Date:   Wed, 9 Feb 2022 19:09:02 +0530
From:   Abhishek Sahu <abhsahu@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, Cornelia Huck <cohuck@...hat.com>,
        Max Gurtovoy <mgurtovoy@...dia.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Jason Gunthorpe <jgg@...dia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vfio/pci: fix memory leak during D3hot to D0
 transition

On 2/9/2022 2:56 AM, Alex Williamson wrote:
> On Tue, 8 Feb 2022 00:50:08 +0530
> Abhishek Sahu <abhsahu@...dia.com> wrote:
> 
>> On 2/2/2022 5:01 AM, Alex Williamson wrote:
>>> On Tue, 1 Feb 2022 17:06:43 +0530
>>> Abhishek Sahu <abhsahu@...dia.com> wrote:
>>>
>>>> On 2/1/2022 1:41 AM, Alex Williamson wrote:
>>>>> On Mon, 31 Jan 2022 16:54:50 +0530
>>>>> Abhishek Sahu <abhsahu@...dia.com> wrote:
>>>>>
>>>>>> If needs_pm_restore is set (PCI device does not have support for no
>>>>>> soft reset), then the current PCI state will be saved during D0->D3hot
>>>>>> transition and same will be restored back during D3hot->D0 transition.
>>>>>> For saving the PCI state locally, pci_store_saved_state() is being
>>>>>> used and the pci_load_and_free_saved_state() will free the allocated
>>>>>> memory.
>>>>>>
>>>>>> But for reset related IOCTLs, vfio driver calls PCI reset related
>>>>>> API's which will internally change the PCI power state back to D0. So,
>>>>>> when the guest resumes, then it will get the current state as D0 and it
>>>>>> will skip the call to vfio_pci_set_power_state() for changing the
>>>>>> power state to D0 explicitly. In this case, the memory pointed by
>>>>>> pm_save will never be freed. In a malicious sequence, the state changing
>>>>>> to D3hot followed by VFIO_DEVICE_RESET/VFIO_DEVICE_PCI_HOT_RESET can be
>>>>>> run in a loop and it can cause an OOM situation.
>>>>>>
>>>>>> Also, pci_pm_reset() returns -EINVAL if we try to reset a device that
>>>>>> isn't currently in D0. Therefore any path where we're triggering a
>>>>>> function reset that could use a PM reset and we don't know if the device
>>>>>> is in D0, should wake up the device before we try that reset.
>>>>>>
>>>>>> This patch changes the device power state to D0 by invoking
>>>>>> vfio_pci_set_power_state() before calling reset related API's.
>>>>>> It will help in fixing the mentioned memory leak and making sure
>>>>>> that the device is in D0 during reset. Also, to prevent any similar
>>>>>> memory leak for future development, this patch frees memory first
>>>>>> before overwriting 'pm_save'.
>>>>>>
>>>>>> Fixes: 51ef3a004b1e ("vfio/pci: Restore device state on PM transition")
>>>>>> Signed-off-by: Abhishek Sahu <abhsahu@...dia.com>
>>>>>> ---
>>>>>>
>>>>>> * Changes in v2
>>>>>>
>>>>>> - Add the Fixes tag and sent this patch independently.
>>>>>> - Invoke vfio_pci_set_power_state() before invoking reset related API's.
>>>>>> - Removed saving of power state locally.
>>>>>> - Removed warning before 'kfree(vdev->pm_save)'.
>>>>>> - Updated comments and commit message according to updated changes.
>>>>>>
>>>>>> * v1 of this patch was sent in
>>>>>> https://lore.kernel.org/lkml/20220124181726.19174-4-abhsahu@nvidia.com/
>>>>>>
>>>>>>  drivers/vfio/pci/vfio_pci_core.c | 27 +++++++++++++++++++++++++++
>>>>>>  1 file changed, 27 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
>>>>>> index f948e6cd2993..d6dd4f7c4b2c 100644
>>>>>> --- a/drivers/vfio/pci/vfio_pci_core.c
>>>>>> +++ b/drivers/vfio/pci/vfio_pci_core.c
>>>>>> @@ -228,6 +228,13 @@ int vfio_pci_set_power_state(struct vfio_pci_core_device *vdev, pci_power_t stat
>>>>>>       if (!ret) {
>>>>>>               /* D3 might be unsupported via quirk, skip unless in D3 */
>>>>>>               if (needs_save && pdev->current_state >= PCI_D3hot) {
>>>>>> +                     /*
>>>>>> +                      * If somehow, the vfio driver was not able to free the
>>>>>> +                      * memory allocated in pm_save, then free the earlier
>>>>>> +                      * memory first before overwriting pm_save to prevent
>>>>>> +                      * memory leak.
>>>>>> +                      */
>>>>>> +                     kfree(vdev->pm_save);
>>>>>>                       vdev->pm_save = pci_store_saved_state(pdev);
>>>>>>               } else if (needs_restore) {
>>>>>>                       pci_load_and_free_saved_state(pdev, &vdev->pm_save);
>>>>>> @@ -322,6 +329,12 @@ void vfio_pci_core_disable(struct vfio_pci_core_device *vdev)
>>>>>>       /* For needs_reset */
>>>>>>       lockdep_assert_held(&vdev->vdev.dev_set->lock);
>>>>>>
>>>>>> +     /*
>>>>>> +      * This function can be invoked while the power state is non-D0,
>>>>>> +      * Change the device power state to D0 first.
>>>>>
>>>>> I think we need to describe more why we're doing this than what we're
>>>>> doing.  We need to make sure the device is in D0 in case we have a
>>>>> reset method that depends on that directly, ex. pci_pm_reset(), or
>>>>> possibly device specific resets that may access device BAR resources.
>>>>> I think it's placed here in the function so that the config space
>>>>> changes below aren't overwritten by restoring the saved state and maybe
>>>>> also because the set_irqs_ioctl() call might access device MMIO space.
>>>>>
>>>>
>>>>  Thanks Alex.
>>>>  I will add more details here in the comment.
>>>>
>>>>>> +      */
>>>>>> +     vfio_pci_set_power_state(vdev, PCI_D0);
>>>>>> +
>>>>>>       /* Stop the device from further DMA */
>>>>>>       pci_clear_master(pdev);
>>>>>>
>>>>>> @@ -921,6 +934,13 @@ long vfio_pci_core_ioctl(struct vfio_device *core_vdev, unsigned int cmd,
>>>>>>                       return -EINVAL;
>>>>>>
>>>>>>               vfio_pci_zap_and_down_write_memory_lock(vdev);
>>>>>> +
>>>>>> +             /*
>>>>>> +              * This function can be invoked while the power state is non-D0,
>>>>>> +              * Change the device power state to D0 before doing reset.
>>>>>> +              */
>>>>>
>>>>> See below, reconsidering this...
>>>>>
>>>>>> +             vfio_pci_set_power_state(vdev, PCI_D0);
>>>>>> +
>>>>>>               ret = pci_try_reset_function(vdev->pdev);
>>>>>>               up_write(&vdev->memory_lock);
>>>>>>
>>>>>> @@ -2055,6 +2075,13 @@ static int vfio_pci_dev_set_hot_reset(struct vfio_device_set *dev_set,
>>>>>>       }
>>>>>>       cur_mem = NULL;
>>>>>>
>>>>>> +     /*
>>>>>> +      * This function can be invoked while the power state is non-D0.
>>>>>> +      * Change power state of all devices to D0 before doing reset.
>>>>>> +      */
>>>>>
>>>>> Here I have trouble convincing myself exactly what we're doing.  As you
>>>>> note in patch 1/ of the RFC series, pci_reset_bus(), or more precisely
>>>>> pci_dev_save_and_disable(), wakes the device to D0 before reset, so we
>>>>> can't be doing this only to get the device into D0.  The function level
>>>>> resets do the same.
>>>>>
>>>>> Actually, now I'm remembering and debugging where I got myself confused
>>>>> previously with pci_pm_reset().  The scenario was a Windows guest with
>>>>> an assigned Intel 82574L NIC.  When doing a shutdown from the guest the
>>>>> device is placed in D3hot and we enter vfio_pci_core_disable() in that
>>>>> state.  That function however uses __pci_reset_function_locked(), which
>>>>> skips the pci_dev_save_and_disable() since much of it is redundant for
>>>>> that call path (I think I generalized this to all flavors of
>>>>> pci_reset_function() in my head).
>>>>
>>>>  Thanks for providing the background related with the original issue.
>>>>
>>>>>
>>>>> The standard call to pci_try_reset_function(), as in the previous
>>>>> chunk, will make use of pci_dev_save_and_disable(), so for either of
>>>>> these latter cases the concern cannot be simply having the device in D0,
>>>>> we need a reason that we want the previously saved state restored on the
>>>>> device before the reset, and thus restored to the device after the
>>>>> reset as the rationale for the change.
>>>>>
>>>>
>>>>  I will add this as a comment.
>>>>
>>>>>> +     list_for_each_entry(cur, &dev_set->device_list, vdev.dev_set_list)
>>>>>> +             vfio_pci_set_power_state(cur, PCI_D0);
>>>>>> +
>>>>>>       ret = pci_reset_bus(pdev);
>>>>>>
>>>>>>  err_undo:
>>>>>
>>>>>
>>>>> We also call pci_reset_bus() in vfio_pci_dev_set_try_reset().  In that
>>>>> case, none of the other devices can be in use by the user, but they can
>>>>> certainly be in D3hot with previous device state saved off into our
>>>>> pm_save cache.  If we don't have a good reason to restore in that case,
>>>>> I'm wondering if we really have a good reason to restore in the above
>>>>> two cases.
>>>>>
>>>>> Perhaps we just need the first chunk above to resolve the memory leak,
>>>>
>>>>  First chunk means only the changes done in vfio_pci_set_power_state()
>>>>  which is calling kfree() before calling pci_store_saved_state().
>>>>  Or I need to include more things in the first patch ?
>>>
>>> Correct, first chunk as is the first change in the patch.  Patch chunks
>>> are delineated by the @@ offset lines.
>>>
>>
>>  Thanks for confirming this.
>>
>>>>
>>>>  With the kfree(), the original memory leak issue should be solved.
>>>>
>>>>> and the second chunk as a separate patch to resolve the issue with
>>>>> devices entering vfio_pci_core_disable() in non-D0 state.  Sorry if I
>>>>
>>>>  And this second patch will contain rest of the things where
>>>>  we will call vfio_pci_set_power_state() explicitly for moving to
>>>>  D0 state ?
>>>
>>> At least the first one in vfio_pci_core_disable(), the others need
>>> justification.
>>>
>>
>>  Yes. First one is needed.
>>
>>>>  Also, We need to explore if setting to D0 state is really required at
>>>>  all these places and If it is not required, then we don't need second
>>>>  patch ?
>>>
>>> We need a second patch, I'm convinced that we don't otherwise wake the
>>> device to D0 before we potentially get to pci_pm_reset() in
>>> vfio_pci_core_disable().  It's the remaining cases of setting D0 that
>>> I'm less clear on.  If it's the case that we need to restore config
>>> space any time a NoSoftRst- device is woken from D3hot and the state
>>> saved and restored around the reset is meaningless otherwise, that's a
>>> valid justification, but is it accurate?  If so, we should recheck the
>>> other case of calling pci_reset_bus() too.  Thanks,
>>>
>>> Alex
>>>
>>
>>  I was analyzing this part in detail and added some debug prints and
>>  made user space program to understand it better. Also, I have gone
>>  through the patch 51ef3a004b1e (“vfio/pci: Restore device state on
>>  PM transition”).
>>
>>  We have 2 cases here:
>>
>>  1. The devices which has NoSoftRst+  (needs_pm_restore is false).
>>     This case should work fine for all the cases (Apart from vfio_pci_core_disable())
>>     without waking-up the device explicitly.
>>
>>  2. The devices which has NoSoftRst- (needs_pm_restore is true).
>>
>>  For case 2, let’s consider following example:
>>
>>  a. The device is in D3hot.
>>  b. User made VFIO_DEVICE_RESET ioctl.
>>  c. pci_try_reset_function() will be called which internally
>>     invokes pci_dev_save_and_disable().
>>  d. pci_set_power_state(dev, PCI_D0) will be called first.
>>  e. pci_save_state() will happen then.
>>
>>  Now, for the devices which has NoSoftRst-,
>>  the pci_set_power_state() should trigger soft reset and
>>  we may lose the original state at step (d) and this state
>>  cannot be restored.
>>
>>  For example, lets assume the case, where SBIOS or host
>>  linux kernel (In the aspm.c) enables PCIe LTR setting for the
>>  PCIe device. When this soft reset will be triggered, then this
>>  LTR setting may be reset, and the device state saved at step (e)
>>  will also have this setting cleared so it cannot be restored.
>>  Same thing can be happened for other PCIe capabilities. Since the
>>  vfio driver only exposes limited enhanced capabilities to its user
>>  So, the vfio-driver user also won’t have option to save and
>>  restore these capabilities state and these original
>>  settings will be permanently lost.
> 
> Yes, this is my concern, thanks for confirming.
> 
>>  So, it seems we need to always move the device explicitly to
>>  D0 state by calling vfio_pci_set_power_state() before
>>  any reset for the reset triggered by IOCTLs. This is mainly to
>>  preserve the state around soft reset.
> 
> The other option would be to test for vdev->pm_save and if found do a
> load-and-free + restore-state after reset.  For simplicity, I'd tend to
> favor your approach to wake the device with vfio_pci_set_power_state()
> before reset.  Either approach seems roughly equal to me.
> 
>>  For vfio_pci_dev_set_try_reset() also, we can have the above
>>  mentioned situation. The other functions/devices can be in D3hot
>>  state and the D0 transition can cause soft reset there also.
>>  For example, in my case, NVIDIA GPU has VGA (func 0) and audio
>>  (func 1) function. I added debug print to dump the current state
>>  before and after pci_reset_bus(). Before pci_reset_bus() the func 1
>>  state was D3hot and after pci_reset_bus() the func 1 state got
>>  changed to D0. This pci_reset_bus() was called during closing of
>>  func 0 device so there are chances of soft reset for other
>>  function/devices.
> 
> Ok, so we'll always wakeup devices for both the pci_reset_function()
> class of resets and bus resets.  This seems to logically fit with the
> fix to wakeup the device on release, so shall we do patch 1 of the
> fixes series includes the kfree only and patch 2 resolves all the cases
> of waking devices before reset?  Thanks,
> 
> Alex
> 

 Thanks. I will make 2 patches and will send for review.

 Regards,
 Abhishek

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