Reinstate some of "swiotlb: rework "fix info leak with DMA_FROM_DEVICE""

Halil Pasic points out [1] that the full revert of that commit (revert
in 71020802bc5b), 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 63e33d1d098e ("swiotlb:
rework "fix info leak with DMA_FROM_DEVICE""), and effectively only
revert the part that caused problems.

Link: https://lore.kernel.org/all/20220328013731.017ae3e3.pasic@linux.ibm.com/
Link: https://lore.kernel.org/all/20220324055732.GB12078@lst.de/
Link: https://lore.kernel.org/all/4386660.LvFx2qVVIh@natalenko.name/
Suggested-by: Halil Pasic <pasic@linux.ibm.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Cc: Christoph Hellwig" <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
index 17706dc91ec9fcf51178017b41f50862072c2c40..1887d92e8e926909b3641ffed82b5129dfde0c4d 100644 (file)
--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -130,11 +130,3 @@ accesses to DMA buffers in both privileged "supervisor" and unprivileged
 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).
-
-DMA_ATTR_OVERWRITE
-------------------
-
-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.

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 6150d11a607e1c5561535bab1224083b455cb9d3..dca2b1355bb133f246d4a5e6d787d86539a18a5d 100644 (file)
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -61,14 +61,6 @@
  */
 #define DMA_ATTR_PRIVILEGED            (1UL << 9)
 
-/*
- * 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.  It can
  * be given to a device to use as a DMA source or target.  It is specific to a

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index bfc56cb217059e930ff79382c59138dab7b391b1..6c350555e5a1ca385fffcc618eefa72acc091587 100644 (file)
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -627,10 +627,14 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
        for (i = 0; i < nr_slots(alloc_size + offset); i++)
                mem->slots[index + i].orig_addr = slot_addr(orig_addr, i);
        tlb_addr = slot_addr(mem->start, index) + offset;
-       if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
-           (!(attrs & DMA_ATTR_OVERWRITE) || dir == DMA_TO_DEVICE ||
-           dir == DMA_BIDIRECTIONAL))
-               swiotlb_bounce(dev, tlb_addr, mapping_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(dev, tlb_addr, mapping_size, DMA_TO_DEVICE);
        return tlb_addr;
 }