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

Add a null pointer check for imsic_state before dereferencing it in
kvm_riscv_aia_imsic_has_attr(). While the function checks that the
vcpu exists, it doesn't verify that the vcpu's imsic_state has been
initialized, leading to a null pointer dereference when accessed.

This issue was discovered during fuzzing of RISC-V KVM code. The
crash occurs when userspace calls KVM_HAS_DEVICE_ATTR ioctl on an
AIA IMSIC device before the IMSIC state has been fully initialized
for a vcpu.

The crash manifests as:
  Unable to handle kernel paging request at virtual address
  dfffffff00000001
  ...
  epc : kvm_riscv_aia_imsic_has_attr+0x464/0x50e
  arch/riscv/kvm/aia_imsic.c:998
  ...
  kvm_riscv_aia_imsic_has_attr+0x464/0x50e arch/riscv/kvm/aia_imsic.c:998
  aia_has_attr+0x128/0x2bc arch/riscv/kvm/aia_device.c:471
  kvm_device_ioctl_attr virt/kvm/kvm_main.c:4722 [inline]
  kvm_device_ioctl+0x296/0x374 virt/kvm/kvm_main.c:4739
  ...

The fix adds a check to return -ENODEV if imsic_state is NULL, which
is consistent with other error handling in the function and prevents
the null pointer dereference.

Fixes: 5463091 ("RISC-V: KVM: Expose IMSIC registers as attributes of AIA irqchip")
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/20260125143344.2515451-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: 6eanut

arch/riscv/kvm/aia_imsic.c

@@ -993,8 +993,11 @@ int kvm_riscv_aia_imsic_has_attr(struct kvm *kvm, unsigned long type)
 	if (!vcpu)
 		return -ENODEV;
 
-	isel = KVM_DEV_RISCV_AIA_IMSIC_GET_ISEL(type);
 	imsic = vcpu->arch.aia_context.imsic_state;
+	if (!imsic)
+		return -ENODEV;
+
+	isel = KVM_DEV_RISCV_AIA_IMSIC_GET_ISEL(type);
 	return imsic_mrif_isel_check(imsic->nr_eix, isel);
 }