Re: [RFC PATCH v2 1/5] dma-mapping: Avoid double decrypting with DMA_RESTRICTED_POOL

[Mostafa Saleh <smostafa@google.com>]

On Mon, Mar 30, 2026 at 12:06:54PM -0300, Jason Gunthorpe wrote:
> On Mon, Mar 30, 2026 at 02:50:39PM +0000, Mostafa Saleh wrote:
> > In case a device have a restricted DMA pool, it will be decrypted
> > by default.
> > 
> > However, in the path of dma_direct_alloc() memory can be allocated
> > from this pool using, __dma_direct_alloc_pages() =>
> > dma_direct_alloc_swiotlb()
> > 
> > After that from the same function, it will attempt to decrypt it
> > using dma_set_decrypted() if force_dma_unencrypted().
> > 
> > Which results in the memory being decrypted twice.
> > 
> > It's not clear how the does realm world/hypervisors deal with that,
> > for example:
> > - CCA: Clear a bit in the page table and call realm IPA_STATE_SET.
> > - TDX: Issue a hypercall.
> > - pKVM: Which doesn't implement force_dma_unencrypted() at the moment,
> >   uses a share hypercall.
> > 
> > Change that to only encrypt/decrypt memory that are not allocated
> > from the restricted dma pools.
> > 
> > Fixes: f4111e39a52a ("swiotlb: Add restricted DMA alloc/free support")
> > Signed-off-by: Mostafa Saleh <smostafa@google.com>
> > ---
> >  kernel/dma/direct.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > index 8f43a930716d..27d804f0473f 100644
> > --- a/kernel/dma/direct.c
> > +++ b/kernel/dma/direct.c
> > @@ -79,7 +79,7 @@ bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
> >  
> >  static int dma_set_decrypted(struct device *dev, void *vaddr, size_t size)
> >  {
> > -	if (!force_dma_unencrypted(dev))
> > +	if (!force_dma_unencrypted(dev) || is_swiotlb_for_alloc(dev))
> >  		return 0;
> 
> This seems really obtuse, I would expect the decryption state of the
> memory to be known by the caller. If dma_direct_alloc_swiotlb() can
> return decrypted or encrypted memory it needs to return a flag saying
> that. It shouldn't be deduced by checking dev flags in random places
> like this.

At the moment restricted dma is always decrypted, also it’s per device
so we don’t have to check this per allocation.
I can change the signature for __dma_direct_alloc_pages() to make it
return an extra flag but that feels more complicated as it changes
dma_direct_alloc_swiotlb() , swiotlb_alloc() with its callers.

I can investigate this approach further.

Thanks,
Mostafa

> 
> Double decryption is certainly a bug, I do not expect that to work.
> 
> Jason