Re: [PATCH V1] nvme-pci: Fix NULL pointer dereference in nvme_pci_prp_iter_next

[Robin Murphy <robin.murphy@arm.com>]

On 2026-02-02 2:35 pm, Christoph Hellwig wrote:
> On Mon, Feb 02, 2026 at 06:27:38PM +0530, Pradeep P V K wrote:
>> Fix a NULL pointer dereference that occurs in nvme_pci_prp_iter_next()
>> when SWIOTLB bounce buffering becomes active during runtime.
>>
>> The issue occurs when SWIOTLB activation changes the device's DMA
>> mapping requirements at runtime,
>>
>> creating a mismatch between
>> iod->dma_vecs allocation and access logic.
>>
>> The problem manifests when:
>> 1. Device initially operates with dma_skip_sync=true
>>     (coherent DMA assumed)
>> 2. First SWIOTLB mapping occurs due to DMA address limitations,
>>     memory encryption, or IOMMU bounce buffering requirements
>> 3. SWIOTLB calls dma_reset_need_sync(), permanently setting
>>     dma_skip_sync=false
>> 4. Subsequent I/Os now have dma_need_unmap()=true, requiring
>>     iod->dma_vecs
> 
> I think this patch just papers over the bug.  If dma_need_unmap
> can't be trusted before the dma_map_* call, we've not saved
> the unmap information and the unmap won't work properly.

The dma_need_unmap() kerneldoc says:

"This function must be called after all mappings that might
  need to be unmapped have been performed."

Trying to infer anything from it beforehand is definitely a bug in the 
caller.

> So we'll need to extend the core code to tell if a mapping
> will set dma_skip_sync=false before doing the mapping.

I don't see that being possible - at best we could reasonably infer that 
a fully-coherent system with no sync ops, no SWIOTLB and no DMA_DEBUG 
shouldn't ever set it to true, but as for the other way round, by the 
time you've run through all the SWIOTLB logic to guess whether a 
particular mapping would be bounced or not, you've basically performed 
the mapping anyway. Thus at best, such an API to potentially do a whole 
dry-run mapping before every actual mapping would seem like a pretty 
pointless anti-optimisation.

Thanks,
Robin.