Re: [PATCH v2] vfio: fix potential deadlock on vfio group lock

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 1/12/23 4:05 PM, Alex Williamson wrote:
> On Thu, 12 Jan 2023 15:38:44 -0500
> Matthew Rosato <mjrosato@linux.ibm.com> wrote:
> 
>> Currently it is possible that the final put of a KVM reference comes from
>> vfio during its device close operation.  This occurs while the vfio group
>> lock is held; however, if the vfio device is still in the kvm device list,
>> then the following call chain could result in a deadlock:
>>
>> kvm_put_kvm
>>  -> kvm_destroy_vm
>>   -> kvm_destroy_devices
>>    -> kvm_vfio_destroy
>>     -> kvm_vfio_file_set_kvm
>>      -> vfio_file_set_kvm
>>       -> group->group_lock/group_rwsem  
>>
>> Avoid this scenario by having vfio core code acquire a KVM reference
>> the first time a device is opened and hold that reference until the
>> device fd is closed, at a point after the group lock has been released.
>>
>> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
>> Reported-by: Alex Williamson <alex.williamson@redhat.com>
>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
>> ---
>> Changes from v1:
>> * Re-write using symbol get logic to get kvm ref during first device
>>   open, release the ref during device fd close after group lock is
>>   released
>> * Drop kvm get/put changes to drivers; now that vfio core holds a
>>   kvm ref until sometime after the device_close op is called, it
>>   should be fine for drivers to get and put their own references to it.
>> ---
>>  drivers/vfio/group.c     |  6 ++---
>>  drivers/vfio/vfio_main.c | 48 +++++++++++++++++++++++++++++++++++++---
>>  include/linux/vfio.h     |  1 -
>>  3 files changed, 48 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
>> index bb24b2f0271e..2b0da82f82f4 100644
>> --- a/drivers/vfio/group.c
>> +++ b/drivers/vfio/group.c
>> @@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
>>  	}
>>  
>>  	/*
>> -	 * Here we pass the KVM pointer with the group under the lock.  If the
>> -	 * device driver will use it, it must obtain a reference and release it
>> -	 * during close_device.
>> +	 * Here we pass the KVM pointer with the group under the lock.  A
>> +	 * reference will be obtained the first time the device is opened and
>> +	 * will be held until the device fd is closed.
>>  	 */
>>  	ret = vfio_device_open(device, device->group->iommufd,
>>  			       device->group->kvm);
>> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
>> index 5177bb061b17..c969e2a0ecd3 100644
>> --- a/drivers/vfio/vfio_main.c
>> +++ b/drivers/vfio/vfio_main.c
>> @@ -16,6 +16,7 @@
>>  #include <linux/fs.h>
>>  #include <linux/idr.h>
>>  #include <linux/iommu.h>
>> +#include <linux/kvm_host.h>
>>  #include <linux/list.h>
>>  #include <linux/miscdevice.h>
>>  #include <linux/module.h>
>> @@ -344,6 +345,35 @@ static bool vfio_assert_device_open(struct vfio_device *device)
>>  	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
>>  }
>>  
>> +static bool vfio_kvm_get_kvm_safe(struct kvm *kvm)
>> +{
>> +	bool (*fn)(struct kvm *kvm);
>> +	bool ret;
>> +
>> +	fn = symbol_get(kvm_get_kvm_safe);
>> +	if (WARN_ON(!fn))
>> +		return false;
>> +
>> +	ret = fn(kvm);
>> +
>> +	symbol_put(kvm_get_kvm_safe);
>> +
>> +	return ret;
>> +}
>> +
>> +static void vfio_kvm_put_kvm(struct kvm *kvm)
>> +{
>> +	void (*fn)(struct kvm *kvm);
>> +
>> +	fn = symbol_get(kvm_put_kvm);
>> +	if (WARN_ON(!fn))
>> +		return;
>> +
>> +	fn(kvm);
>> +
>> +	symbol_put(kvm_put_kvm);
>> +}
>> +
>>  static int vfio_device_first_open(struct vfio_device *device,
>>  				  struct iommufd_ctx *iommufd, struct kvm *kvm)
>>  {
>> @@ -361,16 +391,24 @@ static int vfio_device_first_open(struct vfio_device *device,
>>  	if (ret)
>>  		goto err_module_put;
>>  
>> +	if (kvm && !vfio_kvm_get_kvm_safe(kvm)) {
>> +		ret = -ENOENT;
>> +		goto err_unuse_iommu;
>> +	}
>>  	device->kvm = kvm;
> 
> This could just as easily be:
> 
> 	if (kvm && vfio_kvm_get_kvm_safe(kvm))
> 		device->kvm = kvm;
> 
> Right?  The error path would test device->kvm and we already use
> device->kvm in the release path.

Yeah, with a slight change (see below)

> 
> Otherwise, in the off chance userspace hits this error, what's the
> value in generating a failure here for a device that may or may not
> have a kvm dependency.  A driver with a dependency should fail if
> device->kvm is NULL.

Hmm, you have a point.  Yes, I agree that any driver that needs device->kvm is responsible for checking it for NULL.  I guess I was viewing this case as 'oh, we must already be on the kvm_destroy_vm path for this group' but that just means group->kvm is about to go NULL and doesn't necessarily mean that the vfio group is also going away.

Will change.

> 
>>  	if (device->ops->open_device) {
>>  		ret = device->ops->open_device(device);
>>  		if (ret)
>> -			goto err_unuse_iommu;
>> +			goto err_put_kvm;
>>  	}
>>  	return 0;
>>  
>> +err_put_kvm:
>> +	if (kvm) {

s/kvm/device->kvm/ here to go along with your suggestion above, that way we only do the kvm_put if we previously had a successful kvm_get

>> +		vfio_kvm_put_kvm(kvm);
>> +		device->kvm = NULL;
>> +	}
>>  err_unuse_iommu:
>> -	device->kvm = NULL;
>>  	if (iommufd)
>>  		vfio_iommufd_unbind(device);
>>  	else
>> @@ -387,7 +425,6 @@ static void vfio_device_last_close(struct vfio_device *device,
>>  
>>  	if (device->ops->close_device)
>>  		device->ops->close_device(device);
>> -	device->kvm = NULL;
>>  	if (iommufd)
>>  		vfio_iommufd_unbind(device);
>>  	else
>> @@ -465,6 +502,11 @@ static int vfio_device_fops_release(struct inode *inode, struct file *filep)
>>  
>>  	vfio_device_group_close(device);
>>  
>> +	if (device->open_count == 0 && device->kvm) {
>> +		vfio_kvm_put_kvm(device->kvm);
>> +		device->kvm = NULL;
>> +	}
> 
> IIUC, device->open_count is protected by device->dev_set->lock.  Thanks,

Yep, thanks.  I will surround this bit of code with

mutex_lock(&device->dev_set->lock);
..
mutex_unlock(&device->dev_set->lock);