[Linaro-mm-sig] Re: [PATCH 2/2] dma-buf: heaps: Zero system shared heap pages after conversion

On 8/20/26 12:50, Steven Price wrote:

The system_cc_shared heap allocates pages with __GFP_ZERO before converting them from private to shared with set_memory_decrypted(). This assumes that the conversion preserves the contents of the pages.

For Arm CCA with MEC (Memory Encryption Contexts) the key used to access the page will change, and so by default the visible data will change. The host could ensure that it zeros the page after decryption, but rather than relying on the host's behaviour it's best if the guest simply zeros after the decryption rather than before.

For CC shared buffers, defer zeroing until each page has been converted successfully. For other buffers keep the existing behaviour.

Fixes: 78b30c50a7ac ("dma-buf: heaps: system: add system_cc_shared heap for explicitly shared memory") Signed-off-by: Steven Price steven.price@arm.com

---

drivers/dma-buf/heaps/system_heap.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c index c8959eadc71d..f14930904089 100644 --- a/drivers/dma-buf/heaps/system_heap.c +++ b/drivers/dma-buf/heaps/system_heap.c @@ -376,7 +376,8 @@ static const struct dma_buf_ops system_heap_buf_ops = { }; static struct page *alloc_largest_available(unsigned long size,

• 			    unsigned int max_order)
  

• 			    unsigned int max_order,
  

• 			    bool defer_zero)
  

{ struct page *page; int i; @@ -388,6 +389,9 @@ static struct page *alloc_largest_available(unsigned long size, if (max_order < orders[i]) continue; flags = order_flags[i];

• /* Decryption can change the contents, so clear it afterwards. */
  

• if (defer_zero)
  

• 	flags &= ~__GFP_ZERO;
  

  if (mem_accounting) flags |= __GFP_ACCOUNT; page = alloc_pages(flags, orders[i]);

@@ -438,7 +442,8 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap, goto free_buffer; }

• page = alloc_largest_available(size_remaining, max_order);
  

• page = alloc_largest_available(size_remaining, max_order,
  

• 			       cc_shared_buffer(buffer));
  

  if (!page) goto free_buffer;

@@ -461,9 +466,12 @@ static struct dma_buf *system_heap_allocate(struct dma_heap *heap, if (cc_shared_buffer(buffer)) { for_each_sgtable_sg(table, sg, i) {

• 	ret = system_heap_set_page_decrypted(sg_page(sg));
  

• 	page = sg_page(sg);
  

• 	ret = system_heap_set_page_decrypted(page);
  if (ret)
  	goto free_pages;
  

• 	clear_pages(page_address(page), 1 << compound_order(page));
  

To be honest that looks horrible.

If you need to backport that fine by me, but in the long term we clearly should have that in the initial allocation somehow.

Regards, Christian.

}

}