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

[Andrew Jones]

On Fri, Apr 03, 2026 at 06:13:02AM +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>
> ---
>  arch/riscv/kvm/tlb.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
> index ff1aeac4eb8eb..500e001513a11 100644
> --- a/arch/riscv/kvm/tlb.c
> +++ b/arch/riscv/kvm/tlb.c
> @@ -333,14 +333,17 @@ static void make_xfence_request(struct kvm *kvm,
>  	unsigned long i;
>  	struct kvm_vcpu *vcpu;
>  	unsigned int actual_req = req;
> +	unsigned int idx;
>  	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
>  
>  	bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		if (hbase != -1UL) {
> -			if (vcpu->vcpu_id < hbase)
> +			idx = vcpu->vcpu_id - hbase;
> +
> +			if (idx < 0 || idx >= BITS_PER_LONG)

idx is unsigned, so I would expect this 'idx < 0' to produce a warning
that it's always false.

I wouldn't introduce idx and just change the current condition instead

  if (vcpu->vcpu_id < hbase || vcpu->vcpu_id >= hbase + BITS_PER_LONG)
     continue;

>  				continue;
> -			if (!(hmask & (1UL << (vcpu->vcpu_id - hbase))))
> +			if (!(hmask & (1UL << idx)))
>  				continue;
>  		}
>  
> -- 
> 2.34.1
> 

unsigned arithmetic makes my head hurt. It would hurt less if we
guaranteed the ranges of all the components. vcpu_ids are
guaranteed to be [ 0, min(KVM_MAX_VCPU_IDS, INT_MAX) ), see
kvm_vm_ioctl_create_vcpu(). We should also test and reject hbase
and hmask in all the SBI calls to ensure that hbase is in the
same range as vcpu_ids and hmask is in the [0, BITS_PER_LONG)
range.

Thanks,
drew