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

On Fri, Jan 15, 2021 at 2:09 PM Christian König ckoenig.leichtzumerken@gmail.com wrote:

Am 15.01.21 um 14:02 schrieb Daniel Vetter:

...

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.

Well that's why extracted the helper functions (it would be 4 copies otherwise). It's true that it's still 2x the same operation, but conceptually one of them mangles, the other unmangles the pointers. It's just that with XOR mangling, that's both the same code. Readability feels better that way to me, but I guess I can do another tiny helper function extraction if you insist? -Daniel

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);