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Re: “Backend has not unmapped grant” errors
On 28.08.22 07:15, Demi Marie Obenour wrote:
On Wed, Aug 24, 2022 at 08:11:56AM +0200, Juergen Gross wrote:
On 24.08.22 02:20, Marek Marczykowski-Górecki wrote:
On Tue, Aug 23, 2022 at 09:48:57AM +0200, Juergen Gross wrote:
On 23.08.22 09:40, Demi Marie Obenour wrote:
I recently had a VM’s /dev/xvdb stop working with a “backend has not
unmapped grant” error. Since /dev/xvdb was the VM’s private volume,
that rendered the VM effectively useless. I had to kill it with
qvm-kill.
The backend of /dev/xvdb is dom0, so a malicious backend is clearly not
the cause of this. I believe the actual cause is a race condition, such
as the following:
1. GUI agent in VM allocates grant X.
2. GUI agent tells GUI daemon in dom0 to map X.
3. GUI agent frees grant X.
4. blkfront allocates grant X and passes it to dom0.
5. dom0’s blkback maps grant X.
6. blkback unmaps grant X.
7. GUI daemon maps grant X.
8. blkfront tries to revoke access to grant X and fails. Disaster
ensues.
What could be done to prevent this race? Right now all of the
approaches I can think of are horribly backwards-incompatible. They
require replacing grant IDs with some sort of handle, and requiring
userspace to pass these handles to ioctls. It is also possible that
netfront and blkfront could race against each other in a way that causes
this, though I suspect that race would be much harder to trigger.
This has happened more than once so it is not a fluke due to e.g. cosmic
rays or other random bit-flips.
Marek, do you have any suggestions?
To me that sounds like the interface of the GUI is the culprit.
The GUI agent in the guest should only free a grant, if it got a message
from the backend that it can do so. Just assuming to be able to free it
because it isn't in use currently is the broken assumption here.
FWIW, I hit this issue twice already in this week CI run, while it never
happened before. The difference compared to previous run is Linux
5.15.57 vs 5.15.61. The latter reports persistent grants disabled.
I think this additional bug is just triggering the race in the GUI
interface more easily, as blkfront will allocate new grants with a
much higher frequency.
So fixing the persistent grant issue will just paper over the real
issue.
Indeed so, but making the bug happen much less frequently is still a
significant win for users.
Probably, yes.
In the long term, there is one situation I do not have a good solution
for: recovery from GUI agent crashes. If the GUI agent crashes, the
kernel it is running under has two bad choices. Either the kernel can
reclaim the grants, risking them being mapped at a later time by the GUI
daemon, or it can leak them, which is bad for obvious reasons. I
believe the current implementation makes the former choice.
It does.
I don't have enough information about the GUI architecture you are using.
Which components are involved on the backend side, and which on the
frontend side? Especially the responsibilities and communication regarding
grants is important here.
To fix this problem, I recommend the following changes:
1. Treat “backend has not unmapped grant” errors as non-fatal. The most
likely cause is buggy userspace software, not an attempt to exploit
XSA-396. Instead of disabling the device, just log a warning message
and put the grant on the deferred queue. Even leaking the grant
would be preferable to the current behavior, as disabling a block
device typically leaves the VM unusable.
Sorry, I don't agree. This is a major violation of the normal I/O
architecture. Your reasoning with the disabled block device doesn't make
much sense IMHO, as the mapped grant was due to a bad interface leading to
another component using a grant it was not meant to use.
Shutting down the block device is the right thing to do here, as data
corruption might be happening.
2. Ensure that the same grant being mapped twice is handled correctly.
At least Linux is known to have bugs in this regard.
I agree that this should be repaired.
3. Provide a means for a domain to be notified by Xen whenever one of
its grants is unmapped. Setting an event channel and writing to a
shared ring would suffice. This would allow eliminating the kludgy
deferred freeing mechanism.
Interesting idea.
I believe such an interface would need to be activated per grant, as
otherwise performance could suffer a lot. There are still some unused bits
in the grant flags, one could be used for that purpose.
I'm not sure how often this would be used. In case it is only for the rare
case of unexpectedly long mapped grant pages, a simple event might do the
job, with the event handler just skimming through the pending unmaps to
find the grants being available again.
Juergen
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