[PATCH 5.10 001/599] swiotlb: fix info leak with DMA_FROM_DEVICE

From: Greg Kroah-Hartman
Date: Tue Apr 05 2022 - 17:57:44 EST

From: Halil Pasic <pasic@xxxxxxxxxxxxx>

commit ddbd89deb7d32b1fbb879f48d68fda1a8ac58e8e upstream.

The problem I'm addressing was discovered by the LTP test covering

A short description of what happens follows:
1) The test case issues a command code 00 (TEST UNIT READY) via the SG_IO
interface with: dxfer_len == 524288, dxdfer_dir == SG_DXFER_FROM_DEV
and a corresponding dxferp. The peculiar thing about this is that TUR
is not reading from the device.
2) In sg_start_req() the invocation of blk_rq_map_user() effectively
bounces the user-space buffer. As if the device was to transfer into
it. Since commit a45b599ad808 ("scsi: sg: allocate with __GFP_ZERO in
sg_build_indirect()") we make sure this first bounce buffer is
allocated with GFP_ZERO.
3) For the rest of the story we keep ignoring that we have a TUR, so the
device won't touch the buffer we prepare as if the we had a
DMA_FROM_DEVICE type of situation. My setup uses a virtio-scsi device
and the buffer allocated by SG is mapped by the function
virtqueue_add_split() which uses DMA_FROM_DEVICE for the "in" sgs (here
scatter-gather and not scsi generics). This mapping involves bouncing
via the swiotlb (we need swiotlb to do virtio in protected guest like
s390 Secure Execution, or AMD SEV).
4) When the SCSI TUR is done, we first copy back the content of the second
(that is swiotlb) bounce buffer (which most likely contains some
previous IO data), to the first bounce buffer, which contains all
zeros. Then we copy back the content of the first bounce buffer to
the user-space buffer.
5) The test case detects that the buffer, which it zero-initialized,
ain't all zeros and fails.

One can argue that this is an swiotlb problem, because without swiotlb
we leak all zeros, and the swiotlb should be transparent in a sense that
it does not affect the outcome (if all other participants are well

Copying the content of the original buffer into the swiotlb buffer is
the only way I can think of to make swiotlb transparent in such
scenarios. So let's do just that if in doubt, but allow the driver
to tell us that the whole mapped buffer is going to be overwritten,
in which case we can preserve the old behavior and avoid the performance
impact of the extra bounce.

Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Documentation/core-api/dma-attributes.rst | 8 ++++++++
include/linux/dma-mapping.h | 8 ++++++++
kernel/dma/swiotlb.c | 3 ++-
3 files changed, 18 insertions(+), 1 deletion(-)

--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -130,3 +130,11 @@ 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).
+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
@@ -62,6 +62,14 @@
#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
@@ -598,7 +598,8 @@ phys_addr_t swiotlb_tbl_map_single(struc

tlb_addr = slot_addr(io_tlb_start, index) + offset;
if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
+ (!(attrs & DMA_ATTR_OVERWRITE) || dir == DMA_TO_DEVICE ||
swiotlb_bounce(orig_addr, tlb_addr, mapping_size, DMA_TO_DEVICE);
return tlb_addr;