Re: [PATCH] kvm-arm: Unmap shadow pagetables properly

Itaru Kitayama
2016-09-09T06:02:58Z

VM now shuts down normally on Mustang backed by DEBUG_PAGEALLOC=y
4.8.0-rc5 kernel.

On 9/9/16 12:25 AM, Suzuki K Poulose wrote:
> On arm/arm64, we depend on the kvm_unmap_hva* callbacks (via
> mmu_notifiers::invalidate_*) to unmap the stage2 pagetables when
> the userspace buffer gets unmapped. However, when the Hypervisor
> process exits without explicit unmap of the guest buffers, the only
> notifier we get is kvm_arch_flush_shadow_all() (via mmu_notifier::release
> ) which does nothing on arm. Later this causes us to access pages that
> were already released [via exit_mmap() -> unmap_vmas()] when we actually
> get to unmap the stage2 pagetable [via kvm_arch_destroy_vm() ->
> kvm_free_stage2_pgd()]. This triggers crashes with CONFIG_DEBUG_PAGEALLOC,
> which unmaps any free'd pages from the linear map.
>
> [ 757.644120] Unable to handle kernel paging request at virtual address
> ffff800661e00000
> [ 757.652046] pgd = ffff20000b1a2000
> [ 757.655471] [ffff800661e00000] *pgd=00000047fffe3003, *pud=00000047fcd8c003,
> *pmd=00000047fcc7c003, *pte=00e8004661e00712
> [ 757.666492] Internal error: Oops: 96000147 [#3] PREEMPT SMP
> [ 757.672041] Modules linked in:
> [ 757.675100] CPU: 7 PID: 3630 Comm: qemu-system-aar Tainted: G D
> 4.8.0-rc1 #3
> [ 757.683240] Hardware name: AppliedMicro X-Gene Mustang Board/X-Gene Mustang Board,
> BIOS 3.06.15 Aug 19 2016
> [ 757.692938] task: ffff80069cdd3580 task.stack: ffff8006adb7c000
> [ 757.698840] PC is at __flush_dcache_area+0x1c/0x40
> [ 757.703613] LR is at kvm_flush_dcache_pmd+0x60/0x70
> [ 757.708469] pc : [<ffff20000809dbdc>] lr : [<ffff2000080b4a70>] pstate: 20000145
> ...
> [ 758.357249] [<ffff20000809dbdc>] __flush_dcache_area+0x1c/0x40
> [ 758.363059] [<ffff2000080b6748>] unmap_stage2_range+0x458/0x5f0
> [ 758.368954] [<ffff2000080b708c>] kvm_free_stage2_pgd+0x34/0x60
> [ 758.374761] [<ffff2000080b2280>] kvm_arch_destroy_vm+0x20/0x68
> [ 758.380570] [<ffff2000080aa330>] kvm_put_kvm+0x210/0x358
> [ 758.385860] [<ffff2000080aa524>] kvm_vm_release+0x2c/0x40
> [ 758.391239] [<ffff2000082ad234>] __fput+0x114/0x2e8
> [ 758.396096] [<ffff2000082ad46c>] ____fput+0xc/0x18
> [ 758.400869] [<ffff200008104658>] task_work_run+0x108/0x138
> [ 758.406332] [<ffff2000080dc8ec>] do_exit+0x48c/0x10e8
> [ 758.411363] [<ffff2000080dd5fc>] do_group_exit+0x6c/0x130
> [ 758.416739] [<ffff2000080ed924>] get_signal+0x284/0xa18
> [ 758.421943] [<ffff20000808a098>] do_signal+0x158/0x860
> [ 758.427060] [<ffff20000808aad4>] do_notify_resume+0x6c/0x88
> [ 758.432608] [<ffff200008083624>] work_pending+0x10/0x14
> [ 758.437812] Code: 9ac32042 8b010001 d1000443 8a230000 (d50b7e20)
>
> This patch fixes the issue by moving the kvm_free_stage2_pgd() to
> kvm_arch_flush_shadow_all().
>
> Reported-by: Itaru Kitayama <itaru.kitayama@riken.jp>
> Reported-by: James Morse <james.morse@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Christoffer Dall <christoffer.dall@linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> arch/arm/kvm/arm.c | 2 --
> arch/arm/kvm/mmu.c | 1 +
> 2 files changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 75f130e..c94b90d 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -158,8 +158,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
> {
> int i;
>
> - kvm_free_stage2_pgd(kvm);
> -
> for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> if (kvm->vcpus[i]) {
> kvm_arch_vcpu_free(kvm->vcpus[i]);
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 29d0b23..d626d08 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -1893,6 +1893,7 @@ void kvm_arch_memslots_updated(struct kvm *kvm, struct kvm_memslots *slots)
>
> void kvm_arch_flush_shadow_all(struct kvm *kvm)
> {
> + kvm_free_stage2_pgd(kvm);
> }
>
> void kvm_arch_flush_shadow_memslot(struct kvm *kvm,