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

[Marc Zyngier]

On Wed, 03 Mar 2021 11:29:34 +0000,
Will Deacon <will@xxxxxxxxxx> wrote:
> 
> On Wed, Mar 03, 2021 at 10:42:25AM +0800, Jia He 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.
> > 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.
> > 
> > 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
> > 
> > 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);
> 
> Can you replace both of these lines with:
> 
> 	data->addr = ALIGN(data->addr, kvm_granule_size(level));
> 
> instead?

Seems like a good option. I also took the liberty to rewrite the
commit message in an effort to make it a bit clearer.

Jia, please let me know if you are OK with these cosmetic changes.


Thanks,

	M.