[PATCH] dma-direct: Don't over-decrypt memory

[PATCH] dma-direct: Don't over-decrypt memory

[Robin Murphy]

The original x86 sev_alloc() only called set_memory_decrypted() on
memory returned by alloc_pages_node(), so the page order calculation
fell out of that logic. However, the common dma-direct code has several
potential allocators, not all of which are guaranteed to round up the
underlying allocation to a power-of-two size, so carrying over that
calculation for the encryption/decryption size was a mistake. Fix it by
rounding to a *number* of pages, rather than an order.

Until recently there was an even worse interaction with DMA_DIRECT_REMAP
where we could have ended up decrypting part of the next adjacent
vmalloc area, only averted by no architecture actually supporting both
configs at once. Don't ask how I found that one out...

CC: stable@vger.kernel.org
Fixes: c10f07aa27da ("dma/direct: Handle force decryption for DMA coherent buffers in common code")
Signed-off-by: Robin Murphy <robin.murphy@arm.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 9743c6ccce1a..09d78aa40466 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -79,7 +79,7 @@ static int dma_set_decrypted(struct device *dev, void *vaddr, size_t size)
 {
 	if (!force_dma_unencrypted(dev))
 		return 0;
-	return set_memory_decrypted((unsigned long)vaddr, 1 << get_order(size));
+	return set_memory_decrypted((unsigned long)vaddr, PFN_UP(size));
 }
 
 static int dma_set_encrypted(struct device *dev, void *vaddr, size_t size)
@@ -88,7 +88,7 @@ static int dma_set_encrypted(struct device *dev, void *vaddr, size_t size)
 
 	if (!force_dma_unencrypted(dev))
 		return 0;
-	ret = set_memory_encrypted((unsigned long)vaddr, 1 << get_order(size));
+	ret = set_memory_encrypted((unsigned long)vaddr, PFN_UP(size));
 	if (ret)
 		pr_warn_ratelimited("leaking DMA memory that can't be re-encrypted\n");
 	return ret;
-- 
2.35.3.dirty

Re: [PATCH] dma-direct: Don't over-decrypt memory

[Christoph Hellwig]

Thanks,

applied to the dma-mapping for-next branch.

Re: [PATCH] dma-direct: Don't over-decrypt memory

[David Rientjes]

On Fri, 20 May 2022, Robin Murphy wrote:

> The original x86 sev_alloc() only called set_memory_decrypted() on
> memory returned by alloc_pages_node(), so the page order calculation
> fell out of that logic. However, the common dma-direct code has several
> potential allocators, not all of which are guaranteed to round up the
> underlying allocation to a power-of-two size, so carrying over that
> calculation for the encryption/decryption size was a mistake. Fix it by
> rounding to a *number* of pages, rather than an order.
> 
> Until recently there was an even worse interaction with DMA_DIRECT_REMAP
> where we could have ended up decrypting part of the next adjacent
> vmalloc area, only averted by no architecture actually supporting both
> configs at once. Don't ask how I found that one out...
> 
> CC: stable@vger.kernel.org
> Fixes: c10f07aa27da ("dma/direct: Handle force decryption for DMA coherent buffers in common code")
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

Acked-by: David Rientjes <rientjes@google.com>