Re: [PATCH] KVM: x86/mmu: Take slots_lock when using kvm_mmu_zap_all_fast()

[Paolo Bonzini]

On 13/11/19 20:30, Sean Christopherson wrote:
> Acquire the per-VM slots_lock when zapping all shadow pages as part of
> toggling nx_huge_pages.  The fast zap algorithm relies on exclusivity
> (via slots_lock) to identify obsolete vs. valid shadow pages, e.g. it
> uses a single bit for its generation number.  Holding slots_lock also
> obviates the need to acquire a read lock on the VM's srcu.
> 
> Failing to take slots_lock when toggling nx_huge_pages allows multiple
> instances of kvm_mmu_zap_all_fast() to run concurrently, as the other
> user, KVM_SET_USER_MEMORY_REGION, does not take the global kvm_lock.
> Concurrent fast zap instances causes obsolete shadow pages to be
> incorrectly identified as valid due to the single bit generation number
> wrapping, which results in stale shadow pages being left in KVM's MMU
> and leads to all sorts of undesirable behavior.
> 
> The bug is easily confirmed by running with CONFIG_PROVE_LOCKING and
> toggling nx_huge_pages via its module param.
> 
> Note, the fast zap algorithm could use a 64-bit generation instead of
> relying on exclusivity for correctness, but all callers except the
> recently added set_nx_huge_pages() need to hold slots_lock anyways.
> Given that toggling nx_huge_pages is by no means a fast path, force it
> to conform to the current approach instead of reworking the algorithm to
> support concurrent calls.
> 
> Fixes: b8e8c8303ff28 ("kvm: mmu: ITLB_MULTIHIT mitigation")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>  arch/x86/kvm/mmu.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index cf718fa23dff..2ce9da58611e 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -6285,14 +6285,13 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
>  
>  	if (new_val != old_val) {
>  		struct kvm *kvm;
> -		int idx;
>  
>  		mutex_lock(&kvm_lock);
>  
>  		list_for_each_entry(kvm, &vm_list, vm_list) {
> -			idx = srcu_read_lock(&kvm->srcu);
> +			mutex_lock(&kvm->slots_lock);
>  			kvm_mmu_zap_all_fast(kvm);
> -			srcu_read_unlock(&kvm->srcu, idx);
> +			mutex_unlock(&kvm->slots_lock);
>  
>  			wake_up_process(kvm->arch.nx_lpage_recovery_thread);
>  		}
> 

Queued, thanks.

Paolo