Re: [PATCH v5 14/16] dma-direct: Allocate memory from restricted DMA pool if available

On Fri, Apr 23, 2021 at 9:46 PM Robin Murphy robin.murphy@arm.com wrote:

On 2021-04-22 09:15, Claire Chang wrote:

...

The restricted DMA pool is preferred if available.

The restricted DMA pools provide a basic level of protection against the DMA overwriting buffer contents at unexpected times. However, to protect against general data leakage and system memory corruption, the system needs to provide a way to lock down the memory access, e.g., MPU.

Signed-off-by: Claire Chang tientzu@chromium.org

kernel/dma/direct.c | 35 ++++++++++++++++++++++++++--------- 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c index 7a27f0510fcc..29523d2a9845 100644 --- a/kernel/dma/direct.c +++ b/kernel/dma/direct.c @@ -78,6 +78,10 @@ static bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size) static void __dma_direct_free_pages(struct device *dev, struct page *page, size_t size) { +#ifdef CONFIG_DMA_RESTRICTED_POOL

• if (swiotlb_free(dev, page, size))
  

•         return;
  

+#endif dma_free_contiguous(dev, page, size); }

@@ -92,7 +96,17 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,

  gfp |= dma_direct_optimal_gfp_mask(dev, dev->coherent_dma_mask,
                                     &phys_limit);

• page = dma_alloc_contiguous(dev, size, gfp);
  

+#ifdef CONFIG_DMA_RESTRICTED_POOL

• page = swiotlb_alloc(dev, size);
  

• if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
  

•         __dma_direct_free_pages(dev, page, size);
  

•         page = NULL;
  

• }
  

+#endif

• if (!page)
  

•         page = dma_alloc_contiguous(dev, size, gfp);
  if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
          dma_free_contiguous(dev, page, size);
          page = NULL;
  

@@ -148,7 +162,7 @@ void *dma_direct_alloc(struct device *dev, size_t size, gfp |= __GFP_NOWARN;

  if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&

•     !force_dma_unencrypted(dev)) {
  

•     !force_dma_unencrypted(dev) && !is_dev_swiotlb_force(dev)) {
          page = __dma_direct_alloc_pages(dev, size, gfp & ~__GFP_ZERO);
          if (!page)
                  return NULL;
  

@@ -161,8 +175,8 @@ void *dma_direct_alloc(struct device *dev, size_t size, }

  if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&

•     !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
  

•     !dev_is_dma_coherent(dev))
  

•     !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev) &&
  

•     !is_dev_swiotlb_force(dev))
          return arch_dma_alloc(dev, size, dma_handle, gfp, attrs);
  
  /*
  

@@ -172,7 +186,9 @@ void *dma_direct_alloc(struct device *dev, size_t size, if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) && !gfpflags_allow_blocking(gfp) && (force_dma_unencrypted(dev) ||

•      (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev))))
  

•      (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
  

•       !dev_is_dma_coherent(dev))) &&
  

•     !is_dev_swiotlb_force(dev))
          return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
  
  /* we always manually zero the memory once we are done */
  

@@ -253,15 +269,15 @@ void dma_direct_free(struct device *dev, size_t size, unsigned int page_order = get_order(size);

  if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&

•     !force_dma_unencrypted(dev)) {
  

•     !force_dma_unencrypted(dev) && !is_dev_swiotlb_force(dev)) {
          /* cpu_addr is a struct page cookie, not a kernel address */
          dma_free_contiguous(dev, cpu_addr, size);
          return;
  }
  
  if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&
  

•     !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
  

•     !dev_is_dma_coherent(dev)) {
  

•     !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev) &&
  

•     !is_dev_swiotlb_force(dev)) {
          arch_dma_free(dev, size, cpu_addr, dma_addr, attrs);
          return;
  }
  

@@ -289,7 +305,8 @@ struct page *dma_direct_alloc_pages(struct device *dev, size_t size, void *ret;

  if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&

•     force_dma_unencrypted(dev) && !gfpflags_allow_blocking(gfp))
  

•     force_dma_unencrypted(dev) && !gfpflags_allow_blocking(gfp) &&
  

•     !is_dev_swiotlb_force(dev))
          return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
  

Wait, this seems broken for non-coherent devices - in that case we need to return a non-cacheable address, but we can't simply fall through into the remapping path below in GFP_ATOMIC context. That's why we need the atomic pool concept in the first place :/

Sorry for the late reply. I'm not very familiar with this. I wonder if the memory returned here must be coherent. If yes, could we say for this case, one must set up another device coherent pool (shared-dma-pool) and go with dma_alloc_from_dev_coherent()[1]?

[1] https://elixir.bootlin.com/linux/v5.12/source/kernel/dma/mapping.c#L435

Unless I've overlooked something, we're still using the regular cacheable linear map address of the dma_io_tlb_mem buffer, no?

Robin.

...

  page = __dma_direct_alloc_pages(dev, size, gfp);