +		 * before passing the sgt back to the exporter. */
> +		for_each_sgtable_sg(sg_table, sg, i)
> +			sg->page_link ^= ~0xffUL;
> +	}
> +#endif
> +
> +	return sg_table;
> +}
> +
>   /**
>    * dma_buf_dynamic_attach - Add the device to dma_buf's attachments list
>    * @dmabuf:		[in]	buffer to attach device to.
> @@ -737,7 +761,7 @@ dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
>   				goto err_unlock;
>   		}
>   
> -		sgt = dmabuf->ops->map_dma_buf(attach, DMA_BIDIRECTIONAL);
> +		sgt = __map_dma_buf(attach, DMA_BIDIRECTIONAL);
>   		if (!sgt)
>   			sgt = ERR_PTR(-ENOMEM);
>   		if (IS_ERR(sgt)) {
> @@ -784,6 +808,23 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>   }
>   EXPORT_SYMBOL_GPL(dma_buf_attach);
>   
> +static void __unmap_dma_buf(struct dma_buf_attachment *attach,
> +			    struct sg_table *sg_table,
> +			    enum dma_data_direction direction)
> +{
> +
> +#if CONFIG_DMABUF_DEBUG
> +	if (sg_table) {
> +		int i;
> +		struct scatterlist *sg;
> +
> +		for_each_sgtable_sg(sg_table, sg, i)
> +			sg->page_link ^= ~0xffUL;
> +	}
> +#endif

Instead of duplicating this I would rather structure the code so that we 
have a helper to mangle the sgt when necessary.

This can then be called from both the map() as well as the unmap() path.

Apart from that looks good to me,
Christian.

> +	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
> +}
> +
>   /**
>    * dma_buf_detach - Remove the given attachment from dmabuf's attachments list
>    * @dmabuf:	[in]	buffer to detach from.
> @@ -802,7 +843,7 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>   		if (dma_buf_is_dynamic(attach->dmabuf))
>   			dma_resv_lock(attach->dmabuf->resv, NULL);
>   
> -		dmabuf->ops->unmap_dma_buf(attach, attach->sgt, attach->dir);
> +		__unmap_dma_buf(attach, attach->sgt, attach->dir);
>   
>   		if (dma_buf_is_dynamic(attach->dmabuf)) {
>   			dma_buf_unpin(attach);
> @@ -924,7 +965,7 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
>   		}
>   	}
>   
> -	sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
> +	sg_table = __map_dma_buf(attach, direction);
>   	if (!sg_table)
>   		sg_table = ERR_PTR(-ENOMEM);
>   
> @@ -987,7 +1028,7 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
>   	if (dma_buf_is_dynamic(attach->dmabuf))
>   		dma_resv_assert_held(attach->dmabuf->resv);
>   
> -	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
> +	__unmap_dma_buf(attach, sg_table, direction);
>   
>   	if (dma_buf_is_dynamic(attach->dmabuf) &&
>   	    !IS_ENABLED(CONFIG_DMABUF_MOVE_NOTIFY))
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 628681bf6c99..efdc56b9d95f 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -154,6 +154,12 @@ struct dma_buf_ops {
>   	 * On failure, returns a negative error value wrapped into a pointer.
>   	 * May also return -EINTR when a signal was received while being
>   	 * blocked.
> +	 *
> +	 * Note that exporters should not try to cache the scatter list, or
> +	 * return the same one for multiple calls. Caching is done either by the
> +	 * DMA-BUF code (for non-dynamic importers) or the importer. Ownership
> +	 * of the scatter list is transferred to the caller, and returned by
> +	 * @unmap_dma_buf.
>   	 */
>   	struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
>   					 enum dma_data_direction);

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