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

[Anup Patel]

On Thu, Aug 15, 2024 at 10:39 PM Anup Patel <apatel at ventanamicro.com> wrote:
>
> 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
>   [<ffffffff01590e98>] __kvm_write_guest_page+0x94/0xa6 [kvm]
>   [<ffffffff015943a6>] kvm_vcpu_write_guest+0x56/0x90 [kvm]
>   [<ffffffff015a175c>] kvm_pmu_clear_snapshot_area+0x42/0x7e [kvm]
>   [<ffffffff015a1972>] kvm_riscv_vcpu_pmu_deinit.part.0+0xe0/0x14e [kvm]
>   [<ffffffff015a2ad0>] kvm_riscv_vcpu_pmu_deinit+0x1a/0x24 [kvm]
>   [<ffffffff0159b344>] kvm_arch_vcpu_destroy+0x28/0x4c [kvm]
>   [<ffffffff0158e420>] kvm_destroy_vcpus+0x5a/0xda [kvm]
>   [<ffffffff0159930c>] kvm_arch_destroy_vm+0x14/0x28 [kvm]
>   [<ffffffff01593260>] kvm_destroy_vm+0x168/0x2a0 [kvm]
>   [<ffffffff015933d4>] kvm_put_kvm+0x3c/0x58 [kvm]
>   [<ffffffff01593412>] 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.
>
> Fixes: c2f41ddbcdd7 ("RISC-V: KVM: Implement SBI PMU Snapshot feature")
> Signed-off-by: Anup Patel <apatel at ventanamicro.com>

Queued this patch for Linux-6.11 fixes.

Regards,
Anup

> ---
>  arch/riscv/kvm/vcpu_pmu.c | 14 ++------------
>  1 file changed, 2 insertions(+), 12 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu_pmu.c b/arch/riscv/kvm/vcpu_pmu.c
> index bcf41d6e0df0..2707a51b082c 100644
> --- a/arch/riscv/kvm/vcpu_pmu.c
> +++ b/arch/riscv/kvm/vcpu_pmu.c
> @@ -391,19 +391,9 @@ int kvm_riscv_vcpu_pmu_read_hpm(struct kvm_vcpu *vcpu, unsigned int csr_num,
>  static void kvm_pmu_clear_snapshot_area(struct kvm_vcpu *vcpu)
>  {
>         struct kvm_pmu *kvpmu = vcpu_to_pmu(vcpu);
> -       int snapshot_area_size = sizeof(struct riscv_pmu_snapshot_data);
>
> -       if (kvpmu->sdata) {
> -               if (kvpmu->snapshot_addr != INVALID_GPA) {
> -                       memset(kvpmu->sdata, 0, snapshot_area_size);
> -                       kvm_vcpu_write_guest(vcpu, kvpmu->snapshot_addr,
> -                                            kvpmu->sdata, snapshot_area_size);
> -               } else {
> -                       pr_warn("snapshot address invalid\n");
> -               }
> -               kfree(kvpmu->sdata);
> -               kvpmu->sdata = NULL;
> -       }
> +       kfree(kvpmu->sdata);
> +       kvpmu->sdata = NULL;
>         kvpmu->snapshot_addr = INVALID_GPA;
>  }
>
> --
> 2.34.1
>