[PATCH] KVM: MMU: Handle misaligned accesses to write protected guest page tables

A misaligned access affects two shadow ptes instead of just one.

Since a misaligned access is unlikely to occur on a real page table, just zap
the page out of existence, avoiding further trouble.

Signed-off-by: Avi Kivity <avi@qumranet.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 17 insertions, 2 deletions

diff --git a/drivers/kvm/mmu.c b/drivers/kvm/mmu.c
index 53c3643038bb..50b1432dceee 100644
--- a/drivers/kvm/mmu.c
+++ b/drivers/kvm/mmu.c
@@ -954,21 +954,36 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes)
         gfn_t gfn = gpa >> PAGE_SHIFT;
         struct kvm_mmu_page *page;
         struct kvm_mmu_page *child;
-        struct hlist_node *node;
+        struct hlist_node *node, *n;
         struct hlist_head *bucket;
         unsigned index;
         u64 *spte;
         u64 pte;
         unsigned offset = offset_in_page(gpa);
+        unsigned pte_size;
         unsigned page_offset;
+        unsigned misaligned;
         int level;
 
         pgprintk("%s: gpa %llx bytes %d\n", __FUNCTION__, gpa, bytes);
         index = kvm_page_table_hashfn(gfn) % KVM_NUM_MMU_PAGES;
         bucket = &vcpu->kvm->mmu_page_hash[index];
-        hlist_for_each_entry(page, node, bucket, hash_link) {
+        hlist_for_each_entry_safe(page, node, n, bucket, hash_link) {
                 if (page->gfn != gfn || page->role.metaphysical)
                         continue;
+                pte_size = page->role.glevels == PT32_ROOT_LEVEL ? 4 : 8;
+                misaligned = (offset ^ (offset + bytes - 1)) & ~(pte_size - 1);
+                if (misaligned) {
+                        /*
+                         * Misaligned accesses are too much trouble to fix
+                         * up; also, they usually indicate a page is not used
+                         * as a page table.
+                         */
+                        pgprintk("misaligned: gpa %llx bytes %d role %x\n",
+                                 gpa, bytes, page->role.word);
+                        kvm_mmu_zap_page(vcpu, page);
+                        continue;
+                }
                 page_offset = offset;
                 level = page->role.level;
                 if (page->role.glevels == PT32_ROOT_LEVEL) {