[PATCH 5.16 0967/1017] Reinstate some of "swiotlb: rework "fix info leak with DMA_FROM_DEVICE""

Greg Kroah-Hartman
2022-04-05T14:06:15Z

From: Linus Torvalds <torvalds@linux-foundation.org>

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/20220328013731.017ae3e3.pasic@linux.ibm.com/ [1]
Link: https://lore.kernel.org/all/20220324055732.GB12078@lst.de/ [2]
Link: https://lore.kernel.org/all/4386660.LvFx2qVVIh@natalenko.name/ [3]
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/core-api/dma-attributes.rst | 8 --------
include/linux/dma-mapping.h | 8 --------
kernel/dma/swiotlb.c | 12 ++++++++----
3 files changed, 8 insertions(+), 20 deletions(-)

--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -130,11 +130,3 @@ accesses to DMA buffers in both privileg
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.
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -62,14 +62,6 @@
#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
* given device and there may be a translation between the CPU physical address
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -581,10 +581,14 @@ phys_addr_t swiotlb_tbl_map_single(struc
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;
}