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

In the Linux kernel, the following vulnerability has been resolved:

swiotlb: fix info leak with DMA_FROM_DEVICE

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.