Re: “Backend has not unmapped grant” errors

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

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.

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.

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.

2. Ensure that the same grant being mapped twice is handled correctly.
   At least Linux is known to have bugs in this regard.

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.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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