Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu walking

[Quentin Perret <qperret@google.com>]

On Wednesday 03 Mar 2021 at 09:54:25 (+0000), Marc Zyngier wrote:
> Hi Jia,
> 
> On Wed, 03 Mar 2021 02:42:25 +0000,
> Jia He <justin.he@arm.com> wrote:
> > 
> > If the start addr is not aligned with the granule size of that level.
> > loop step size should be adjusted to boundary instead of simple
> > kvm_granual_size(level) increment. Otherwise, some mmu entries might miss
> > the chance to be walked through.
> > E.g. Assume the unmap range [data->addr, data->end] is
> > [0xff00ab2000,0xff00cb2000] in level 2 walking and NOT block mapping.
> 
> When does this occur? Upgrade from page mappings to block? Swap out?
> 
> > And the 1st part of that pmd entry is [0xff00ab2000,0xff00c00000]. The
> > pmd value is 0x83fbd2c1002 (not valid entry). In this case, data->addr
> > should be adjusted to 0xff00c00000 instead of 0xff00cb2000.
> 
> Let me see if I understand this. Assuming 4k pages, the region
> described above spans *two* 2M entries:
> 
> (a) ff00ab2000-ff00c00000, part of ff00a00000-ff00c00000
> (b) ff00c00000-ff00db2000, part of ff00c00000-ff00e00000
> 
> (a) has no valid mapping, but (b) does. Because we fail to correctly
> align on a block boundary when skipping (a), we also skip (b), which
> is then left mapped.
> 
> Did I get it right? If so, yes, this is... annoying.
> 
> Understanding the circumstances this triggers in would be most
> interesting. This current code seems to assume that we get ranges
> aligned to mapping boundaries, but I seem to remember that the old
> code did use the stage2_*_addr_end() helpers to deal with this case.
> 
> Will: I don't think things have changed in that respect, right?

Indeed we should still use stage2_*_addr_end(), especially in the unmap
path that is mentioned here, so it would be helpful to have a little bit
more context.

> > Without this fix, userspace "segment fault" error can be easily
> > triggered by running simple gVisor runsc cases on an Ampere Altra
> > server:
> >     docker run --runtime=runsc -it --rm  ubuntu /bin/bash
> > 
> > In container:
> >     for i in `seq 1 100`;do ls;done
> 
> The workload on its own isn't that interesting. What I'd like to
> understand is what happens on the host during that time.
> 
> > 
> > Reported-by: Howard Zhang <Howard.Zhang@arm.com>
> > Signed-off-by: Jia He <justin.he@arm.com>
> > ---
> >  arch/arm64/kvm/hyp/pgtable.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index bdf8e55ed308..4d99d07c610c 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -225,6 +225,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
> >  		goto out;
> >  
> >  	if (!table) {
> > +		data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level));
> >  		data->addr += kvm_granule_size(level);
> >  		goto out;
> >  	}
> 
> It otherwise looks good to me. Quentin, Will: unless you object to
> this, I plan to take it in the next round of fixes with

Though I'm still unsure how we hit that today, the change makes sense on
its own I think, so no objection from me.

Thanks,
Quentin
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