Re: [PATCH for-4.15 2/4] xen/iommu: x86: Free the IOMMU page-tables with the pgtables.lock held

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 23/12/2020 13:48, Jan Beulich wrote:
> On 22.12.2020 16:43, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > The pgtables.lock is protecting access to the page list pgtables.list.
> > However, iommu_free_pgtables() will not held it. I guess it was assumed
> > that page-tables cannot be allocated while the domain is dying.
> >
> > Unfortunately, there is no guarantee that iommu_map() will not be
> > called while a domain is dying (it looks like to be possible from
> > XEN_DOMCTL_memory_mapping).
> I'd rather disallow any new allocations for a dying domain, not
> the least because ...
Patch #4 will disallow such allocation. However...

> > So it would be possible to be concurrently
> > allocate memory and free the page-tables.
> >
> > Therefore, we need to held the lock when freeing the page tables.
> ... we should try to avoid holding locks across allocation /
> freeing functions wherever possible. >
> As to where to place a respective check - I wonder if we wouldn't
> be better off disallowing a majority of domctl-s (and perhaps
> other operations) on dying domains. Thoughts?
... this is still pretty racy because you need to guarantee that 
d->is_dying is seen by the other processors to prevent allocation.
As to whether we can forbid most of the domctl-s, I would agree this is 
a good move. But this doesn't remove the underlying problem here.
We are hoping that a top-level function will protect us against the 
race. Given the IOMMU code is quite deep in the callstack, this is 
something pretty hard to guarantee with future change.
So I still think we need a way to mitigate the issue.

Cheers,

--
Julien Grall