Subject: [03/12] Reinstate some of "swiotlb: rework "fix info leak with DMA_FROM_DEVICE""

From: Linus Torvalds <[email-masked]>
Date: Mon, 28 Mar 2022 11:37:05 -0700

commit 901c7280ca0d5e2b4a8929fbe0bfb007ac2a6544 upstream.

Halil Pasic points out [1] that the full revert of that commit (revert
in bddac7c1e02b), and that a partial revert that only reverts the
problematic case, but still keeps some of the cleanups is probably
better.  ￼

And that partial revert [2] had already been verified by Oleksandr
Natalenko to also fix the issue, I had just missed that in the long
discussion.

So let's reinstate the cleanups from commit aa6f8dcbab47 ("swiotlb:
rework "fix info leak with DMA_FROM_DEVICE""), and effectively only
revert the part that caused problems.

Link: https://lore.kernel.org/all/[email-masked]/ [1]
Link: https://lore.kernel.org/all/[email-masked]/ [2]
Link: https://lore.kernel.org/all/[email-masked]/ [3]
Suggested-by: Halil Pasic <[email-masked]>
Tested-by: Oleksandr Natalenko <[email-masked]>
Cc: Christoph Hellwig" <[email-masked]>
Signed-off-by: Linus Torvalds <[email-masked]>
[OP: backport to 4.14: apply swiotlb_tbl_map_single() changes in lib/swiotlb.c]
Signed-off-by: Ovidiu Panait <[email-masked]>
Signed-off-by: Greg Kroah-Hartman <[email-masked]>
[bwh: Backported to 4.9: adjust context]
Signed-off-by: Ben Hutchings <[email-masked]>
---
 Documentation/DMA-attributes.txt | 10 ----------
 include/linux/dma-mapping.h      |  7 -------
 lib/swiotlb.c                    | 12 ++++++++----
 3 files changed, 8 insertions(+), 21 deletions(-)

diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
index c2f7567e0094..98bf7ac29aad 100644
--- a/Documentation/DMA-attributes.txt
+++ b/Documentation/DMA-attributes.txt
@@ -143,13 +143,3 @@ So, this provides a way for drivers to avoid those error messages on calls
 where allocation failures are not a problem, and shouldn't bother the logs.
 
 NOTE: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC.
-
-DMA_ATTR_PRIVILEGED
--------------------
-
-Some advanced peripherals such as remote processors and GPUs perform
-accesses to DMA buffers in both privileged "supervisor" and unprivileged
-"user" modes.  This attribute is used to indicate to the DMA-mapping
-subsystem that the buffer is fully accessible at the elevated privilege
-level (and ideally inaccessible or at least read-only at the
-lesser-privileged levels).
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index e069e2fd0c9f..97f817f4eb78 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -61,13 +61,6 @@
  * allocation failure reports (similarly to __GFP_NOWARN).
  */
 #define DMA_ATTR_NO_WARN	(1UL << 8)
-/*
- * This is a hint to the DMA-mapping subsystem that the device is expected
- * to overwrite the entire mapped size, thus the caller does not require any
- * of the previous buffer contents to be preserved. This allows
- * bounce-buffering implementations to optimise DMA_FROM_DEVICE transfers.
- */
-#define DMA_ATTR_OVERWRITE		(1UL << 10)
 
 /*
  * A dma_addr_t can hold any valid DMA or bus address for the platform.
diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index 30c6715f4110..277a2f6fde6b 100644
--- a/lib/swiotlb.c
+++ b/lib/swiotlb.c
@@ -532,10 +532,14 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 	 */
 	for (i = 0; i < nslots; i++)
 		io_tlb_orig_addr[index+i] = orig_addr + (i << IO_TLB_SHIFT);
-	if (!(attrs & DMA_ATTR_OVERWRITE) || dir == DMA_TO_DEVICE ||
-	    dir == DMA_BIDIRECTIONAL)
-		swiotlb_bounce(orig_addr, tlb_addr, size, DMA_TO_DEVICE);
-
+	/*
+	 * When dir == DMA_FROM_DEVICE we could omit the copy from the orig
+	 * to the tlb buffer, if we knew for sure the device will
+	 * overwirte the entire current content. But we don't. Thus
+	 * unconditional bounce may prevent leaking swiotlb content (i.e.
+	 * kernel memory) to user-space.
+	 */
+	swiotlb_bounce(orig_addr, tlb_addr, size, DMA_TO_DEVICE);
 	return tlb_addr;
 }
 EXPORT_SYMBOL_GPL(swiotlb_tbl_map_single);