On Tue, Mar 22, 2022 at 11:02:17AM +0100, Halil Pasic wrote:
> The problem I'm addressing was discovered by the LTP test covering
> cve-2018-1000204.
>
> 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
> behaved).
>
> 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>
> Cc: stable@xxxxxxxxxxxxxxx
> [pasic@xxxxxxxxxxxxx: resolved merge conflicts]
> ---
> 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(-)
What is the git commit id of this patch in Linus's tree?
