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 [off-list ref] wrote:

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

quoted

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 <redacted>
---
  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.

quoted

      page = __dma_direct_alloc_pages(dev, size, gfp);