Re: KVM: MMU: update sp->gfns on pte update path

Avi Kivity
2011-01-25T14:32:38Z

On 01/25/2011 03:07 PM, Marcelo Tosatti wrote:
> If an emulated pte write modifies the gpa of a present spte, sp->gfns is
> not updated, retaining a stale value which later leads to:
>
> rmap_remove: ffff8807d245fff8 0->BUG
> ------------[ cut here ]------------
> kernel BUG at arch/x86/kvm/mmu.c:695!
>
> Fix by updating sp->gfns even if spte was present.
>
> Resolves: https://bugzilla.kernel.org/show_bug.cgi?id=27052
> Reported-and-tested-by: Nicolas Prochazka<prochazka.nicolas@gmail.com>
> KVM-Stable-Tag.
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index cc1bada..37d0886 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2054,6 +2054,12 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> rmap_count = rmap_add(vcpu, sptep, gfn);
> if (rmap_count> RMAP_RECYCLE_THRESHOLD)
> rmap_recycle(vcpu, sptep, gfn);
> + } else {
> + struct kvm_mmu_page *sp = page_header(__pa(sptep));
> + int index = sptep - sp->spt;
> +
> + if (!sp->role.direct&& sp->gfns[index] != gfn)
> + sp->gfns[index] = gfn;
> }
> kvm_release_pfn_clean(pfn);
> if (speculative) {
>

Should be done by a call to kvm_mmu_page_set_gfn(). But I don't
understand how it could become inconsistent in the first place.

if (is_rmap_spte(*sptep)) {
/*
* If we overwrite a PTE page pointer with a 2MB PMD, unlink
* the parent of the now unreachable PTE.
*/
if (level > PT_PAGE_TABLE_LEVEL &&
!is_large_pte(*sptep)) {
struct kvm_mmu_page *child;
u64 pte = *sptep;

child = page_header(pte & PT64_BASE_ADDR_MASK);
mmu_page_remove_parent_pte(child, sptep);
__set_spte(sptep, shadow_trap_nonpresent_pte);
kvm_flush_remote_tlbs(vcpu->kvm);
} else if (pfn != spte_to_pfn(*sptep)) {
pgprintk("hfn old %llx new %llx\n",
spte_to_pfn(*sptep), pfn);
drop_spte(vcpu->kvm, sptep, shadow_trap_nonpresent_pte);
kvm_flush_remote_tlbs(vcpu->kvm);
} else
was_rmapped = 1;
}

If we set was_rmapped, that means rmap_add() was previously called for
this spte/gfn/pfn pair, and all that changes is permissions, no?

--
error compiling committee.c: too many arguments to function