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

[Ashish Mhetre <amhetre@nvidia.com>]

On 2/25/2026 2:27 AM, Pranjal Shrivastava wrote:
> External email: Use caution opening links or attachments
>
>
> On Tue, Feb 24, 2026 at 02:32:21PM +0200, Leon Romanovsky wrote:
>> 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.
>>

Thanks Leon for the review. This crash started after commit 30280eee2db1
("iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg").

> Doesn't folio_is_pci_p2pdma() also check for zone device?
> I see[1] that it does:
>
> static inline bool folio_is_pci_p2pdma(const struct folio *folio)
> {
>          return IS_ENABLED(CONFIG_PCI_P2PDMA) &&
>                  folio_is_zone_device(folio) &&
>                  folio->pgmap->type == MEMORY_DEVICE_PCI_P2PDMA;
> }
>
> I believe the problem arises due to the page_folio() call in
> folio_is_pci_p2pdma(page_folio(page)); within is_pci_p2pdma_page().
> page_folio() assumes it has a valid struct page to work with. For these
> carveouts, that isn't true.
>
> Potentially something like the following would stop the crash:
>
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index e3c2ccf872a8..e47876021afa 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -197,7 +197,8 @@ static inline void folio_set_zone_device_data(struct folio *folio, void *data)
>
>   static inline bool is_pci_p2pdma_page(const struct page *page)
>   {
> -       return IS_ENABLED(CONFIG_PCI_P2PDMA) &&
> +       return IS_ENABLED(CONFIG_PCI_P2PDMA) && page &&
> +               pfn_valid(page_to_pfn(page)) &&
>                  folio_is_pci_p2pdma(page_folio(page));
>   }
>

Yes, this will also fix the crash.

> But my broader question is: why are we calling a page-based API like
> is_pci_p2pdma_page() on non-struct-page memory in the first place?
> Could we instead add a helper to verify if the sg_page() return value
> is actually backed by a struct page? If it isn't, we should arguably
> skip the P2PDMA logic entirely and fall back to a dma_map_phys style
> path. Isn't handling these "pageless" physical ranges the primary reason
> dma_map_phys exists?

Thanks for the feedback, Pranjal.

To clarify: are you suggesting we handle non-page-backed mappings inside
iommu_dma_map_sg (within dma-iommu), or that callers should detect
non-page-backed memory and use dma_map_phys instead of dma_map_sg?
Former approach sounds better so that existing iommu_dma_map_sg callers
don't need changes, but I'd like to confirm your preference.

> +mm list
>
> Thanks,
> Praan
>
> [1] https://elixir.bootlin.com/linux/v6.19.3/source/include/linux/memremap.h#L179
>
>
>> 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@nvidia.com>
>>> ---
>>>   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
>>>
>>>