KVM: Ensure local memslot copies operate on up-to-date arch-specific data

When modifying memslots, snapshot the "old" memslot and copy it to the
"new" memslot's arch data after (re)acquiring slots_arch_lock.  x86 can
change a memslot's arch data while memslot updates are in-progress so
long as it holds slots_arch_lock, thus snapshotting a memslot without
holding the lock can result in the consumption of stale data.

Fixes: b10a038e84d1 ("KVM: mmu: Add slots_arch_lock for memslot arch fields")
Cc: stable@vger.kernel.org
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211104002531.1176691-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9646bb9..2104fc2 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1531,11 +1531,10 @@ static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
 
 static int kvm_set_memslot(struct kvm *kvm,
                           const struct kvm_userspace_memory_region *mem,
-                          struct kvm_memory_slot *old,
                           struct kvm_memory_slot *new, int as_id,
                           enum kvm_mr_change change)
 {
-       struct kvm_memory_slot *slot;
+       struct kvm_memory_slot *slot, old;
        struct kvm_memslots *slots;
        int r;
 
@@ -1566,7 +1565,7 @@ static int kvm_set_memslot(struct kvm *kvm,
                 * Note, the INVALID flag needs to be in the appropriate entry
                 * in the freshly allocated memslots, not in @old or @new.
                 */
-               slot = id_to_memslot(slots, old->id);
+               slot = id_to_memslot(slots, new->id);
                slot->flags |= KVM_MEMSLOT_INVALID;
 
                /*
@@ -1597,6 +1596,26 @@ static int kvm_set_memslot(struct kvm *kvm,
                kvm_copy_memslots(slots, __kvm_memslots(kvm, as_id));
        }
 
+       /*
+        * Make a full copy of the old memslot, the pointer will become stale
+        * when the memslots are re-sorted by update_memslots(), and the old
+        * memslot needs to be referenced after calling update_memslots(), e.g.
+        * to free its resources and for arch specific behavior.  This needs to
+        * happen *after* (re)acquiring slots_arch_lock.
+        */
+       slot = id_to_memslot(slots, new->id);
+       if (slot) {
+               old = *slot;
+       } else {
+               WARN_ON_ONCE(change != KVM_MR_CREATE);
+               memset(&old, 0, sizeof(old));
+               old.id = new->id;
+               old.as_id = as_id;
+       }
+
+       /* Copy the arch-specific data, again after (re)acquiring slots_arch_lock. */
+       memcpy(&new->arch, &old.arch, sizeof(old.arch));
+
        r = kvm_arch_prepare_memory_region(kvm, new, mem, change);
        if (r)
                goto out_slots;
@@ -1604,14 +1623,18 @@ static int kvm_set_memslot(struct kvm *kvm,
        update_memslots(slots, new, change);
        slots = install_new_memslots(kvm, as_id, slots);
 
-       kvm_arch_commit_memory_region(kvm, mem, old, new, change);
+       kvm_arch_commit_memory_region(kvm, mem, &old, new, change);
+
+       /* Free the old memslot's metadata.  Note, this is the full copy!!! */
+       if (change == KVM_MR_DELETE)
+               kvm_free_memslot(kvm, &old);
 
        kvfree(slots);
        return 0;
 
 out_slots:
        if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
-               slot = id_to_memslot(slots, old->id);
+               slot = id_to_memslot(slots, new->id);
                slot->flags &= ~KVM_MEMSLOT_INVALID;
                slots = install_new_memslots(kvm, as_id, slots);
        } else {
@@ -1626,7 +1649,6 @@ static int kvm_delete_memslot(struct kvm *kvm,
                              struct kvm_memory_slot *old, int as_id)
 {
        struct kvm_memory_slot new;
-       int r;
 
        if (!old->npages)
                return -EINVAL;
@@ -1639,12 +1661,7 @@ static int kvm_delete_memslot(struct kvm *kvm,
         */
        new.as_id = as_id;
 
-       r = kvm_set_memslot(kvm, mem, old, &new, as_id, KVM_MR_DELETE);
-       if (r)
-               return r;
-
-       kvm_free_memslot(kvm, old);
-       return 0;
+       return kvm_set_memslot(kvm, mem, &new, as_id, KVM_MR_DELETE);
 }
 
 /*
@@ -1718,7 +1735,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
        if (!old.npages) {
                change = KVM_MR_CREATE;
                new.dirty_bitmap = NULL;
-               memset(&new.arch, 0, sizeof(new.arch));
        } else { /* Modify an existing slot. */
                if ((new.userspace_addr != old.userspace_addr) ||
                    (new.npages != old.npages) ||
@@ -1732,9 +1748,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
                else /* Nothing to change. */
                        return 0;
 
-               /* Copy dirty_bitmap and arch from the current memslot. */
+               /* Copy dirty_bitmap from the current memslot. */
                new.dirty_bitmap = old.dirty_bitmap;
-               memcpy(&new.arch, &old.arch, sizeof(new.arch));
        }
 
        if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {
@@ -1760,7 +1775,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
                        bitmap_set(new.dirty_bitmap, 0, new.npages);
        }
 
-       r = kvm_set_memslot(kvm, mem, &old, &new, as_id, change);
+       r = kvm_set_memslot(kvm, mem, &new, as_id, change);
        if (r)
                goto out_bitmap;