Re: [PATCH v2] RISC-V: KVM: Document a TOCTOU race in SBI system suspend handler

On Mon, May 25, 2026 at 7:06 AM Jiakai Xu <[email protected]> wrote:
>
> The SUSP handler checks that all other vCPUs are stopped before
> entering system suspend, but a concurrent HSM HART_START can start
> a vCPU after it has already passed the check.
>
> This is a known TOCTOU race. We do not fix it because:
> 1. Triggering it requires a pathological guest.
> 2. Only guest state is at risk, not host integrity.
> 3. Userspace can double-check vCPU states before suspend.
>
> Add a comment documenting the race and the rationale for not fixing it.
>
> Signed-off-by: Jiakai Xu <[email protected]>
> Signed-off-by: Jiakai Xu <[email protected]>
> Assisted-by: YuanSheng:DeepSeek-V3.2

Queued this patch for Linux-7.2

Thanks,
Anup

> ---
> V1 -> V2:
> - Replaced the fix with a comment.
> Link: https://lore.kernel.org/linux-riscv/[email protected]/t/#u
> ---
>  arch/riscv/kvm/vcpu_sbi_system.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/arch/riscv/kvm/vcpu_sbi_system.c b/arch/riscv/kvm/vcpu_sbi_system.c
> index c6f7e609ac794..6f64a59e5d3c4 100644
> --- a/arch/riscv/kvm/vcpu_sbi_system.c
> +++ b/arch/riscv/kvm/vcpu_sbi_system.c
> @@ -35,6 +35,20 @@ static int kvm_sbi_ext_susp_handler(struct kvm_vcpu *vcpu, struct kvm_run *run,
>                         return 0;
>                 }
>
> +               /*
> +                * Check that all other vCPUs are stopped before entering
> +                * system suspend.
> +                *
> +                * There is a known TOCTOU race here: a concurrent HSM
> +                * HART_START on another vCPU can start a vCPU after it
> +                * has already passed this check, violating the invariant.
> +                *
> +                * We do not fix this because:
> +                * 1. Triggering the race requires a pathological guest.
> +                * 2. Only guest state is at risk, not host integrity.
> +                * 3. Userspace can double-check vCPU states before
> +                *    proceeding with suspend.
> +                */
>                 kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
>                         if (tmp == vcpu)
>                                 continue;
> --
> 2.34.1
>

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