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

[Justin He]

Hi Quentin and Marc
I noticed Marc had sent out new version on behalf of me, thanks for the help.
I hated the time difference, sorry for the late.

Just answer the comments below to make it clear.

> -----Original Message-----
> From: Quentin Perret <qperret@xxxxxxxxxx>
> Sent: Wednesday, March 3, 2021 7:09 PM
> To: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Justin He <Justin.He@xxxxxxx>; kvmarm@xxxxxxxxxxxxxxxxxxxxx; James
> Morse <James.Morse@xxxxxxx>; Julien Thierry <julien.thierry.kdev@xxxxxxxxx>;
> Suzuki Poulose <Suzuki.Poulose@xxxxxxx>; Catalin Marinas
> <Catalin.Marinas@xxxxxxx>; Will Deacon <will@xxxxxxxxxx>; Gavin Shan
> <gshan@xxxxxxxxxx>; Yanan Wang <wangyanan55@xxxxxxxxxx>; linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu walking
> 
> 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@xxxxxxx> 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.
> >

Yes, exactly the case

> > 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.

Yes, stage2_pgd_addr_end() was still there but the stage2_pmd_addr_end() was removed.
> 
> > > 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.

Okay

> >
> > >
> > > Reported-by: Howard Zhang <Howard.Zhang@xxxxxxx>
> > > Signed-off-by: Jia He <justin.he@xxxxxxx>
> > > ---
> > >  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