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.

Reproducer and detailed analysis available at:
https://github.com/j1akai/temp/tree/main/20260125

Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
---
 arch/riscv/kvm/aia_imsic.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
index e597e86491c3b..9c58a66068447 100644
--- a/arch/riscv/kvm/aia_imsic.c
+++ b/arch/riscv/kvm/aia_imsic.c
@@ -995,6 +995,9 @@ int kvm_riscv_aia_imsic_has_attr(struct kvm *kvm, unsigned long type)
 
 	isel = KVM_DEV_RISCV_AIA_IMSIC_GET_ISEL(type);
 	imsic = vcpu->arch.aia_context.imsic_state;
+	if (!imsic)
+		return -ENODEV;
+
 	return imsic_mrif_isel_check(imsic->nr_eix, isel);
 }
 
-- 
2.34.1

On Sun, Jan 25, 2026 at 11:34 AM Jiakai Xu <jiakaipeanut@gmail.com> wrote:
>
> 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.
>
> Reproducer and detailed analysis available at:
> https://github.com/j1akai/temp/tree/main/20260125

No need for a link to your personal repo over here.

Please add Fixes tag here.

>
> Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>

Otherwise, LGTM.

Reviewed-by: Anup Patel <anup@brainfault.org>

Regards,
Anup

> ---
>  arch/riscv/kvm/aia_imsic.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> index e597e86491c3b..9c58a66068447 100644
> --- a/arch/riscv/kvm/aia_imsic.c
> +++ b/arch/riscv/kvm/aia_imsic.c
> @@ -995,6 +995,9 @@ int kvm_riscv_aia_imsic_has_attr(struct kvm *kvm, unsigned long type)
>
>         isel = KVM_DEV_RISCV_AIA_IMSIC_GET_ISEL(type);
>         imsic = vcpu->arch.aia_context.imsic_state;
> +       if (!imsic)
> +               return -ENODEV;
> +
>         return imsic_mrif_isel_check(imsic->nr_eix, isel);
>  }
>
> --
> 2.34.1
>