On 05/15/2019 11:04 AM, Eric Farman wrote:
On 5/15/19 8:23 AM, Cornelia Huck wrote:
On Wed, 15 May 2019 01:42:46 +0200
Eric Farman <farman@xxxxxxxxxxxxx> wrote:
It is possible that a guest might issue a CCW with a length of zero,
and will expect a particular response. Consider this chain:
Address Format-1 CCW
-------- -----------------
0 33110EC0 346022CC 33177468
1 33110EC8 CF200000 3318300C
CCW[0] moves a little more than two pages, but also has the
Suppress Length Indication (SLI) bit set to handle the expectation
that considerably less data will be moved. CCW[1] also has the SLI
bit set, and has a length of zero. Once vfio-ccw does its magic,
the kernel issues a start subchannel on behalf of the guest with this:
Address Format-1 CCW
-------- -----------------
0 021EDED0 346422CC 021F0000
1 021EDED8 CF240000 3318300C
Both CCWs were converted to an IDAL and have the corresponding flags
set (which is by design), but only the address of the first data
address is converted to something the host is aware of. The second
CCW still has the address used by the guest, which happens to be (A)
(probably) an invalid address for the host, and (B) an invalid IDAW
address (doubleword boundary, etc.).
While the I/O fails, it doesn't fail correctly. In this example, we
would receive a program check for an invalid IDAW address, instead of
a unit check for an invalid command.
To fix this, revert commit 4cebc5d6a6ff ("vfio: ccw: validate the
count field of a ccw before pinning") and allow the individual fetch
routines to process them like anything else. We'll make a slight
adjustment to our allocation of the pfn_array (for direct CCWs) or
IDAL (for IDAL CCWs) memory, so that we have room for at least one
address even though no data will be transferred.
Note that this doesn't provide us with a channel program that will
fail in the expected way. Since our length is zero, vfio_pin_pages()
s/is/was/
returns -EINVAL and cp_prefetch() will thus fail. This will be fixed
in the next patch.
So, this failed before, and still fails, just differently?
Probably. If the guest gave us a valid address, the pin might actually
work now whereas before it would fail because the length was zero. If
the address were also invalid,
>IOW, this
has no effect on bisectability?
I think so, but I suppose that either (A) patch 5 and 6 could be
squashed together, or (B) I could move the "set pa_nr to zero" (or more
accurately, set it to ccw->count) pieces from patch 6 into this patch,
so that the vfio_pin_pages() call occurs like it does today.
While going through patch 5, I was confused as to why we need to pin
pages if we are only trying to translate the addresses and no data
transfer will take place with count==0. Well, you answer that in patch 6 :)
So maybe it might be better to move parts of patch 6 to 5 or squash
them, or maybe reverse the order.
Thanks
Farhan
Signed-off-by: Eric Farman <farman@xxxxxxxxxxxxx>
---
drivers/s390/cio/vfio_ccw_cp.c | 26 ++++++++------------------
1 file changed, 8 insertions(+), 18 deletions(-)
