On Thu, 9 Jul 2020 07:16:31 +0000
"Liu, Yi L" <yi.l....@intel.com> wrote:
> Hi Alex,
>
> After more thinking, looks like adding a r-b tree is still not enough to
> solve the potential problem for free a range of PASID in one ioctl. If
> caller gives [0, MAX_UNIT] in the free request, kernel anyhow should
> loop all the PASIDs and search in the r-b tree. Even VFIO can track the
> smallest/largest allocated PASID, and limit the free range to an accurate
> range, it is still no efficient. For example, user has allocated two PASIDs
> ( 1 and 999), and user gives the [0, MAX_UNIT] range in free request. VFIO
> will limit the free range to be [1, 999], but still needs to loop PASID 1 -
> 999, and search in r-b tree.
That sounds like a poor tree implementation. Look at vfio_find_dma()
for instance, it returns a node within the specified range. If the
tree has two nodes within the specified range we should never need to
call a search function like vfio_find_dma() more than three times. We
call it once, get the first node, remove it. Call it again, get the
other node, remove it. Call a third time, find no matches, we're done.
So such an implementation limits searches to N+1 where N is the number
of nodes within the range.
> So I'm wondering can we fall back to prior proposal which only free one
> PASID for a free request. how about your opinion?
Doesn't it still seem like it would be a useful user interface to have
a mechanism to free all pasids, by calling with exactly [0, MAX_UINT]?
I'm not sure if there's another use case for this given than the user
doesn't have strict control of the pasid values they get. Thanks,
Alex
> > From: Liu, Yi L <yi.l....@intel.com>
> > Sent: Thursday, July 9, 2020 10:26 AM
> >
> > Hi Kevin,
> >
> > > From: Tian, Kevin <kevin.t...@intel.com>
> > > Sent: Thursday, July 9, 2020 10:18 AM
> > >
> > > > From: Liu, Yi L <yi.l....@intel.com>
> > > > Sent: Thursday, July 9, 2020 10:08 AM
> > > >
> > > > Hi Kevin,
> > > >
> > > > > From: Tian, Kevin <kevin.t...@intel.com>
> > > > > Sent: Thursday, July 9, 2020 9:57 AM
> > > > >
> > > > > > From: Liu, Yi L <yi.l....@intel.com>
> > > > > > Sent: Thursday, July 9, 2020 8:32 AM
> > > > > >
> > > > > > Hi Alex,
> > > > > >
> > > > > > > Alex Williamson <alex.william...@redhat.com>
> > > > > > > Sent: Thursday, July 9, 2020 3:55 AM
> > > > > > >
> > > > > > > On Wed, 8 Jul 2020 08:16:16 +0000 "Liu, Yi L"
> > > > > > > <yi.l....@intel.com> wrote:
> > > > > > >
> > > > > > > > Hi Alex,
> > > > > > > >
> > > > > > > > > From: Liu, Yi L < yi.l....@intel.com>
> > > > > > > > > Sent: Friday, July 3, 2020 2:28 PM
> > > > > > > > >
> > > > > > > > > Hi Alex,
> > > > > > > > >
> > > > > > > > > > From: Alex Williamson <alex.william...@redhat.com>
> > > > > > > > > > Sent: Friday, July 3, 2020 5:19 AM
> > > > > > > > > >
> > > > > > > > > > On Wed, 24 Jun 2020 01:55:19 -0700 Liu Yi L
> > > > > > > > > > <yi.l....@intel.com> wrote:
> > > > > > > > > >
> > > > > > > > > > > This patch allows user space to request PASID
> > > > > > > > > > > allocation/free,
> > > > e.g.
> > > > > > > > > > > when serving the request from the guest.
> > > > > > > > > > >
> > > > > > > > > > > PASIDs that are not freed by userspace are
> > > > > > > > > > > automatically freed
> > > > > > when
> > > > > > > > > > > the IOASID set is destroyed when process exits.
> > > > > > > > [...]
> > > > > > > > > > > +static int vfio_iommu_type1_pasid_request(struct
> > > > > > > > > > > +vfio_iommu
> > > > > > *iommu,
> > > > > > > > > > > + unsigned long arg) {
> > > > > > > > > > > + struct vfio_iommu_type1_pasid_request req;
> > > > > > > > > > > + unsigned long minsz;
> > > > > > > > > > > +
> > > > > > > > > > > + minsz = offsetofend(struct
> > > vfio_iommu_type1_pasid_request,
> > > > > > > range);
> > > > > > > > > > > +
> > > > > > > > > > > + if (copy_from_user(&req, (void __user *)arg, minsz))
> > > > > > > > > > > + return -EFAULT;
> > > > > > > > > > > +
> > > > > > > > > > > + if (req.argsz < minsz || (req.flags &
> > > > > > > ~VFIO_PASID_REQUEST_MASK))
> > > > > > > > > > > + return -EINVAL;
> > > > > > > > > > > +
> > > > > > > > > > > + if (req.range.min > req.range.max)
> > > > > > > > > >
> > > > > > > > > > Is it exploitable that a user can spin the kernel for a
> > > > > > > > > > long time in the case of a free by calling this with [0,
> > > > > > > > > > MAX_UINT] regardless of their
> > > > > > > actual
> > > > > > > > > allocations?
> > > > > > > > >
> > > > > > > > > IOASID can ensure that user can only free the PASIDs
> > > > > > > > > allocated to the
> > > > > > user.
> > > > > > > but
> > > > > > > > > it's true, kernel needs to loop all the PASIDs within the
> > > > > > > > > range provided by user.
> > > > > > > it
> > > > > > > > > may take a long time. is there anything we can do? one
> > > > > > > > > thing may limit
> > > > > > the
> > > > > > > range
> > > > > > > > > provided by user?
> > > > > > > >
> > > > > > > > thought about it more, we have per-VM pasid quota (say
> > > > > > > > 1000), so even if user passed down [0, MAX_UNIT], kernel
> > > > > > > > will only loop the
> > > > > > > > 1000 pasids at most. do you think we still need to do something
> > > > > > > > on it?
> > > > > > >
> > > > > > > How do you figure that? vfio_iommu_type1_pasid_request()
> > > > > > > accepts the user's min/max so long as (max > min) and passes
> > > > > > > that to vfio_iommu_type1_pasid_free(), then to
> > > > > > > vfio_pasid_free_range() which loops as:
> > > > > > >
> > > > > > > ioasid_t pasid = min;
> > > > > > > for (; pasid <= max; pasid++)
> > > > > > > ioasid_free(pasid);
> > > > > > >
> > > > > > > A user might only be able to allocate 1000 pasids, but
> > > > > > > apparently they can ask to free all they want.
> > > > > > >
> > > > > > > It's also not obvious to me that calling ioasid_free() is only
> > > > > > > allowing the user to free their own passid. Does it? It
> > > > > > > would be a pretty
> > > > >
> > > > > Agree. I thought ioasid_free should at least carry a token since
> > > > > the user
> > > > space is
> > > > > only allowed to manage PASIDs in its own set...
> > > > >
> > > > > > > gaping hole if a user could free arbitrary pasids. A r-b tree
> > > > > > > of passids might help both for security and to bound spinning in
> > > > > > > a loop.
> > > > > >
> > > > > > oh, yes. BTW. instead of r-b tree in VFIO, maybe we can add an
> > > > > > ioasid_set parameter for ioasid_free(), thus to prevent the user
> > > > > > from freeing PASIDs that doesn't belong to it. I remember Jacob
> > > > > > mentioned it
> > > > before.
> > > > > >
> > > > >
> > > > > check current ioasid_free:
> > > > >
> > > > > spin_lock(&ioasid_allocator_lock);
> > > > > ioasid_data = xa_load(&active_allocator->xa, ioasid);
> > > > > if (!ioasid_data) {
> > > > > pr_err("Trying to free unknown IOASID %u\n", ioasid);
> > > > > goto exit_unlock;
> > > > > }
> > > > >
> > > > > Allow an user to trigger above lock paths with MAX_UINT times
> > > > > might still
> > > > be bad.
> > > >
> > > > yeah, how about the below two options:
> > > >
> > > > - comparing the max - min with the quota before calling ioasid_free().
> > > > If max - min > current quota of the user, then should fail it. If
> > > > max - min < quota, then call ioasid_free() one by one. still trigger
> > > > the above lock path with quota times.
> > >
> > > This is definitely wrong. [min, max] is about the range of the PASID
> > > value, while quota is about the number of allocated PASIDs. It's a bit
> > > weird to mix two together.
> >
> > got it.
> >
> > > btw what is the main purpose of allowing batch PASID free requests?
> > > Can we just simplify to allow one PASID in each free just like how is
> > > it done in allocation path?
> >
> > it's an intention to reuse the [min, max] range as allocation path.
> > currently, we
> > don't have such request as far as I can see.
> >
> > > >
> > > > - pass the max and min to ioasid_free(), let ioasid_free() decide.
> > > > should
> > > > be able to avoid trigger the lock multiple times, and ioasid has have
> > > > a
> > > > track on how may PASIDs have been allocated, if max - min is larger
> > > > than
> > > > the allocated number, should fail anyway.
> > >
> > > What about Alex's r-b tree suggestion? Is there any downside in you mind?
> > >
> >
> > no downside, I was just wanting to reuse the tracks in ioasid_set. I can
> > add a r-b
> > for allocated PASIDs and find the PASIDs in the r-b tree only do free for
> > the
> > PASIDs found in r-b tree, others in the range would be ignored.
> > does it look good?
> >
> > Regards,
> > Yi Liu
> >
> > > Thanks,
> > > Kevin
>
