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

Jason Gunthorpe
2026-03-30T15:06:57Z

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.

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

Jason