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

Mostafa Saleh
2026-03-30T14:51:25Z

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;
return set_memory_decrypted((unsigned long)vaddr, PFN_UP(size));
}
@@ -88,7 +88,7 @@ static int dma_set_encrypted(struct device *dev, void *vaddr, size_t size)
{
int ret;

- if (!force_dma_unencrypted(dev))
+ if (!force_dma_unencrypted(dev) || is_swiotlb_for_alloc(dev))
return 0;
ret = set_memory_encrypted((unsigned long)vaddr, PFN_UP(size));
if (ret)
--
2.53.0.1185.g05d4b7b318-goog