Re: [PATCH v2] RISC-V: KVM: Fix null pointer dereference in kvm_riscv_vcpu_aia_rmw_topei()

[Nutty.Liu]

On 2/26/2026 4:51 PM, Jiakai Xu wrote:
> kvm_riscv_vcpu_aia_rmw_topei() assumes that the per-vCPU IMSIC state has
s/kvm_riscv_vcpu_aia_rmw_topei/kvm_riscv_vcpu_aia_imsic_rmw ?
> been initialized once AIA is reported as available and initialized at
> the VM level. This assumption does not always hold.
>
> Under fuzzed ioctl sequences, a guest may access the IMSIC TOPEI CSR
> before the vCPU IMSIC state is set up. In this case,
> vcpu->arch.aia_context.imsic_state is still NULL, and the TOPEI RMW path
> dereferences it unconditionally, leading to a host kernel crash.
>
> The crash manifests as:
>    Unable to handle kernel paging request at virtual address
>    dfffffff0000000e
>    ...
>    kvm_riscv_vcpu_aia_imsic_rmw arch/riscv/kvm/aia_imsic.c:909
>    kvm_riscv_vcpu_aia_rmw_topei arch/riscv/kvm/aia.c:231
>    csr_insn arch/riscv/kvm/vcpu_insn.c:208
>    system_opcode_insn arch/riscv/kvm/vcpu_insn.c:281
>    kvm_riscv_vcpu_virtual_insn arch/riscv/kvm/vcpu_insn.c:355
>    kvm_riscv_vcpu_exit arch/riscv/kvm/vcpu_exit.c:230
>    kvm_arch_vcpu_ioctl_run arch/riscv/kvm/vcpu.c:1008
>    ...
>
> Fix this by explicitly checking whether the vCPU IMSIC state has been
> initialized before handling TOPEI CSR accesses. If not, forward the CSR
> emulation to user space.
>
> Fixes: db8b7e97d6137 ("RISC-V: KVM: Add in-kernel virtualization of AIA IMSIC")
> Signed-off-by: Jiakai Xu <xujiakai2025@xxxxxxxxxxx>
> Signed-off-by: Jiakai Xu <jiakaiPeanut@xxxxxxxxx>
> ---
> V1 -> V2: Moved imsic_state NULL check into kvm_riscv_vcpu_aia_imsic_rmw().
>            Updated Fixes tag to db8b7e97d6137.
> ---
>   arch/riscv/kvm/aia_imsic.c | 4 ++++
>   1 file changed, 4 insertions(+)
Reviewed-by: Nutty Liu <nutty.liu@xxxxxxxxxxx>

Thanks,
Nutty
> diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> index fda0346f0ea1f..60917f4789d84 100644
> --- a/arch/riscv/kvm/aia_imsic.c
> +++ b/arch/riscv/kvm/aia_imsic.c
> @@ -892,6 +892,10 @@ int kvm_riscv_vcpu_aia_imsic_rmw(struct kvm_vcpu *vcpu, unsigned long isel,
>   	int r, rc = KVM_INSN_CONTINUE_NEXT_SEPC;
>   	struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
>   
> +	/* If IMSIC vCPU state not initialized then forward to user space */
> +	if (!imsic)
> +		return KVM_INSN_EXIT_TO_USER_SPACE;
> +
>   	if (isel == KVM_RISCV_AIA_IMSIC_TOPEI) {
>   		/* Read pending and enabled interrupt with highest priority */
>   		topei = imsic_mrif_topei(imsic->swfile, imsic->nr_eix,