[PATCH 5.15 073/207] KVM: Ensure local memslot copies operate on up-to-date arch-specific data

Greg Kroah-Hartman
2021-12-06T15:43:06Z

From: Sean Christopherson <seanjc@google.com>

commit bda44d844758c70c8dc1478e6fc9c25efa90c5a7 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
virt/kvm/kvm_main.c | 47 +++++++++++++++++++++++++++++++----------------
1 file changed, 31 insertions(+), 16 deletions(-)

--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1523,11 +1523,10 @@ static struct kvm_memslots *kvm_dup_mems

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;

@@ -1558,7 +1557,7 @@ static int kvm_set_memslot(struct kvm *k
* 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;

/*
@@ -1589,6 +1588,26 @@ static int kvm_set_memslot(struct kvm *k
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;
@@ -1596,14 +1615,18 @@ static int kvm_set_memslot(struct kvm *k
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 {
@@ -1618,7 +1641,6 @@ static int kvm_delete_memslot(struct kvm
struct kvm_memory_slot *old, int as_id)
{
struct kvm_memory_slot new;
- int r;

if (!old->npages)
return -EINVAL;
@@ -1631,12 +1653,7 @@ static int kvm_delete_memslot(struct 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);
}

/*
@@ -1711,7 +1728,6 @@ int __kvm_set_memory_region(struct 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) ||
@@ -1725,9 +1741,8 @@ int __kvm_set_memory_region(struct 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)) {
@@ -1753,7 +1768,7 @@ int __kvm_set_memory_region(struct 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;