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

[Ashish Mhetre <amhetre@nvidia.com>]

On 2/26/2026 1:28 PM, Leon Romanovsky wrote:
> External email: Use caution opening links or attachments
>
>
> On Wed, Feb 25, 2026 at 08:11:29PM +0000, Pranjal Shrivastava wrote:
>> On Wed, Feb 25, 2026 at 09:56:09AM +0200, Leon Romanovsky wrote:
>>> On Wed, Feb 25, 2026 at 10:19:41AM +0530, Ashish Mhetre wrote:
>>>>
>>>> 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?
>>> The latter one.
>>>
>> Yup, I meant the latter.
>>
>>>> Former approach sounds better so that existing iommu_dma_map_sg callers
>>>> don't need changes, but I'd like to confirm your preference.
>>> The bug is in callers which used wrong API, they need to be adapted.
>> Yes, the thing is, if the caller already knows that the region to be
>> mapped is NOT struct page-backed, then why does it use dma_map_sg
>> variants?
> Before dma_map_phys() was added, there was no reliable way to DMA‑map
> such memory, and using dma_map_sg() was a workaround that happened to
> work. I'm not sure whether it worked by design or by accident, but the
> correct approach now is to use dma_map_phys().

Thanks Leon and Pranjal for the detailed feedback. I'll update our 
callers to use
dma_map_phys() for non-page-backed buffers.

One question: would it make sense to add a check in iommu_dma_map_sg to
fail gracefully when non-page-backed buffers are passed, instead of crashing
the kernel?

Thanks,
Ashish Mhetre

> Thanks
>
>> Thanks
>> Praan