RISC-V: KVM: Fix double-free of sdata in kvm_pmu_clear_snapshot_area()

In kvm_riscv_vcpu_pmu_snapshot_set_shmem(), when kvm_vcpu_write_guest()
fails, kvpmu->sdata is freed but not set to NULL. This leaves a dangling
pointer that will be freed again when kvm_pmu_clear_snapshot_area() is
called during vcpu teardown, triggering a KASAN double-free report.

First free occurs in kvm_riscv_vcpu_pmu_snapshot_set_shmem():
 kvm_riscv_vcpu_pmu_snapshot_set_shmem arch/riscv/kvm/vcpu_pmu.c:443
 kvm_sbi_ext_pmu_handler arch/riscv/kvm/vcpu_sbi_pmu.c:74
 kvm_riscv_vcpu_sbi_ecall arch/riscv/kvm/vcpu_sbi.c:608
 kvm_riscv_vcpu_exit arch/riscv/kvm/vcpu_exit.c:240
 kvm_arch_vcpu_ioctl_run arch/riscv/kvm/vcpu.c:1008
 kvm_vcpu_ioctl virt/kvm/kvm_main.c:4476

Second free (double-free) occurs in kvm_pmu_clear_snapshot_area():
 kvm_pmu_clear_snapshot_area arch/riscv/kvm/vcpu_pmu.c:403 [inline]
 kvm_riscv_vcpu_pmu_deinit.part arch/riscv/kvm/vcpu_pmu.c:905
 kvm_riscv_vcpu_pmu_deinit arch/riscv/kvm/vcpu_pmu.c:893
 kvm_arch_vcpu_destroy arch/riscv/kvm/vcpu.c:199
 kvm_vcpu_destroy virt/kvm/kvm_main.c:469 [inline]
 kvm_destroy_vcpus virt/kvm/kvm_main.c:489
 kvm_arch_destroy_vm arch/riscv/kvm/vm.c:54
 kvm_destroy_vm virt/kvm/kvm_main.c:1301 [inline]
 kvm_put_kvm virt/kvm/kvm_main.c:1338
 kvm_vm_release virt/kvm/kvm_main.c:1361

Fix it by setting kvpmu->sdata to NULL after kfree() in
kvm_riscv_vcpu_pmu_snapshot_set_shmem(), so that the subsequent
kfree(NULL) in kvm_pmu_clear_snapshot_area() becomes a safe no-op.

This bug was found by fuzzing the KVM RISC-V PMU interface.

Fixes: c2f41ddbcd ("RISC-V: KVM: Implement SBI PMU Snapshot feature")
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Reviewed-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260318092956.708246-1-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/kvm/vcpu_pmu.c

@@ -456,6 +456,7 @@ int kvm_riscv_vcpu_pmu_snapshot_set_shmem(struct kvm_vcpu *vcpu, unsigned long s
 	/* No need to check writable slot explicitly as kvm_vcpu_write_guest does it internally */
 	if (kvm_vcpu_write_guest(vcpu, saddr, kvpmu->sdata, snapshot_area_size)) {
 		kfree(kvpmu->sdata);
+		kvpmu->sdata = NULL;
 		sbiret = SBI_ERR_INVALID_ADDRESS;
 		goto out;
 	}