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@xxxxxxxxxx Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- virt/kvm/kvm_main.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 10bfc88a69f7..8f03b56dafbd 100644 --- 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); /* base-commit: 1c6d984f523f67ecfad1083bb04c55d91977bb15 -- 2.43.0.472.g3155946c3a-goog
