Re: [PATCH 3/5] KVM: x86/mmu: Harden TDP MMU iteration against root w/o shadow page

[Sean Christopherson]

On Tue, Jul 25, 2023, Yu Zhang wrote:
> On Fri, Jul 21, 2023 at 06:23:48PM -0700, Sean Christopherson wrote:
> > Explicitly check that tdp_iter_start() is handed a valid shadow page
> > to harden KVM against bugs where
> 
> Sorry, where? 

Gah, I must have seen something shiny when writing the changelog.

> It's not about guest using an invisible GFN, it's about a KVM bug, right?

Yes, the intent is to guard against a KVM bug, e.g. if KVM managed to get into
the TDP MMU with an invalid root, or a root belonging to a shadow MMU.  I'll fix
the changelog in v2.

> > Opportunistically stop the TDP MMU iteration instead of continuing on
> > with garbage if the incoming root is bogus.  Attempting to walk a garbage
> > root is more likely to caused major problems than doing nothing.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/tdp_iter.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
> > index d2eb0d4f8710..bd30ebfb2f2c 100644
> > --- a/arch/x86/kvm/mmu/tdp_iter.c
> > +++ b/arch/x86/kvm/mmu/tdp_iter.c
> > @@ -39,13 +39,14 @@ void tdp_iter_restart(struct tdp_iter *iter)
> >  void tdp_iter_start(struct tdp_iter *iter, struct kvm_mmu_page *root,
> >  		    int min_level, gfn_t next_last_level_gfn)
> >  {
> > -	int root_level = root->role.level;
> > -
> > -	WARN_ON(root_level < 1);
> > -	WARN_ON(root_level > PT64_ROOT_MAX_LEVEL);
> > +	if (WARN_ON_ONCE(!root || (root->role.level < 1) ||
> > +			 (root->role.level > PT64_ROOT_MAX_LEVEL))) {
> > +		iter->valid = false;
> > +		return;
> > +	}
> >  
> 
> I saw many usages of WARN_ON_ONCE() and WARN_ON() in KVM. And just wonder,
> is there any criteria for KVM when to use which?

Heh, already a step ahead of you :-)

https://lore.kernel.org/all/20230721230006.2337941-5-seanjc@xxxxxxxxxx