Re: [PATCH v2 5/8] KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Acquiring `mmu_lock` with preemption disabled causes a sleep-in-atomic bug on PREEMPT_RT kernels.
--

--- Patch [5]: [PATCH v2 5/8] KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping ---
commit 7a7c469ac4158e77c3548a526ab62a924ed75161
Author: Marc Zyngier <[email protected]>

KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping

This patch addresses a race condition between VNCR TLB invalidation and VNCR
unmapping during vcpu_put(). It introduces an atomic exchange for resetting
the cpu tracking field and a common function for unmapping, ensuring these
operations happen atomically. This closes the TOCTOU race and prevents
concurrent mapping faults.

Link: https://lore.kernel.org/r/[email protected]

> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 67cc5b6d80705..05f0e7e89e982 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c

[ ... ]

> @@ -1584,8 +1607,6 @@ static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)

[Severity: High]
This is a pre-existing issue, but does acquiring mmu_lock inside a
preemption-disabled region here cause a sleep-in-atomic bug on RT kernels?

Looking at kvm_map_l1_vncr() from the context, it starts with:

static void kvm_map_l1_vncr(struct kvm_vcpu *vcpu)
{
        struct vncr_tlb *vt = vcpu->arch.vncr_tlb;
        pgprot_t prot;

        guard(preempt)();
        guard(read_lock)(&vcpu->kvm->mmu_lock);

Since read_lock() is implemented as an RT-mutex on PREEMPT_RT and can block,
could this sequence trigger a scheduling-while-atomic panic if the lock is
contended?

>  	if (vt->wr.nG && get_asid_by_regime(vcpu, TR_EL20) != vt->wr.asid)
>  		return;
>  
> -	vt->cpu = smp_processor_id();
> -
>  	if (vt->hpa_writable && vt->wr.pw && vt->wr.pr)
>  		prot = PAGE_KERNEL;
>  	else if (vt->wr.pr)

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=5