KVM: MMU: fix direct sp's access corrupted

If the mapping is writable but the dirty flag is not set, we will find
the read-only direct sp and setup the mapping, then if the write #PF
occur, we will mark this mapping writable in the read-only direct sp,
now, other real read-only mapping will happily write it without #PF.

It may hurt guest's COW

Fixed by re-install the mapping when write #PF occur.

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 26 insertions, 2 deletions

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index f4e4aaa65ff3..117d63f6304d 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -325,8 +325,32 @@ static u64 *FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
                         break;
                 }
 
-                if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep))
-                        continue;
+                if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) {
+                        struct kvm_mmu_page *child;
+                        unsigned direct_access;
+
+                        if (level != gw->level)
+                                continue;
+
+                        /*
+                         * For the direct sp, if the guest pte's dirty bit
+                         * changed form clean to dirty, it will corrupt the
+                         * sp's access: allow writable in the read-only sp,
+                         * so we should update the spte at this point to get
+                         * a new sp with the correct access.
+                         */
+                        direct_access = gw->pt_access & gw->pte_access;
+                        if (!is_dirty_gpte(gw->ptes[gw->level - 1]))
+                                direct_access &= ~ACC_WRITE_MASK;
+
+                        child = page_header(*sptep & PT64_BASE_ADDR_MASK);
+                        if (child->role.access == direct_access)
+                                continue;
+
+                        mmu_page_remove_parent_pte(child, sptep);
+                        __set_spte(sptep, shadow_trap_nonpresent_pte);
+                        kvm_flush_remote_tlbs(vcpu->kvm);
+                }
 
                 if (is_large_pte(*sptep)) {
                         rmap_remove(vcpu->kvm, sptep);