On Fri, Oct 30, 2026 at 1:44 PM Chris Wiseman chriswiseman@google.com wrote:

When commit 381295758792 ("dma-buf: system_heap: Rework system heap to use sgtables instead of pagelists") stopped using heap_helpers.c and added a custom system_heap_mmap(), the check rejecting private mappings was dropped.

Other exporters and buffer allocators reject private or copy-on-write mappings in their mmap handlers: cma_heap_mmap() and mmap_udmabuf() check that VM_SHARED or VM_MAYSHARE is set, and drm_gem_shmem_mmap() and ttm_bo_mmap_obj() reject vma_is_cow_mapping(vma).

However, system_heap_mmap() omits this check and calls remap_pfn_range() even when userspace requests MAP_PRIVATE with PROT_WRITE. As a result, subsequent CPU writes to a MAP_PRIVATE mapping of a system heap buffer fault into do_wp_page() -> wp_page_copy(), silently allocating private anonymous copy-on-write pages whose writes never reach the underlying system heap buffer pages.

Reject non-shared mappings in system_heap_mmap() with -EINVAL to match cma_heap_mmap() and mmap_udmabuf().

Fixes: 381295758792 ("dma-buf: system_heap: Rework system heap to use sgtables instead of pagelists") Cc: stable@vger.kernel.org Signed-off-by: Chris Wiseman chriswiseman@google.com

Reviewed-by: T.J. Mercier tjmercier@google.com

I wonder if we should have this in dma_buf_mmap_internal() though. Any thoughts Sumit or Christian?