Re: [PATCH v2] RISC-V: KVM: Fix shift-out-of-bounds in make_xfence_request()

[Andrew Jones]

On Fri, Apr 03, 2026 at 11:20:11PM +0000, Jiakai Xu wrote:
> The make_xfence_request() function uses a shift operation to check if a
> vCPU is in the hart mask:
> 
>   if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
> 
> However, when the difference between vcpu_id and hbase
> is >= BITS_PER_LONG, the shift operation causes undefined behavior.
> 
> This was detected by UBSAN:
>   UBSAN: shift-out-of-bounds in arch/riscv/kvm/tlb.c:343:23
>   shift exponent 256 is too large for 64-bit type 'long unsigned int'
> 
> Fix this by adding a bounds check before the shift operation.
> 
> This bug was found by fuzzing the KVM RISC-V interface.
> 
> Fixes: 13acfec2dbcc ("RISC-V: KVM: Add remote HFENCE functions based on VCPU requests")
> Signed-off-by: Jiakai Xu <jiakaiPeanut@xxxxxxxxx>
> Signed-off-by: Jiakai Xu <xujiakai2025@xxxxxxxxxxx>
> ---
> V1 -> V2:
> - Dropped 'idx' variable and compared vcpu_id against hbase directly,
>   as suggested by Andrew Jones.
> ---
>  arch/riscv/kvm/tlb.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
> index ff1aeac4eb8eb..439c20c2775ab 100644
> --- a/arch/riscv/kvm/tlb.c
> +++ b/arch/riscv/kvm/tlb.c
> @@ -338,7 +338,8 @@ static void make_xfence_request(struct kvm *kvm,
>  	bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		if (hbase != -1UL) {
> -			if (vcpu->vcpu_id < hbase)
> +			if (vcpu->vcpu_id < hbase ||
> +				vcpu->vcpu_id >= hbase + BITS_PER_LONG)
>  				continue;
>  			if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
>  				continue;
> -- 
> 2.34.1
>

Reviewed-by: Andrew Jones <andrew.jones@xxxxxxxxxxxxxxxx>