Re: [PATCH RFC] iommu/dma: Validate page before accessing P2PDMA state

[Leon Romanovsky]

On Tue, Feb 24, 2026 at 10:42:57AM +0000, Ashish Mhetre wrote:
> When mapping scatter-gather entries that reference reserved
> memory regions without struct page backing (e.g., bootloader created
> carveouts), is_pci_p2pdma_page() dereferences the page pointer
> returned by sg_page() without first verifying its validity.

I believe this behavior started after commit 88df6ab2f34b  
("mm: add folio_is_pci_p2pdma()"). Prior to that change, the
is_zone_device_page(page) check would return false when given a
non‑existent page pointer.

If any fix is needed, the is_pci_p2pdma_page() must be changed and not iommu.

Thanks

> 
> This causes a kernel paging fault when CONFIG_PCI_P2PDMA is enabled
> and dma_map_sg_attrs() is called for memory regions that have no
> associated struct page:
> 
> Unable to handle kernel paging request at virtual address fffffc007d100000
>   ...
>   Call trace:
>    iommu_dma_map_sg+0x118/0x414
>    dma_map_sg_attrs+0x38/0x44
> 
> Fix this by adding a pfn_valid() check before calling
> is_pci_p2pdma_page(). If the page frame number is invalid, skip the
> P2PDMA check entirely as such memory cannot be P2PDMA memory anyway.
> 
> Signed-off-by: Ashish Mhetre <amhetre@xxxxxxxxxx>
> ---
>  drivers/iommu/dma-iommu.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 5dac64be61bb..5f45f33b23c2 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1423,6 +1423,9 @@ int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
>  		size_t s_length = s->length;
>  		size_t pad_len = (mask - iova_len + 1) & mask;
>  
> +		if (!pfn_valid(page_to_pfn(sg_page(s))))
> +			goto post_pci_p2pdma;
> +
>  		switch (pci_p2pdma_state(&p2pdma_state, dev, sg_page(s))) {
>  		case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
>  			/*
> @@ -1449,6 +1452,7 @@ int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
>  			goto out_restore_sg;
>  		}
>  
> +post_pci_p2pdma:
>  		sg_dma_address(s) = s_iova_off;
>  		sg_dma_len(s) = s_length;
>  		s->offset -= s_iova_off;
> -- 
> 2.25.1
> 
>