Re: [PATCH 5.10.y 04/17] KVM: x86/mmu: Ensure MMU pages are available when allocating roots

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The patch introduces a sleep-in-atomic bug by holding `mmu_lock` (a spinlock) while reading guest PDPTRs, which can sleep.
--

commit 2d0be26b413397b6f1a85adc0319fa2727427ab6
Author: Sean Christopherson <seanjc@google.com>

KVM: x86/mmu: Ensure MMU pages are available when allocating roots

This patch modifies the root allocation flow to hold the mmu_lock for write
for the entire duration of allocating and initializing an MMU's roots. This
prevents root allocations from failing by ensuring MMU pages are available.

> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index a506c0818e77a..9b1f63b5e86ec 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c

[ ... ]

> @@ -3315,6 +3306,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  
>  	for (i = 0; i < 4; ++i) {
>  		MMU_WARN_ON(VALID_PAGE(mmu->pae_root[i]));
> +
>  		if (mmu->root_level == PT32E_ROOT_LEVEL) {
>  			pdptr = mmu->get_pdptr(vcpu, i);

[Severity: High]
Does this code sleep while holding a spinlock?

Since kvm_mmu_load() now holds vcpu->kvm->mmu_lock when calling
mmu_alloc_shadow_roots(), this ends up calling mmu->get_pdptr()
under a spinlock.

On SVM, this resolves to kvm_pdptr_read(), which can sleep:

arch/x86/kvm/kvm_cache_regs.h:kvm_pdptr_read() {
    might_sleep();
    ...
}

Could this lead to a "scheduling while atomic" panic?

>  			if (!(pdptr & PT_PRESENT_MASK)) {

[ ... ]

> @@ -4872,7 +4854,15 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
>  	r = mmu_alloc_special_roots(vcpu);
>  	if (r)
>  		goto out;
> -	r = mmu_alloc_roots(vcpu);
> +	spin_lock(&vcpu->kvm->mmu_lock);
> +	if (make_mmu_pages_available(vcpu))
> +		r = -ENOSPC;
> +	else if (vcpu->arch.mmu->direct_map)
> +		r = mmu_alloc_direct_roots(vcpu);
> +	else
> +		r = mmu_alloc_shadow_roots(vcpu);
> +	spin_unlock(&vcpu->kvm->mmu_lock);
> +
>  	kvm_mmu_sync_roots(vcpu);
>  	if (r)
>  		goto out;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260626112634.1778506-1-pbonzini@redhat.com?part=4