RE: [PATCH v2 08/13] vfio/pci: protect cap/ecap_perm bits alloc/free with atomic op

[Liu, Yi L]

Hi Alex,

> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Thursday, September 26, 2019 10:36 AM
> To: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Subject: Re: [PATCH v2 08/13] vfio/pci: protect cap/ecap_perm bits alloc/free with
> atomic op
> 
> On Thu,  5 Sep 2019 15:59:25 +0800
> Liu Yi L <yi.l.liu@xxxxxxxxx> wrote:
> 
> > There is a case in which cap_perms and ecap_perms can be reallocated
> > by different modules. e.g. the vfio-mdev-pci sample driver. To secure
> > the initialization of cap_perms and ecap_perms, this patch adds an
> > atomic variable to track the user of cap/ecap_perms bits. First caller
> > of vfio_pci_init_perm_bits() will initialize the bits. While the last
> > caller of vfio_pci_uninit_perm_bits() will free the bits.
> 
> Yes, but it still allows races; we're not really protecting the data.
> If driver A begins freeing the shared data in the uninit path, driver B could start
> allocating shared data in the init path and we're left with either use after free issues
> or memory leaks.  Probably better to hold a semaphore around the allocation/free
> and a non-atomic for reference counting.  Thanks,

That's true. We just want to have only one copy of the bits. As long as the
race is under control, it is acceptable. Let me make this change. Thanks.

> Alex
> 
Regards,
Yi Liu

> > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Cc: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > Suggested-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
> > ---
> >  drivers/vfio/pci/vfio_pci_config.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_config.c
> > b/drivers/vfio/pci/vfio_pci_config.c
> > index f0891bd..1b3e6e5 100644
> > --- a/drivers/vfio/pci/vfio_pci_config.c
> > +++ b/drivers/vfio/pci/vfio_pci_config.c
> > @@ -992,11 +992,17 @@ static int __init init_pci_ext_cap_pwr_perm(struct
> perm_bits *perm)
> >  	return 0;
> >  }
> >
> > +/* Track the user number of the cap/ecap perm_bits */ atomic_t
> > +vfio_pci_perm_bits_users = ATOMIC_INIT(0);
> > +
> >  /*
> >   * Initialize the shared permission tables
> >   */
> >  void vfio_pci_uninit_perm_bits(void)
> >  {
> > +	if (atomic_dec_return(&vfio_pci_perm_bits_users))
> > +		return;
> > +
> >  	free_perm_bits(&cap_perms[PCI_CAP_ID_BASIC]);
> >
> >  	free_perm_bits(&cap_perms[PCI_CAP_ID_PM]);
> > @@ -1013,6 +1019,9 @@ int __init vfio_pci_init_perm_bits(void)  {
> >  	int ret;
> >
> > +	if (atomic_inc_return(&vfio_pci_perm_bits_users) != 1)
> > +		return 0;
> > +
> >  	/* Basic config space */
> >  	ret = init_pci_cap_basic_perm(&cap_perms[PCI_CAP_ID_BASIC]);
> >