Re: [PATCH v3 02/12] vfio/pci: Only check ownership of opened devices in hot reset

[Eric Auger <eric.auger@redhat.com>]

Hi Yi,

On 4/4/23 16:37, Liu, Yi L wrote:
> Hi Eric,
>
>> From: Eric Auger <eric.auger@redhat.com>
>> Sent: Tuesday, April 4, 2023 10:00 PM
>>
>> Hi YI,
>>
>> On 4/1/23 16:44, Yi Liu wrote:
>>> If the affected device is not opened by any user, it's safe to reset it
>>> given it's not in use.
>>>
>>> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
>>> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
>>> Tested-by: Yanting Jiang <yanting.jiang@intel.com>
>>> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
>>> ---
>>>  drivers/vfio/pci/vfio_pci_core.c | 14 +++++++++++---
>>>  include/uapi/linux/vfio.h        |  8 ++++++++
>>>  2 files changed, 19 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
>>> index 65bbef562268..5d745c9abf05 100644
>>> --- a/drivers/vfio/pci/vfio_pci_core.c
>>> +++ b/drivers/vfio/pci/vfio_pci_core.c
>>> @@ -2429,10 +2429,18 @@ static int vfio_pci_dev_set_hot_reset(struct
>> vfio_device_set *dev_set,
>>>  	list_for_each_entry(cur_vma, &dev_set->device_list, vdev.dev_set_list) {
>>>  		/*
>>> -		 * Test whether all the affected devices are contained by the
>>> -		 * set of groups provided by the user.
>>> +		 * Test whether all the affected devices can be reset by the
>>> +		 * user.
>>> +		 *
>>> +		 * Resetting an unused device (not opened) is safe, because
>>> +		 * dev_set->lock is held in hot reset path so this device
>>> +		 * cannot race being opened by another user simultaneously.
>>> +		 *
>>> +		 * Otherwise all opened devices in the dev_set must be
>>> +		 * contained by the set of groups provided by the user.
>>>  		 */
>>> -		if (!vfio_dev_in_groups(cur_vma, groups)) {
>>> +		if (cur_vma->vdev.open_count &&
>>> +		    !vfio_dev_in_groups(cur_vma, groups)) {
>>>  			ret = -EINVAL;
>>>  			goto err_undo;
>>>  		}
>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>>> index 0552e8dcf0cb..f96e5689cffc 100644
>>> --- a/include/uapi/linux/vfio.h
>>> +++ b/include/uapi/linux/vfio.h
>>> @@ -673,6 +673,14 @@ struct vfio_pci_hot_reset_info {
>>>   * VFIO_DEVICE_PCI_HOT_RESET - _IOW(VFIO_TYPE, VFIO_BASE + 13,
>>>   *				    struct vfio_pci_hot_reset)
>>>   *
>>> + * Userspace requests hot reset for the devices it uses.  Due to the
>>> + * underlying topology, multiple devices can be affected in the reset
>> by the reset
>>> + * while some might be opened by another user.  To avoid interference
>> s/interference/hot reset failure?
> I don’t think user can really avoid hot reset failure since there may
> be new devices plugged into the affected slot. Even user has opened
I don't know the legacy wrt that issue but this sounds a serious issue,
meaning the reset of an assigned device could impact another device
belonging to another group not not owned by the user?
> all the groups/devices reported by VFIO_DEVICE_GET_PCI_HOT_RESET_INFO,
> the hot reset can fail if new device is plugged in and has not been
> bound to vfio or opened by another user during the window of
> _INFO and HOT_RESET.
with respect to the latter isn't the dev_set lock held during the hot
reset and sufficient to prevent any new opening to occur?
>
> maybe the whole statement should be as below:
>
> To avoid interference, the hot reset can only be conducted when all
> the affected devices are either opened by the calling user or not
> opened yet at the moment of the hot reset attempt.

OK

Eric
>
>>> + * the calling user must ensure all affected devices, if opened, are
>>> + * owned by itself.
>>> + *
>>> + * The ownership is proved by an array of group fds.
>>> + *
>>>   * Return: 0 on success, -errno on failure.
>>>   */
>>>  struct vfio_pci_hot_reset {
> Regards,
> Yi Liu