RISC-V: KVM: Fix null pointer dereference in kvm_riscv_vcpu_aia_rmw_topei()

kvm_riscv_vcpu_aia_rmw_topei() assumes that the per-vCPU IMSIC state has
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: db8b7e9 ("RISC-V: KVM: Add in-kernel virtualization of AIA IMSIC")
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260226085119.643295-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: 6eanut

arch/riscv/kvm/aia_imsic.c

@@ -908,6 +908,10 @@ 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,