KVM: Harden against unpaired kvm_mmu_notifier_invalidate_range_end() calls

When handling the end of an mmu_notifier invalidation, WARN if
mn_active_invalidate_count is already 0 do not decrement it further, i.e.
avoid causing mn_active_invalidate_count to underflow/wrap.  In the worst
case scenario, effectively corrupting mn_active_invalidate_count could
cause kvm_swap_active_memslots() to hang indefinitely.

end() calls are *supposed* to be paired with start(), i.e. underflow can
only happen if there is a bug elsewhere in the kernel, but due to lack of
lockdep assertions in the mmu_notifier helpers, it's all too easy for a
bug to go unnoticed for some time, e.g. see the recently introduced
PAGEMAP_SCAN ioctl().

Ideally, mmu_notifiers would incorporate lockdep assertions, but users of
mmu_notifiers aren't required to hold any one specific lock, i.e. adding
the necessary annotations to make lockdep aware of all locks that are
mutally exclusive with mm_take_all_locks() isn't trivial.

Link: https://lore.kernel.org/all/000000000000f6d051060c6785bc@google.com
Link: https://lore.kernel.org/r/20240110004239.491290-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 10bfc88a69f72b6a0e310cca043fb04882e24eb1..8f03b56dafbdab27b0f5085c3ac7e82cc6531f99 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -890,7 +890,9 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
 
        /* Pairs with the increment in range_start(). */
        spin_lock(&kvm->mn_invalidate_lock);
-       wake = (--kvm->mn_active_invalidate_count == 0);
+       if (!WARN_ON_ONCE(!kvm->mn_active_invalidate_count))
+               --kvm->mn_active_invalidate_count;
+       wake = !kvm->mn_active_invalidate_count;
        spin_unlock(&kvm->mn_invalidate_lock);
 
        /*