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SubjectRe: [PATCH v1 08/16] kvm: arm/arm64: Clean up stage2 pgd life time
On Tue, Jan 09, 2018 at 07:04:03PM +0000, Suzuki K Poulose wrote:
> On arm/arm64 we pre-allocate the entry level page tables when
> a VM is created and is free'd when either all the mm users are
> gone or the KVM is about to get destroyed. i.e, kvm_free_stage2_pgd
> is triggered via kvm_arch_flush_shadow_all() which can be invoked
> from two different paths :
>
> 1) do_exit()-> .-> mmu_notifier->release()-> ..-> kvm_arch_flush_shadow_all()
> OR
> 2) kvm_destroy_vm()-> mmu_notifier_unregister-> kvm_arch_flush_shadow_all()
>
> This has created lot of race conditions in the past as some of
> the VCPUs could be active when we free the stage2 via path (1).

How?? mmu_notifier->release() is called via __mput->exit_mmap(), which
is only called if mm_users == 0, which means there are no more threads
left than the one currently doing exit().

>
> On a closer look, all we need to do with kvm_arch_flush_shadow_all() is,
> to ensure that the stage2 mappings are cleared. This doesn't mean we
> have to free up the stage2 entry level page tables yet, which could
> be delayed until the kvm is destroyed. This would avoid issues
> of use-after-free,

do we have any of those left?

> This will be later used for delaying
> the allocation of the stage2 entry level page tables until we really
> need to do something with it.

Fine, but you don't actually explain why this change of flow is
necessary for what you're trying to do later?

>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <cdall@linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> virt/kvm/arm/arm.c | 1 +
> virt/kvm/arm/mmu.c | 56 ++++++++++++++++++++++++++++--------------------------
> 2 files changed, 30 insertions(+), 27 deletions(-)
>
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index c8d49879307f..19b720ddedce 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -189,6 +189,7 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
> }
> }
> atomic_set(&kvm->online_vcpus, 0);
> + kvm_free_stage2_pgd(kvm);
> }
>
> int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 78253fe00fc4..c94c61ac38b9 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -298,11 +298,10 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
> pgd = kvm->arch.pgd + stage2_pgd_index(addr);
> do {
> /*
> - * Make sure the page table is still active, as another thread
> - * could have possibly freed the page table, while we released
> - * the lock.
> + * The page table shouldn't be free'd as we still hold a reference
> + * to the KVM.

To avoid confusion about references to the kernel module KVM and a
specific KVM VM instance, please s/KVM/VM/.

> */
> - if (!READ_ONCE(kvm->arch.pgd))
> + if (WARN_ON(!READ_ONCE(kvm->arch.pgd)))

This reads a lot like a defensive implementation now, and I think for
this patch to make sense, we shouldn't try to handle a buggy super-racy
implementation gracefully, but rather have VM_BUG_ON() (if we care about
performance of the check) or simply BUG_ON().

The rationale being that if we've gotten this flow incorrect and freed
the pgd at the wrong time, we don't want to leave a ticking bomb to blow
up somewhere else randomly (which it will!), but instead crash and burn.

> break;
> next = stage2_pgd_addr_end(addr, end);
> if (!stage2_pgd_none(*pgd))
> @@ -837,30 +836,33 @@ void stage2_unmap_vm(struct kvm *kvm)
> up_read(&current->mm->mmap_sem);
> srcu_read_unlock(&kvm->srcu, idx);
> }
> -
> -/**
> - * kvm_free_stage2_pgd - free all stage-2 tables
> - * @kvm: The KVM struct pointer for the VM.
> - *
> - * Walks the level-1 page table pointed to by kvm->arch.pgd and frees all
> - * underlying level-2 and level-3 tables before freeing the actual level-1 table
> - * and setting the struct pointer to NULL.
> +/*
> + * kvm_flush_stage2_all: Unmap the entire stage2 mappings including
> + * device and regular RAM backing memory.
> */
> -void kvm_free_stage2_pgd(struct kvm *kvm)
> +static void kvm_flush_stage2_all(struct kvm *kvm)
> {
> - void *pgd = NULL;
> -
> spin_lock(&kvm->mmu_lock);
> - if (kvm->arch.pgd) {
> + if (kvm->arch.pgd)
> unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
> - pgd = READ_ONCE(kvm->arch.pgd);
> - kvm->arch.pgd = NULL;
> - }
> spin_unlock(&kvm->mmu_lock);
> +}
>
> - /* Free the HW pgd, one page at a time */
> - if (pgd)
> - free_pages_exact(pgd, S2_PGD_SIZE);
> +/**
> + * kvm_free_stage2_pgd - Free the entry level page tables in stage-2.

nit: you should put the parameter description here and leave a blank
line before the lengthy discussion.

> + * This is called when all reference to the KVM has gone away and we
> + * really don't need any protection in resetting the PGD. This also

I don't think I understand the last part of this sentence.

This function is pretty self-explanatory really, and I think we can
either drop the documentation all together or simply say that this
function clears all stage 2 page table entries to release the memory of
the lower-level page tables themselves and then frees the pgd in the
end. The VM is known to go away and no more VCPUs exist at this point.

> + * means that nobody should be touching stage2 at this point, as we
> + * have unmapped the entire stage2 already and all dynamic entities,
> + * (VCPUs and devices) are no longer active.
> + *
> + * @kvm: The KVM struct pointer for the VM.
> + */
> +void kvm_free_stage2_pgd(struct kvm *kvm)
> +{
> + kvm_flush_stage2_all(kvm);
> + free_pages_exact(kvm->arch.pgd, S2_PGD_SIZE);
> + kvm->arch.pgd = NULL;
> }
>
> static pud_t *stage2_get_pud(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
> @@ -1189,12 +1191,12 @@ static void stage2_wp_range(struct kvm *kvm, phys_addr_t addr, phys_addr_t end)
> * large. Otherwise, we may see kernel panics with
> * CONFIG_DETECT_HUNG_TASK, CONFIG_LOCKUP_DETECTOR,
> * CONFIG_LOCKDEP. Additionally, holding the lock too long
> - * will also starve other vCPUs. We have to also make sure
> - * that the page tables are not freed while we released
> - * the lock.
> + * will also starve other vCPUs.
> + * The page tables shouldn't be free'd while we released the

s/shouldn't/can't/

> + * lock, since we hold a reference to the KVM.

s/KVM/VM/

> */
> cond_resched_lock(&kvm->mmu_lock);
> - if (!READ_ONCE(kvm->arch.pgd))
> + if (WARN_ON(!READ_ONCE(kvm->arch.pgd)))
> break;
> next = stage2_pgd_addr_end(addr, end);
> if (stage2_pgd_present(*pgd))
> @@ -1950,7 +1952,7 @@ 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);
> + kvm_flush_stage2_all(kvm);
> }
>
> void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
> --
> 2.13.6
>

Thanks,
-Christoffer

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