RISC-V: KVM: Don't zero-out PMU snapshot area before freeing data

In the Linux kernel, the following vulnerability has been resolved:

RISC-V: KVM: Don't zero-out PMU snapshot area before freeing data

With the latest Linux-6.11-rc3, the below NULL pointer crash is observed when SBI PMU snapshot is enabled for the guest and the guest is forcefully powered-off.

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000508 Oops [#1] Modules linked in: kvm CPU: 0 UID: 0 PID: 61 Comm: term-poll Not tainted 6.11.0-rc3-00018-g44d7178dd77a #3 Hardware name: riscv-virtio,qemu (DT) epc : __kvm_write_guest_page+0x94/0xa6 [kvm] ra : __kvm_write_guest_page+0x54/0xa6 [kvm] epc : ffffffff01590e98 ra : ffffffff01590e58 sp : ffff8f80001f39b0 gp : ffffffff81512a60 tp : ffffaf80024872c0 t0 : ffffaf800247e000 t1 : 00000000000007e0 t2 : 0000000000000000 s0 : ffff8f80001f39f0 s1 : 00007fff89ac4000 a0 : ffffffff015dd7e8 a1 : 0000000000000086 a2 : 0000000000000000 a3 : ffffaf8000000000 a4 : ffffaf80024882c0 a5 : 0000000000000000 a6 : ffffaf800328d780 a7 : 00000000000001cc s2 : ffffaf800197bd00 s3 : 00000000000828c4 s4 : ffffaf800248c000 s5 : ffffaf800247d000 s6 : 0000000000001000 s7 : 0000000000001000 s8 : 0000000000000000 s9 : 00007fff861fd500 s10: 0000000000000001 s11: 0000000000800000 t3 : 00000000000004d3 t4 : 00000000000004d3 t5 : ffffffff814126e0 t6 : ffffffff81412700 status: 0000000200000120 badaddr: 0000000000000508 cause: 000000000000000d [] __kvm_write_guest_page+0x94/0xa6 [kvm] [] kvm_vcpu_write_guest+0x56/0x90 [kvm] [] kvm_pmu_clear_snapshot_area+0x42/0x7e [kvm] [] kvm_riscv_vcpu_pmu_deinit.part.0+0xe0/0x14e [kvm] [] kvm_riscv_vcpu_pmu_deinit+0x1a/0x24 [kvm] [] kvm_arch_vcpu_destroy+0x28/0x4c [kvm] [] kvm_destroy_vcpus+0x5a/0xda [kvm] [] kvm_arch_destroy_vm+0x14/0x28 [kvm] [] kvm_destroy_vm+0x168/0x2a0 [kvm] [] kvm_put_kvm+0x3c/0x58 [kvm] [] kvm_vm_release+0x22/0x2e [kvm]

Clearly, the kvm_vcpu_write_guest() function is crashing because it is being called from kvm_pmu_clear_snapshot_area() upon guest tear down.

To address the above issue, simplify the kvm_pmu_clear_snapshot_area() to not zero-out PMU snapshot area from kvm_pmu_clear_snapshot_area() because the guest is anyway being tore down.

The kvm_pmu_clear_snapshot_area() is also called when guest changes PMU snapshot area of a VCPU but even in this case the previous PMU snaphsot area must not be zeroed-out because the guest might have reclaimed the pervious PMU snapshot area for some other purpose.