[PATCH] tee: shm: sync DMA memory for the device before it is lent

[PATCH] tee: shm: sync DMA memory for the device before it is lent

[Tyler Baker]

tee_shm_alloc_dma_mem() returns dma_alloc_pages() memory without the
dma_sync_single_for_device() that the DMA API requires before a device
uses it. dma_alloc_pages() zeroes the pages through the cacheable
linear map, so the zeroes can still be dirty in the cache when the
optee protected memory pool lends the pages and a Secure Data Path TA
writes them through its secure mapping. A later eviction of those
lines overwrites the TA's data.

On an Eliza EVK with OP-TEE as the FF-A SPMC, a TA decrypts into a
protected buffer, a read of all RAM through /proc/kcore evicts the
lines, and the TA's digest of the buffer no longer matches:

  frame 0: 0e534d8c..., reference d98ddba1...

Sync the allocation for the device before returning it.

Fixes: ab09dd6d9201 ("tee: add tee_shm_alloc_dma_mem()")
Cc: stable@vger.kernel.org
Signed-off-by: Tyler Baker <tyler.baker@oss.qualcomm.com>
---
Vincent's direct-map series [1] cleans the range in
dma_zap_direct_map() before the lend, which also covers this when it
can unmap. It returns -ENODEV without any cache maintenance when there
is no lendable pool and the direct map cannot change (arm64 with
rodata=noalias or rodata=off), and it is not a stable candidate.

On a dma-coherent TEE device the sync does nothing. teedev->dev is not
configured from firmware, so it is not coherent today.

Without the fix the TA's data is lost:
  https://lava.infra.foundries.io/scheduler/job/554467#L2262
With it, the data is intact after the scan and 50 lend rounds:
  https://lava.infra.foundries.io/scheduler/job/563840#L1979
  https://lava.infra.foundries.io/scheduler/job/563840#L2008

[1] https://lore.kernel.org/all/20261111110738.872947-1-vdonnefort@google.com/
---
 drivers/tee/tee_shm.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
index 5c47a36916c45..70a273a425f88 100644
--- a/drivers/tee/tee_shm.c
+++ b/drivers/tee/tee_shm.c
@@ -297,6 +297,9 @@ struct tee_shm *tee_shm_alloc_dma_mem(struct tee_context *ctx,
 	if (!page)
 		goto err_put_teedev;
 
+	dma_sync_single_for_device(&teedev->dev, dma_addr,
+				   page_count * PAGE_SIZE, DMA_BIDIRECTIONAL);
+
 	dma_mem = kzalloc_obj(*dma_mem);
 	if (!dma_mem)
 		goto err_free_pages;

---
base-commit: e21989d08e0d6abacfff71387d0cb36dd96a4079
change-id: 20261113-tee-shm-dma-sync-12f58435b81e

Best regards,
--  
Tyler Baker <tyler.baker@oss.qualcomm.com>