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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/[email protected] Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Sean Christopherson <[email protected]>
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