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

[Leon Romanovsky <leon@kernel.org>]

On Mon, Feb 02, 2026 at 03:35:48PM +0100, 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.  

Agree

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

There are two paths that lead to SWIOTLB in dma_direct_map_phys().  
The first is is_swiotlb_force_bounce(dev), which dma_need_unmap() can  
easily evaluate. The second is more problematic, as it depends on  
dma_addr and size, neither of which is available in dma_need_unmap():

  102                 if (unlikely(!dma_capable(dev, dma_addr, size, true)) ||
  103                     dma_kmalloc_needs_bounce(dev, size, dir)) {
  104                         if (is_swiotlb_active(dev))

What about the following change?

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 37163eb49f9f..1510b93a8791 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -461,6 +461,8 @@ bool dma_need_unmap(struct device *dev)
 {
        if (!dma_map_direct(dev, get_dma_ops(dev)))
                return true;
+       if (is_swiotlb_force_bounce(dev) || is_swiotlb_active(dev))
+               return true;
        if (!dev->dma_skip_sync)
                return true;
        return IS_ENABLED(CONFIG_DMA_API_DEBUG);