[PATCH 2/4] drm/panthor: Fix iova_mapped_as_huge_page() for imported BOs

[Boris Brezillon <boris.brezillon@collabora.com>]

Imported BOs have no backing.pages array allocated, leading to a NULL
deref when iova_mapped_as_huge_page() gets called on them.

Implement this check through and sgt walk to reach the position of the
sgt targeted by a VA, and check that the DMA address is properly aligned
and the size remaining in the SG entry is bigger than a huge page.
This is basically matching the logic in vm_map_pages(), with get_pgsize()
being replaced by a simpler test, because we don't care about the pgcount
info.

Reported-by: Akash Goel <akash.goel@arm.com>
Fixes: 8e7460eac786 ("drm/panthor: Support partial unmaps of huge pages")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
---
 drivers/gpu/drm/panthor/panthor_mmu.c | 44 ++++++++++++++++++++++++++++-------
 1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
index b0a7033480e6..6cef954e2cba 100644
--- a/drivers/gpu/drm/panthor/panthor_mmu.c
+++ b/drivers/gpu/drm/panthor/panthor_mmu.c
@@ -2299,7 +2299,6 @@ iova_mapped_as_huge_page(struct drm_gpuva *mapping, u64 va)
 {
 	struct panthor_gem_object *bo = to_panthor_bo(mapping->gem.obj);
 	u64 aligned_va = ALIGN_DOWN(va, SZ_2M);
-	const struct page *pg;
 	pgoff_t bo_offset;
 
 	/* If the 2M-aligned VA is outside the mapping being tested, we know
@@ -2309,16 +2308,45 @@ iova_mapped_as_huge_page(struct drm_gpuva *mapping, u64 va)
 		return false;
 
 	bo_offset = aligned_va - mapping->va.addr + mapping->gem.offset;
-	pg = bo->backing.pages[bo_offset >> PAGE_SHIFT];
 
-	/* In case of shmem backing, we know we can only have a huge mapping
-	 * if the bo_offset is 2M aligned, meaning we can skip the folio size
-	 * check if it's not the case.
-	 */
-	if (!IS_ALIGNED(bo_offset, SZ_2M))
+	if (drm_gem_is_imported(&bo->base)) {
+		struct sg_table *sgt = bo->dmap.sgt;
+		struct scatterlist *sgl;
+		unsigned int count;
+
+		/* If this is an imported BO, we have to walk the SGT and
+		 * check the dma address/size alignment to determine if it's
+		 * a huge map or not.
+		 */
+		for_each_sgtable_dma_sg(sgt, sgl, count) {
+			size_t len = sg_dma_len(sgl);
+			dma_addr_t daddr;
+
+			if (len <= bo_offset) {
+				bo_offset -= len;
+				continue;
+			}
+
+			len -= bo_offset;
+			daddr = sg_dma_address(sgl) + bo_offset;
+
+			return IS_ALIGNED(daddr, SZ_2M) &&
+			       len >= SZ_2M;
+		}
+
 		return false;
+	} else {
+		const struct page *pg = bo->backing.pages[bo_offset >> PAGE_SHIFT];
 
-	return folio_size(page_folio(pg)) >= SZ_2M;
+		/* In case of shmem backing, we know we can only have a huge mapping
+		 * if the bo_offset is 2M aligned, meaning we can skip the folio size
+		 * check if it's not the case.
+		 */
+		if (!IS_ALIGNED(bo_offset, SZ_2M))
+			return false;
+
+		return folio_size(page_folio(pg)) >= SZ_2M;
+	}
 }
 
 static void

-- 
2.55.0