linux-kernel - Re: [PATCH v4] RISC-V: KVM: Fix null pointer dereference in kvm_riscv_aia_imsic_has

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Message-ID:
 <KUZPR04MB92652C4D69EF09E324916DE9F393A@KUZPR04MB9265.apcprd04.prod.outlook.com>
Date: Mon, 26 Jan 2026 12:31:13 +0800
From: "Nutty.Liu" <nutty.liu@...mail.com>
To: Jiakai Xu <jiakaipeanut@...il.com>, linux-kernel@...r.kernel.org,
 linux-riscv@...ts.infradead.org, kvm-riscv@...ts.infradead.org,
 kvm@...r.kernel.org
Cc: Alexandre Ghiti <alex@...ti.fr>, Albert Ou <aou@...s.berkeley.edu>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <pjw@...nel.org>,
 Atish Patra <atish.patra@...ux.dev>, Anup Patel <anup@...infault.org>,
 Jiakai Xu <xujiakai2025@...as.ac.cn>
Subject: Re: [PATCH v4] RISC-V: KVM: Fix null pointer dereference in
 kvm_riscv_aia_imsic_has_attr()


On 1/25/2026 10:33 PM, Jiakai Xu 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.
>
> Fixes: 5463091a51cf ("RISC-V: KVM: Expose IMSIC registers as attributes of AIA irqchip")
> Signed-off-by: Jiakai Xu <xujiakai2025@...as.ac.cn>
> Signed-off-by: Jiakai Xu <jiakaiPeanut@...il.com>
> ---
> V3 -> V4: Fix typo in Signed-off-by email address.
> V2 -> V3: Moved isel assignment after imsic_state NULL check.
>            Placed patch version history after '---' separator.
>            Added parentheses to function name in subject.
> V1 -> V2: Added Fixes tag and drop external link as suggested.
>
>   arch/riscv/kvm/aia_imsic.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
Reviewed-by: Nutty Liu <nutty.liu@...mail.com>

Thanks,
Nutty
> diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> index e597e86491c3b..cd070d83663a9 100644
> --- a/arch/riscv/kvm/aia_imsic.c
> +++ b/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);
>   }
>