Re: [PATCH] KVM: PPC: Book3S HV: Protect updates to spapr_tce_tables list

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Aug 28, 2017 at 02:42:29PM +1000, Paul Mackerras wrote:
> Al Viro pointed out that while one thread of a process is executing
> in kvm_vm_ioctl_create_spapr_tce(), another thread could guess the
> file descriptor returned by anon_inode_getfd() and close() it before
> the first thread has added it to the kvm->arch.spapr_tce_tables list.
> That highlights a more general problem: there is no mutual exclusion
> between writers to the spapr_tce_tables list, leading to the
> possibility of the list becoming corrupted, which could cause a
> host kernel crash.
> 
> To fix the mutual exclusion problem, we add a mutex_lock/unlock
> pair around the list_del_rce in kvm_spapr_tce_release().  Also,
> this moves the call to anon_inode_getfd() inside the region
> protected by the kvm->lock mutex, after we have done the check for
> a duplicate LIOBN.  This means that if another thread does guess the
> file descriptor and closes it, its call to kvm_spapr_tce_release()
> will not do any harm because it will have to wait until the first
> thread has released kvm->lock.
> 
> The other things that the second thread could do with the guessed
> file descriptor are to mmap it or to pass it as a parameter to a
> KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl on a KVM device fd.  An mmap
> call won't cause any harm because kvm_spapr_tce_mmap() and
> kvm_spapr_tce_fault() don't access the spapr_tce_tables list or
> the kvmppc_spapr_tce_table.list field, and the fields that they do use
> have been properly initialized by the time of the anon_inode_getfd()
> call.
> 
> The KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE ioctl calls
> kvm_spapr_tce_attach_iommu_group(), which scans the spapr_tce_tables
> list looking for the kvmppc_spapr_tce_table struct corresponding to
> the fd given as the parameter.  Either it will find the new entry
> or it won't; if it doesn't, it just returns an error, and if it
> does, it will function normally.  So, in each case there is no
> harmful effect.
> 
> Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Although, as you know, I've missed bugs in here several times
previously, so I'm not sure how much the above is worth :/

> ---
>  arch/powerpc/kvm/book3s_64_vio.c | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index 53766e2..8f2da8b 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -265,8 +265,11 @@ static int kvm_spapr_tce_release(struct inode *inode, struct file *filp)
>  {
>  	struct kvmppc_spapr_tce_table *stt = filp->private_data;
>  	struct kvmppc_spapr_tce_iommu_table *stit, *tmp;
> +	struct kvm *kvm = stt->kvm;
>  
> +	mutex_lock(&kvm->lock);
>  	list_del_rcu(&stt->list);
> +	mutex_unlock(&kvm->lock);
>  
>  	list_for_each_entry_safe(stit, tmp, &stt->iommu_tables, next) {
>  		WARN_ON(!kref_read(&stit->kref));
> @@ -298,7 +301,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  	unsigned long npages, size;
>  	int ret = -ENOMEM;
>  	int i;
> -	int fd = -1;
>  
>  	if (!args->size)
>  		return -EINVAL;
> @@ -328,11 +330,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  			goto fail;
>  	}
>  
> -	ret = fd = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops,
> -				    stt, O_RDWR | O_CLOEXEC);
> -	if (ret < 0)
> -		goto fail;
> -
>  	mutex_lock(&kvm->lock);
>  
>  	/* Check this LIOBN hasn't been previously allocated */
> @@ -344,17 +341,19 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  		}
>  	}
>  
> -	if (!ret) {
> +	if (!ret)
> +		ret = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops,
> +				       stt, O_RDWR | O_CLOEXEC);
> +
> +	if (ret >= 0) {
>  		list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
>  		kvm_get_kvm(kvm);
>  	}
>  
>  	mutex_unlock(&kvm->lock);
>  
> -	if (!ret)
> -		return fd;
> -
> -	put_unused_fd(fd);
> +	if (ret >= 0)
> +		return ret;
>  
>   fail:
>  	for (i = 0; i < npages; i++)

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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