[PATCH] drm/panthor: Fix race when retrieving the new chunk in panthor_heap_grow()

[Osama Abdelkader <osama.abdelkader@gmail.com>]

panthor_heap_grow() allocates a new chunk with panthor_alloc_heap_chunk(),
which adds the chunk at the head of heap->chunks under heap->lock, and
then drops the lock. panthor_heap_grow() then assumes the head of the
list is the chunk it just added, and reads its GPU VA through
list_first_entry() without holding heap->lock.

panthor_heap_grow() and panthor_heap_return_chunk() only take pool->lock
in read mode, and tiler OOM events of different groups are processed in
parallel on the unbound heap_alloc_wq. If two groups use the same heap
context, they can modify the chunk list concurrently:

  group A OOM work                     group B OOM work
  alloc C_A, list_add()
                                       alloc C_B, list_add()
  list_first_entry() -> C_B
                                       group scheduled out:
                                       panthor_heap_return_chunk(C_B)
                                         list_del(), destroy BO, kfree()
  C_B->bo                              <- use-after-free

Even without the free, both groups end up being handed the same chunk,
and the chunk allocated by group A is never linked to the heap context.

Fix this by having panthor_alloc_heap_chunk() return the GPU VA of the
chunk it allocated, retrieved before the chunk is published in the list,
instead of looking it up afterwards.

Fixes: 9cca48fa4f89 ("drm/panthor: Add the heap logical block")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5.5
Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
---
 drivers/gpu/drm/panthor/panthor_heap.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_heap.c b/drivers/gpu/drm/panthor/panthor_heap.c
index 99311abdf1e9..d80f514366fd 100644
--- a/drivers/gpu/drm/panthor/panthor_heap.c
+++ b/drivers/gpu/drm/panthor/panthor_heap.c
@@ -139,7 +139,8 @@ static void panthor_free_heap_chunk(struct panthor_heap_pool *pool,
 
 static int panthor_alloc_heap_chunk(struct panthor_heap_pool *pool,
 				    struct panthor_heap *heap,
-				    bool initial_chunk)
+				    bool initial_chunk,
+				    u64 *chunk_gpu_va)
 {
 	struct panthor_heap_chunk *chunk;
 	struct panthor_heap_chunk_header *hdr;
@@ -181,6 +182,15 @@ static int panthor_alloc_heap_chunk(struct panthor_heap_pool *pool,
 
 	panthor_kernel_bo_vunmap(chunk->bo);
 
+	/*
+	 * Retrieve the GPU VA before the chunk becomes visible in the heap
+	 * chunk list: once heap->lock is released, a concurrent grow or
+	 * return_chunk on the same heap can modify the list, and the chunk
+	 * can be freed.
+	 */
+	if (chunk_gpu_va)
+		*chunk_gpu_va = panthor_kernel_bo_gpuva(chunk->bo);
+
 	mutex_lock(&heap->lock);
 	list_add(&chunk->node, &heap->chunks);
 	heap->chunk_count++;
@@ -216,7 +226,7 @@ static int panthor_alloc_heap_chunks(struct panthor_heap_pool *pool,
 	u32 i;
 
 	for (i = 0; i < chunk_count; i++) {
-		ret = panthor_alloc_heap_chunk(pool, heap, true);
+		ret = panthor_alloc_heap_chunk(pool, heap, true, NULL);
 		if (ret)
 			return ret;
 	}
@@ -437,8 +447,8 @@ int panthor_heap_grow(struct panthor_heap_pool *pool,
 {
 	u64 offset = heap_gpu_va - panthor_kernel_bo_gpuva(pool->gpu_contexts);
 	u32 heap_id = (u32)offset / panthor_heap_ctx_stride(pool->ptdev);
-	struct panthor_heap_chunk *chunk;
 	struct panthor_heap *heap;
+	u64 chunk_gpu_va;
 	int ret;
 
 	if (offset > U32_MAX || heap_id >= MAX_HEAPS_PER_POOL)
@@ -473,14 +483,11 @@ int panthor_heap_grow(struct panthor_heap_pool *pool,
 	 * further jobs in this queue fail immediately instead of having to
 	 * wait for the job timeout.
 	 */
-	ret = panthor_alloc_heap_chunk(pool, heap, false);
+	ret = panthor_alloc_heap_chunk(pool, heap, false, &chunk_gpu_va);
 	if (ret)
 		goto out_unlock;
 
-	chunk = list_first_entry(&heap->chunks,
-				 struct panthor_heap_chunk,
-				 node);
-	*new_chunk_gpu_va = (panthor_kernel_bo_gpuva(chunk->bo) & GENMASK_ULL(63, 12)) |
+	*new_chunk_gpu_va = (chunk_gpu_va & GENMASK_ULL(63, 12)) |
 			    (heap->chunk_size >> 12);
 	ret = 0;
 
-- 
2.53.0