Re: [PATCH 3/7] drm/panfrost: Give back the pages of a heap chunk that failed to map

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] drm/panfrost: partial cleanup of >2MB folios in
  panfrost_heap_put_chunk()
It also noticed 1 related pre-existing issue this patch does not
introduce:
- [Critical] drm/panfrost: NULL pointer dereference in
  panfrost_gem_sync() on heap BOs [1]
--

commit afb30e586f33dffb885482044302a7b61909475c
Author: Arun Iyengar <aiyengar1990@gmail.com>

drm/panfrost: Give back the pages of a heap chunk that failed to map

This commit adds panfrost_heap_put_chunk() to release folios and clear page
array entries when mapping a 2 MB heap chunk fails in
panfrost_mmu_map_fault_addr().

> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index ba37da4992307..13728b0c46efc 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -597,6 +597,28 @@ addr_to_mapping(struct panfrost_device *pfdev, int as, u64 addr)
>  
>  #define NUM_FAULT_PAGES (SZ_2M / PAGE_SIZE)
>  
> +static void panfrost_heap_put_chunk(struct page **pages, pgoff_t start,
> +				    pgoff_t end)
> +{
> +	pgoff_t pg = start;
> +
> +	while (pg < end) {
> +		struct folio *folio;
> +		pgoff_t n;
> +
> +		if (!pages[pg]) {
> +			pg++;
> +			continue;
> +		}
> +
> +		folio = page_folio(pages[pg]);
> +		n = folio_nr_pages(folio);
> +		for (pgoff_t i = 0; i < n && pg < end; i++)
> +			pages[pg++] = NULL;
> +		folio_put(folio);
> +	}
> +}
> +
>  static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
>  				       u64 addr)
>  {
> @@ -671,7 +693,7 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
>  		folio = shmem_read_folio(mapping, pg);
>  		if (IS_ERR(folio)) {
>  			ret = PTR_ERR(folio);
> -			goto err_unlock;
> +			goto err_put_chunk;
>  		}

[Severity: Critical]
This problem wasn't introduced by this patch, but can calling
panfrost_gem_sync() on a heap BO [1] cause a NULL pointer dereference?

On coherent devices (pfdev->coherent is true), should_map_wc() returns
false, so panfrost_gem_sync() does not return early on shmem->map_wc and
calls drm_gem_shmem_get_pages_sgt():

drivers/gpu/drm/panfrost/panfrost_gem.c:panfrost_gem_sync() {
	...
	if (shmem->map_wc)
		return 0;

	/* Nothing to do if the size is zero. */
	if (size == 0)
		return 0;

	sgt = drm_gem_shmem_get_pages_sgt(shmem);
	...
}

If a heap BO has already faulted in at least one chunk, bo->base.pages is
non-NULL with NULL entries for unfaulted chunks and bo->base.sgt is NULL, so
drm_gem_shmem_get_pages_sgt() passes the sparse bo->base.pages array to
drm_prime_pages_to_sg() and dereferences NULL page pointers.  Conversely, if
panfrost_gem_sync() runs before the first GPU fault on the heap BO,
drm_gem_shmem_get_pages_sgt() populates bo->base.pages while leaving
bo->sgts NULL, and a later fault in panfrost_mmu_map_fault_addr() skips
allocating bo->sgts and dereferences NULL when indexing bo->sgts.

[ ... ]

> @@ -720,6 +742,14 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
>  	dma_unmap_sgtable(pfdev->base.dev, sgt, DMA_BIDIRECTIONAL, 0);
>  err_map:
>  	sg_free_table(sgt);
> +err_put_chunk:
> +	/*
> +	 * Give back the folios read for this chunk, so that the next fault in
> +	 * it (the chunk has no sg table yet) starts from an empty page range
> +	 * instead of finding it partly owned.
> +	 */
> +	panfrost_heap_put_chunk(pages, page_offset,
> +				min_t(pgoff_t, page_offset + NUM_FAULT_PAGES, nr_pages));

[Severity: Medium]
Can panfrost_heap_put_chunk() leave dangling page pointers or drop an active
folio reference when shmem_read_folio() returns a folio larger than 2 MB?

With PAGE_SIZE > 4 KB (such as 16 KB or 64 KB pages on arm64) and
transparent hugepages enabled, shmem_read_folio() on the huge=within_size
mount can return a folio with folio_nr_pages(folio) > NUM_FAULT_PAGES.  The
fill loop in panfrost_mmu_map_fault_addr() aligns pg down to the folio
boundary and populates pages[] across the entire folio:

drivers/gpu/drm/panfrost/panfrost_mmu.c:panfrost_mmu_map_fault_addr() {
		...
		pg &= ~(folio_nr_pages(folio) - 1);
		for (u32 i = 0; i < folio_nr_pages(folio) && pg < nr_pages; i++) {
			if (pages[pg])
				already_owned = true;

			pages[pg++] = folio_page(folio, i);
		}
}

On error, panfrost_heap_put_chunk() only clears pages[] within
[page_offset, page_offset + NUM_FAULT_PAGES) before calling
folio_put(folio).  Any pages[] slots populated for that folio in adjacent
2 MB chunks remain non-NULL after the folio reference is dropped (or if an
adjacent chunk was already mapped and the duplicate reference was dropped via
already_owned, folio_put(folio) drops the mapped chunk's reference), leading
to a second folio_put() in drm_gem_put_pages() when the BO is freed and a
folio use-after-free.

[1] https://sashiko.dev/#/bug/linux-fe4f761b-d268-4774-87c0-3f0de658e362

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261110-panfrost-long-jobs-v1-0-24508939ae8d@gmail.com?part=3