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

[Jiakai Xu]

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: 2f4d58f7635ae ("RISC-V: KVM: Virtualize per-HART AIA CSRs")
Signed-off-by: Jiakai Xu <xujiakai2025 at iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut at gmail.com>
---
 arch/riscv/kvm/aia.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/riscv/kvm/aia.c b/arch/riscv/kvm/aia.c
index dad3181856600..c176b960d8a40 100644
--- a/arch/riscv/kvm/aia.c
+++ b/arch/riscv/kvm/aia.c
@@ -228,6 +228,10 @@ int kvm_riscv_vcpu_aia_rmw_topei(struct kvm_vcpu *vcpu,
 	if (!kvm_riscv_aia_initialized(vcpu->kvm))
 		return KVM_INSN_EXIT_TO_USER_SPACE;
 
+	/* If IMSIC vCPU state not initialized then forward to user space */
+	if (!vcpu->arch.aia_context.imsic_state)
+		return KVM_INSN_EXIT_TO_USER_SPACE;
+
 	return kvm_riscv_vcpu_aia_imsic_rmw(vcpu, KVM_RISCV_AIA_IMSIC_TOPEI,
 					    val, new_val, wr_mask);
 }
-- 
2.34.1