Re: [PATCH 1/2] RISC-V: KVM: Fix array out-of-bounds in pmu_ctr_read()

[Andrew Jones]

On Fri, Mar 06, 2026 at 07:37:38AM +0000, Jiakai Xu wrote:
> When a guest invokes SBI_EXT_PMU_COUNTER_FW_READ on a firmware counter
> that has not been configured via SBI_EXT_PMU_COUNTER_CFG_MATCH, the
> pmc->event_idx remains SBI_PMU_EVENT_IDX_INVALID (0xFFFFFFFF).
> get_event_code() extracts the lower 16 bits, yielding 0xFFFF (65535),
> which is then used to index into kvpmu->fw_event[]. Since fw_event is
> only RISCV_KVM_MAX_FW_CTRS (32) entries, this triggers an
> array-index-out-of-bounds:
> 
>   UBSAN: array-index-out-of-bounds in arch/riscv/kvm/vcpu_pmu.c:255:37
>   index 65535 is out of range for type 'kvm_fw_event [32]'
> 
> Add a bounds check on fevent_code before accessing the fw_event array,
> returning -EINVAL for invalid event codes.
> 
> Fixes: badc386869e2c ("RISC-V: KVM: Support firmware events")
> Signed-off-by: Jiakai Xu <xujiakai2025@xxxxxxxxxxx>
> Signed-off-by: Jiakai Xu <jiakaiPeanut@xxxxxxxxx>
> ---
>  arch/riscv/kvm/vcpu_pmu.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/riscv/kvm/vcpu_pmu.c b/arch/riscv/kvm/vcpu_pmu.c
> index e873430e596b..c6d42459c2a1 100644
> --- a/arch/riscv/kvm/vcpu_pmu.c
> +++ b/arch/riscv/kvm/vcpu_pmu.c
> @@ -252,6 +252,10 @@ static int pmu_ctr_read(struct kvm_vcpu *vcpu, unsigned long cidx,
>  
>  	if (pmc->cinfo.type == SBI_PMU_CTR_TYPE_FW) {
>  		fevent_code = get_event_code(pmc->event_idx);
> +		if (fevent_code >= SBI_PMU_FW_MAX) {
> +			pr_warn("Invalid firmware event code [%d] for counter [%ld]\n", fevent_code, cidx);

We shouldn't add warnings for these types of things since a guest could
fill the host's log buffer. At most it could be a warn-once, but I think
we should just return the error as quickly as possible. It looks like
there are other pr_warns in the pmu emulation code that should probably
be audited to see if they should be removed or changed to warn-once as
well.

Thanks,
drew

> +			return -EINVAL;
> +		}
>  		pmc->counter_val = kvpmu->fw_event[fevent_code].value;
>  	} else if (pmc->perf_event) {
>  		pmc->counter_val += perf_event_read_value(pmc->perf_event, &enabled, &running);
> -- 
> 2.34.1
> 
> 
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