Re: [PATCH v2 3/3] kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd

[Christoffer Dall]

On Thu, Mar 16, 2017 at 06:20:51PM +0000, Suzuki K Poulose wrote:
> In kvm_free_stage2_pgd() we don't hold the kvm->mmu_lock while calling
> unmap_stage2_range() on the entire memory range for the guest. This could
> cause problems with other callers (e.g, munmap on a memslot) trying to
> unmap a range. And since we have to unmap the entire Guest memory range
> holding a spinlock, make sure we yield the lock if necessary, after we
> unmap each PUD range.
> 
> Fixes: commit d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
> Cc: stable@xxxxxxxxxxxxxxx # v3.10+
> Cc: Paolo Bonzini <pbonzin@xxxxxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> [ Avoid vCPU starvation and lockup detector warnings ]
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>

Reviewed-by: Christoffer Dall <cdall@xxxxxxxxxx>

> ---
>  arch/arm/kvm/mmu.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 13b9c1f..7628ef1 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -292,8 +292,14 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
>  	phys_addr_t addr = start, end = start + size;
>  	phys_addr_t next;
>  
> +	assert_spin_locked(&kvm->mmu_lock);
>  	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
>  	do {
> +		/*
> +		 * If the range is too large, release the kvm->mmu_lock
> +		 * to prevent starvation and lockup detector warnings.
> +		 */
> +		cond_resched_lock(&kvm->mmu_lock);
>  		next = stage2_pgd_addr_end(addr, end);
>  		if (!stage2_pgd_none(*pgd))
>  			unmap_stage2_puds(kvm, pgd, addr, next);
> @@ -831,7 +837,10 @@ void kvm_free_stage2_pgd(struct kvm *kvm)
>  	if (kvm->arch.pgd == NULL)
>  		return;
>  
> +	spin_lock(&kvm->mmu_lock);
>  	unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
> +	spin_unlock(&kvm->mmu_lock);
> +
>  	/* Free the HW pgd, one page at a time */
>  	free_pages_exact(kvm->arch.pgd, S2_PGD_SIZE);
>  	kvm->arch.pgd = NULL;
> -- 
> 2.7.4
>