Re: [PATCH] KVM: Add separate helper for putting borrowed reference to kvm

[Leonardo Bras <leonardo@linux.ibm.com>]

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On Mon, 2019-10-21 at 15:58 -0700, Sean Christopherson wrote:
> Add a new helper, kvm_put_kvm_no_destroy(), to handle putting a
> borrowed
> reference[*] to the VM when installing a new file descriptor
> fails.  KVM
> expects the refcount to remain valid in this case, as the in-progress
> ioctl() has an explicit reference to the VM.  The primary motiviation
> for the helper is to document that the 'kvm' pointer is still valid
> after putting the borrowed reference, e.g. to document that doing
> mutex(&kvm->lock) immediately after putting a ref to kvm isn't
> broken.
> 
> [*] When exposing a new object to userspace via a file descriptor,
> e.g.
>     a new vcpu, KVM grabs a reference to itself (the VM) prior to
> making
>     the object visible to userspace to avoid prematurely freeing the
> VM
>     in the scenario where userspace immediately closes file
> descriptor.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c |  2 +-
>  arch/powerpc/kvm/book3s_64_vio.c    |  2 +-
>  include/linux/kvm_host.h            |  1 +
>  virt/kvm/kvm_main.c                 | 16 ++++++++++++++--
>  4 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 9a75f0e1933b..68678e31c84c 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -2000,7 +2000,7 @@ int kvm_vm_ioctl_get_htab_fd(struct kvm *kvm,
> struct kvm_get_htab_fd *ghf)
>  	ret = anon_inode_getfd("kvm-htab", &kvm_htab_fops, ctx, rwflag
> | O_CLOEXEC);
>  	if (ret < 0) {
>  		kfree(ctx);
> -		kvm_put_kvm(kvm);
> +		kvm_put_kvm_no_destroy(kvm);
>  		return ret;
>  	}
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c
> b/arch/powerpc/kvm/book3s_64_vio.c
> index 5834db0a54c6..883a66e76638 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -317,7 +317,7 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm
> *kvm,
>  	if (ret >= 0)
>  		list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
>  	else
> -		kvm_put_kvm(kvm);
> +		kvm_put_kvm_no_destroy(kvm);
> 
>  	mutex_unlock(&kvm->lock);
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 719fc3e15ea4..90a2102605ef 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -622,6 +622,7 @@ void kvm_exit(void);
> 
>  void kvm_get_kvm(struct kvm *kvm);
>  void kvm_put_kvm(struct kvm *kvm);
> +void kvm_put_kvm_no_destroy(struct kvm *kvm);
> 
>  static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm,
> int as_id)
>  {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 67ef3f2e19e8..b8534c6b8cf6 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -772,6 +772,18 @@ void kvm_put_kvm(struct kvm *kvm)
>  }
>  EXPORT_SYMBOL_GPL(kvm_put_kvm);
> 
> +/*
> + * Used to put a reference that was taken on behalf of an object
> associated
> + * with a user-visible file descriptor, e.g. a vcpu or device, if
> installation
> + * of the new file descriptor fails and the reference cannot be
> transferred to
> + * its final owner.  In such cases, the caller is still actively
> using @kvm and
> + * will fail miserably if the refcount unexpectedly hits zero.
> + */
> +void kvm_put_kvm_no_destroy(struct kvm *kvm)
> +{
> +	WARN_ON(refcount_dec_and_test(&kvm->users_count));
> +}
> +EXPORT_SYMBOL_GPL(kvm_put_kvm_no_destroy);
> 
>  static int kvm_vm_release(struct inode *inode, struct file *filp)
>  {
> @@ -2679,7 +2691,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm
> *kvm, u32 id)
>  	kvm_get_kvm(kvm);
>  	r = create_vcpu_fd(vcpu);
>  	if (r < 0) {
> -		kvm_put_kvm(kvm);
> +		kvm_put_kvm_no_destroy(kvm);
>  		goto unlock_vcpu_destroy;
>  	}
> 
> @@ -3117,7 +3129,7 @@ static int kvm_ioctl_create_device(struct kvm
> *kvm,
>  	kvm_get_kvm(kvm);
>  	ret = anon_inode_getfd(ops->name, &kvm_device_fops, dev, O_RDWR
> | O_CLOEXEC);
>  	if (ret < 0) {
> -		kvm_put_kvm(kvm);
> +		kvm_put_kvm_no_destroy(kvm);
>  		mutex_lock(&kvm->lock);
>  		list_del(&dev->vm_node);
>  		mutex_unlock(&kvm->lock);

Hello,

I see what are you solving here, but would not this behavior cause the
refcount to reach negative values?

If so, is not there a problem? I mean, in some archs (powerpc included)
refcount_dec_and_test() will decrement and then test if the value is
equal 0. If we ever reach a negative value, this will cause that memory
to never be released. 

An example is that refcount_dec_and_test(), on other archs than x86,
will call atomic_dec_and_test(), which on include/linux/atomic-
fallback.h will do:

return atomic_dec_return(v) == 0;

To change this behavior, it would mean change the whole atomic_*_test
behavior, or do a copy function in order to change this '== 0' to 
'<= 0'. 

Does it make sense? Do you need any help on this?

Kind regards,
Leonardo Brás

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