Re: [PATCH] iommu/arm-smmu-v3: Align size in __arm_smmu_tlb_inv_range

[Nicolin Chen <nicolinc@nvidia.com>]

On Thu, Apr 14, 2022 at 11:32:38AM +0100, Robin Murphy wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 2022-04-13 21:19, Nicolin Chen wrote:
> > Hi Robin,
> > 
> > On Wed, Apr 13, 2022 at 02:40:31PM +0100, Robin Murphy wrote:
> > > On 2022-04-13 05:17, Nicolin Chen wrote:
> > > > To calculate num_pages, the size should be aligned with
> > > > "page size", determined by the tg value. Otherwise, its
> > > > following "while (iova < end)" might become an infinite
> > > > loop if unaligned size is slightly greater than 1 << tg.
> > > 
> > > Hmm, how does a non-page-aligned invalidation request get generated in
> > > the first place?
> > 
> > I don't have the testing environment because it was a bug,
> > reported by a client who uses SVA feature on top of SMMU.
> > 
> > But judging from the log, the non-page-aligned inv request
> > was coming from an likely incorrect end address, e.g.
> >       { start = 0xff10000, end = 0xff20000 }
> > So the size turned out to be 0x10001, unaligned.
> > 
> > I don't have a full call trace on hand right now to see if
> > upper callers are doing something wrong when calculate the
> > end address, though I've asked the owner to check.
> > 
> > By looking at the call trace within arm_smmu_* functions:
> >    __arm_smmu_tlb_inv_range
> >    arm_smmu_tlb_inv_range_asid
> >    arm_smmu_mm_invalidate_range
> >    {from mm_notifier_* functions}
> > 
> > There's no address alignment check. Although I do think we
> > should fix the source who passes down the non-page-aligned
> > parameter, the SMMU driver shouldn't silently dead loop if
> > a set of unaligned inputs are given, IMHO.
> 
> Oh, sure, I'm not saying we definitely don't need to fix anything, I'd
> just like to get a better understanding of *what* we're fixing. I'd have
> (naively) expected the mm layer to give us page-aligned quantities even
> in the SVA notifier case, so if we've got a clear off-by-one somewhere
> in that path we should fix that before just blindly over-invalidating to
> paper over it;

I see. Yea, definitely should fix the source too. I've asked
the owner to track it down. I sent the change, thinking that
we could do it in parallel.

> if we still also want to be robust at the SMMU driver end
> just in case, something like "if (WARN_ON(num_pages == 0)) num_pages =
> 1;" might be more appropriate. However if it turns out that we *can*
> actually end up with unsanitised input from some userspace unmap
> interface getting this far, then a silent fixup is the best option, but
> if so I'd still like to confirm that we're rounding in the same
> direction as whoever touched the pagetables (since it can't have been us).

I see. I'll give an update once I have the debugging result.

Thanks!
Nic