Re: [PATCH] RISC-V: KVM: Fix integer overflow in kvm_pmu_validate_counter_mask()

[Nutty.Liu]

On 3/19/2026 11:59 AM, Jiakai Xu wrote:
> When a guest initiates an SBI_EXT_PMU_COUNTER_CFG_MATCH call with
> ctr_base=0xfffffffffffffffe, ctr_mask=0xeb5f and flags=0x1
> (SBI_PMU_CFG_FLAG_SKIP_MATCH), kvm_riscv_vcpu_pmu_ctr_cfg_match()
> first invokes kvm_pmu_validate_counter_mask() to verify whether
> ctr_base and ctr_mask are valid, by evaluating:
>   !ctr_mask || (ctr_base + __fls(ctr_mask) >= kvm_pmu_num_counters(kvpmu))
>
> With the above inputs, __fls(0xeb5f) equals 15, and adding 15 to
> 0xfffffffffffffffe causes an integer overflow, wrapping around to 13.
> Since 13 is less than kvm_pmu_num_counters(), the validation wrongly
> succeeds.
>
> Thereafter, since flags & SBI_PMU_CFG_FLAG_SKIP_MATCH is satisfied,
> the code evaluates:
>   !test_bit(ctr_base + __ffs(ctr_mask), kvpmu->pmc_in_use)
>
> Here __ffs(0xeb5f) equals 0, so test_bit() receives 0xfffffffffffffffe
> as the bit index and attempts to access the corresponding element of
> the kvpmu->pmc_in_use, which results in an invalid memory access. This
> triggers the following Oops:
>   Unable to handle kernel paging request at virtual address e3ebffff12abba89
>    generic_test_bit include/asm-generic/bitops/generic-non-atomic.h:128
>    kvm_riscv_vcpu_pmu_ctr_cfg_match arch/riscv/kvm/vcpu_pmu.c:758
>    kvm_sbi_ext_pmu_handler arch/riscv/kvm/vcpu_sbi_pmu.c:49
>    kvm_riscv_vcpu_sbi_ecall arch/riscv/kvm/vcpu_sbi.c:608
>    kvm_riscv_vcpu_exit arch/riscv/kvm/vcpu_exit.c:240
>
> The root cause is that kvm_pmu_validate_counter_mask() does not account
> for the case where ctr_base itself is out of range, allowing the
> subsequent addition to silently overflow and bypass the check.
>
> Fix this by explicitly validating ctr_base against kvm_pmu_num_counters()
> before performing the addition.
>
> This bug was found by fuzzing the KVM RISC-V PMU interface.
>
> Fixes: 0cb74b65d2e5e6 ("RISC-V: KVM: Implement perf support without sampling")
> Signed-off-by: Jiakai Xu <jiakaiPeanut@xxxxxxxxx>
> Signed-off-by: Jiakai Xu <xujiakai2025@xxxxxxxxxxx>
Reviewed-by: Nutty Liu <nutty.liu@xxxxxxxxxxx>

Thanks,
Nutty
> ---
>   arch/riscv/kvm/vcpu_pmu.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu_pmu.c b/arch/riscv/kvm/vcpu_pmu.c
> index e873430e596b2..a098a9b417ad8 100644
> --- a/arch/riscv/kvm/vcpu_pmu.c
> +++ b/arch/riscv/kvm/vcpu_pmu.c
> @@ -266,8 +266,10 @@ static int pmu_ctr_read(struct kvm_vcpu *vcpu, unsigned long cidx,
>   static int kvm_pmu_validate_counter_mask(struct kvm_pmu *kvpmu, unsigned long ctr_base,
>   					 unsigned long ctr_mask)
>   {
> -	/* Make sure the we have a valid counter mask requested from the caller */
> -	if (!ctr_mask || (ctr_base + __fls(ctr_mask) >= kvm_pmu_num_counters(kvpmu)))
> +	unsigned long num_ctrs = kvm_pmu_num_counters(kvpmu);
> +
> +	/* Make sure we have a valid counter mask requested from the caller */
> +	if (!ctr_mask || ctr_base >= num_ctrs || (ctr_base + __fls(ctr_mask) >= num_ctrs))
>   		return -EINVAL;
>   
>   	return 0;