Re: [RFC v2 2/3] vfio/type1: VFIO_IOMMU_PASID_REQUEST(alloc/free)

Jacob Pan
2019-11-13T19:40:57Z

On Wed, 13 Nov 2019 08:29:40 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Wed, 13 Nov 2019 11:03:17 +0000
> "Liu, Yi L" <yi.l.liu@intel.com> wrote:
>
> > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > Sent: Friday, November 8, 2019 11:15 PM
> > > To: Liu, Yi L <yi.l.liu@intel.com>
> > > Subject: Re: [RFC v2 2/3] vfio/type1:
> > > VFIO_IOMMU_PASID_REQUEST(alloc/free)
> > >
> > > On Fri, 8 Nov 2019 12:23:41 +0000
> > > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> > >
> > > > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > > > Sent: Friday, November 8, 2019 6:07 AM
> > > > > To: Liu, Yi L <yi.l.liu@intel.com>
> > > > > Subject: Re: [RFC v2 2/3] vfio/type1:
> > > > > VFIO_IOMMU_PASID_REQUEST(alloc/free)
> > > > >
> > > > > On Wed, 6 Nov 2019 13:27:26 +0000
> > > > > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> > > > >
> > > > > > > From: Alex Williamson <alex.williamson@redhat.com>
> > > > > > > Sent: Wednesday, November 6, 2019 7:36 AM
> > > > > > > To: Liu, Yi L <yi.l.liu@intel.com>
> > > > > > > Subject: Re: [RFC v2 2/3] vfio/type1:
> > > VFIO_IOMMU_PASID_REQUEST(alloc/free)
> > > > > > >
> > > > > > > On Thu, 24 Oct 2019 08:26:22 -0400
> > > > > > > Liu Yi L <yi.l.liu@intel.com> wrote:
> > > > > > >
> > > > > > > > This patch adds VFIO_IOMMU_PASID_REQUEST ioctl which
> > > > > > > > aims to passdown PASID allocation/free request from the
> > > > > > > > virtual iommu. This is required to get PASID managed in
> > > > > > > > system-wide.
> > > > > > > >
> > > > > > > > Cc: Kevin Tian <kevin.tian@intel.com>
> > > > > > > > Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> > > > > > > > Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>
> > > > > > > > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > > > > > > ---
> > > > > > > > drivers/vfio/vfio_iommu_type1.c | 114
> > > > > > > ++++++++++++++++++++++++++++++++++++++++
> > > > > > > > include/uapi/linux/vfio.h | 25 +++++++++
> > > > > > > > 2 files changed, 139 insertions(+)
> > > > > > > >
> > > > > > > > diff --git a/drivers/vfio/vfio_iommu_type1.c
> > > > > b/drivers/vfio/vfio_iommu_type1.c
> > > > > > > > index cd8d3a5..3d73a7d 100644
> > > > > > > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > > > > > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > > > > > > @@ -2248,6 +2248,83 @@ static int
> > > > > > > > vfio_cache_inv_fn(struct device *dev,
> > > > > void
> > > > > > > *data)
> > > > > > > > return iommu_cache_invalidate(dc->domain, dev,
> > > > > > > > &ustruct->info); }
> > > > > > > >
> > > > > > > > +static int vfio_iommu_type1_pasid_alloc(struct
> > > > > > > > vfio_iommu *iommu,
> > > > > > > > + int min_pasid,
> > > > > > > > + int max_pasid)
> > > > > > > > +{
> > > > > > > > + int ret;
> > > > > > > > + ioasid_t pasid;
> > > > > > > > + struct mm_struct *mm = NULL;
> > > > > > > > +
> > > > > > > > + mutex_lock(&iommu->lock);
> > > > > > > > + if (!IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu)) {
> > > > > > > > + ret = -EINVAL;
> > > > > > > > + goto out_unlock;
> > > > > > > > + }
> > > > > > > > + mm = get_task_mm(current);
> > > > > > > > + /* Track ioasid allocation owner by mm */
> > > > > > > > + pasid = ioasid_alloc((struct ioasid_set *)mm,
> > > > > > > > min_pasid,
> > > > > > > > + max_pasid, NULL);
> > > > > > >
> > > > > > > Are we sure we want to tie this to the task mm vs perhaps
> > > > > > > the vfio_iommu pointer?
> > > > > >
> > > > > > Here we want to have a kind of per-VM mark, which can be
> > > > > > used to do ownership check on whether a pasid is held by a
> > > > > > specific VM. This is very important to prevent across VM
> > > > > > affect. vfio_iommu pointer is competent for vfio as vfio is
> > > > > > both pasid alloc requester and pasid consumer. e.g. vfio
> > > > > > requests pasid alloc from ioasid and also it will invoke
> > > > > > bind_gpasid(). vfio can either check ownership before
> > > > > > invoking bind_gpasid() or pass vfio_iommu pointer to iommu
> > > > > > driver. But in future, there may be other modules which are
> > > > > > just consumers of pasid. And they also want to do ownership
> > > > > > check for a pasid. Then, it would be hard for them as they
> > > > > > are not the pasid alloc requester. So here better to have a
> > > > > > system wide structure to perform as the per-VM mark. task
> > > > > > mm looks to be much competent.
> > > > >
> > > > > Ok, so it's intentional to have a VM-wide token. Elsewhere
> > > > > in the type1 code (vfio_dma_do_map) we record the task_struct
> > > > > per dma mapping so that we can get the task mm as needed.
> > > > > Would the task_struct pointer provide any advantage?
> > > >
> > > > I think we may use task_struct pointer to make type1 code
> > > > consistent. How do you think?
> > >
> > > If it has the same utility, sure.
> >
> > thanks, I'll make this change.
> >
> > > > > Also, an overall question, this provides userspace with pasid
> > > > > alloc and free ioctls, (1) what prevents a userspace process
> > > > > from consuming every available pasid, and (2) if the process
> > > > > exits or crashes without freeing pasids, how are they
> > > > > recovered aside from a reboot?
> > > >
> > > > For question (1), I think we only need to take care about
> > > > malicious userspace process. As vfio usage is under privilege
> > > > mode, so we may be safe on it so far.
> > >
> > > No, where else do we ever make this assumption? vfio requires a
> > > privileged entity to configure the system for vfio, bind devices
> > > for user use, and grant those devices to the user, but the usage
> > > of the device is always assumed to be by an unprivileged user.
> > > It is absolutely not acceptable require a privileged user. It's
> > > vfio's responsibility to protect the system from the user.
> >
> > My assumption is not precise here. sorry for it... Maybe to further
> > check with you to better understand your point. I think the user
> > (QEMU) of vfio needs to have a root permission. Thus it can open
> > the vfio fds. At this point, the user is a privileged one. Also I
> > guess that's why vfio can grant the user with the usage of
> > VFIO_MAP/UNMAP to config mappings into iommu page tables. But I'm
> > not quite sure when will the user be an unprivileged one.
>
> QEMU does NOT need to be run as root to use vfio. This is NOT the
> model libvirt follows. libvirt grants a user access to a device, or
> rather a set of one or more devices (ie. the group) via standard file
> permission access to the group file (/dev/vfio/$GROUP). Ownership of
> a device allows the user permission to make use of the IOMMU. The
> user's ability to create DMA mappings is restricted by their process
> locked memory limits, where libvirt elevates the user limit
> sufficient for the size of the VM. QEMU should never need to be run
> as root and doing so is entirely unacceptable from a security
> perspective. The only mode of vfio that requires elevated privilege
> for use is when making use of no-iommu, where we have no IOMMU
> protection or translation.
>
> > > > However, we may need to introduce a kind of credit
> > > > mechanism to protect it. I've thought it, but no good idea yet.
> > > > Would be happy to hear from you.
> > >
> > > It's a limited system resource and it's unclear how many might
> > > reasonably used by a user. I don't have an easy answer.
> >
> > How about the below method? based on some offline chat with Jacob.
> > a. some reasonable defaults for the initial per VM quota, e.g. 1000
> > per process
> > b. IOASID should be able to enforce per ioasid_set (it is kind of
> > per VM mark) limit
>
> We support large numbers of assigned devices, how many IOASIDs might
> be reasonably used per device? Is the mm or the task still the
> correct "set" in this scenario? I don't have any better ideas than
> setting a limit, but it probably needs a kernel or module tunable,
> and it needs to match the scaling we expect to see when multiple
> devices are involved.
>
I think mm/task is still the correct set in that we try to prevent
abuse based on mm not device. Or we need to have some notion of super
container (Ashok proposed a while ago) that maps to a VM.

I am guessing you are suggesting the per mm quota should also be
scaled against number of devices assigned. I think that is very
reasonable. Perhaps we can do:

1. A tunable per iommu group PASID quota with a default of 1000, e.g.
nr_pasid_per_group. Since we are dealing with nested translation and
each device has its own second level so we pretty much have one device
per group. Call it nr_pasid_per_group?

2. Limit number of PASIDs per VM with nr_pasid_per_group * nr_groups.
Probably update the limit when group is added to a container with the
same mm.

I guess we could also use a cgroup controller for PASIDs.

> > > > For question (2), I think we need to reclaim the allocated
> > > > pasids when the vfio container fd is released just like what
> > > > vfio does to the domain mappings. I didn't add it yet. But I
> > > > can add it in next version if you think it would make the pasid
> > > > alloc/free be much sound.
> > >
> > > Consider it required, the interface is susceptible to abuse
> > > without it.
> >
> > sure, let me add it in next version.
> >
> > > > > > > > + if (pasid == INVALID_IOASID) {
> > > > > > > > + ret = -ENOSPC;
> > > > > > > > + goto out_unlock;
> > > > > > > > + }
> > > > > > > > + ret = pasid;
> > > > > > > > +out_unlock:
> > > > > > > > + mutex_unlock(&iommu->lock);
> > > > >
> > > > > What does holding this lock protect? That the vfio_iommu
> > > > > remains backed by an iommu during this operation, even though
> > > > > we don't do anything to release allocated pasids when that
> > > > > iommu backing is removed?
> > > >
> > > > yes, it is unnecessary to hold the lock here. At least for the
> > > > operations in this patch. will remove it. :-)
> > > >
> > > > > > > > + if (mm)
> > > > > > > > + mmput(mm);
> > > > > > > > + return ret;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +static int vfio_iommu_type1_pasid_free(struct
> > > > > > > > vfio_iommu *iommu,
> > > > > > > > + unsigned int
> > > > > > > > pasid) +{
> > > > > > > > + struct mm_struct *mm = NULL;
> > > > > > > > + void *pdata;
> > > > > > > > + int ret = 0;
> > > > > > > > +
> > > > > > > > + mutex_lock(&iommu->lock);
> > > > > > > > + if (!IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu)) {
> > > > > > > > + ret = -EINVAL;
> > > > > > > > + goto out_unlock;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + /**
> > > > > > > > + * REVISIT:
> > > > > > > > + * There are two cases free could fail:
> > > > > > > > + * 1. free pasid by non-owner, we use
> > > > > > > > ioasid_set to track mm, if
> > > > > > > > + * the set does not match, caller is not
> > > > > > > > permitted to free.
> > > > > > > > + * 2. free before unbind all devices, we can
> > > > > > > > check if ioasid private
> > > > > > > > + * data, if data != NULL, then fail to free.
> > > > > > > > + */
> > > > > > > > + mm = get_task_mm(current);
> > > > > > > > + pdata = ioasid_find((struct ioasid_set *)mm,
> > > > > > > > pasid, NULL);
> > > > > > > > + if (IS_ERR(pdata)) {
> > > > > > > > + if (pdata == ERR_PTR(-ENOENT))
> > > > > > > > + pr_err("PASID %u is not
> > > > > > > > allocated\n", pasid);
> > > > > > > > + else if (pdata == ERR_PTR(-EACCES))
> > > > > > > > + pr_err("Free PASID %u by
> > > > > > > > non-owner, denied",
> > > pasid);
> > > > > > > > + else
> > > > > > > > + pr_err("Error searching PASID
> > > > > > > > %u\n", pasid);
> > > > > > >
> > > > > > > This should be removed, errno is sufficient for the user,
> > > > > > > this just provides the user with a trivial DoS vector
> > > > > > > filling logs.
> > > > > >
> > > > > > sure, will fix it. thanks.
> > > > > >
> > > > > > > > + ret = -EPERM;
> > > > > > >
> > > > > > > But why not return PTR_ERR(pdata)?
> > > > > >
> > > > > > aha, would do it.
> > > > > >
> > > > > > > > + goto out_unlock;
> > > > > > > > + }
> > > > > > > > + if (pdata) {
> > > > > > > > + pr_debug("Cannot free pasid %d with
> > > > > > > > private data\n", pasid);
> > > > > > > > + /* Expect PASID has no private data if
> > > > > > > > not bond */
> > > > > > > > + ret = -EBUSY;
> > > > > > > > + goto out_unlock;
> > > > > > > > + }
> > > > > > > > + ioasid_free(pasid);
> > > > > > >
> > > > > > > We only ever get here with pasid == NULL?!
> > > > > >
> > > > > > I guess you meant only when pdata==NULL.
> > > > > >
> > > > > > > Something is wrong. Should
> > > > > > > that be 'if (!pdata)'? (which also makes that pr_debug
> > > > > > > another DoS vector)
> > > > > >
> > > > > > Oh, yes, just do it as below:
> > > > > >
> > > > > > if (!pdata) {
> > > > > > ioasid_free(pasid);
> > > > > > ret = SUCCESS;
> > > > > > } else
> > > > > > ret = -EBUSY;
> > > > > >
> > > > > > Is it what you mean?
> > > > >
> > > > > No, I think I was just confusing pdata and pasid, but I am
> > > > > still confused about testing pdata. We call ioasid_alloc()
> > > > > with private = NULL, and I don't see any of your patches
> > > > > calling ioasid_set_data() to change the private data after
> > > > > allocation, so how could this ever be set? Should this just
> > > > > be a BUG_ON(pdata) as the integrity of the system is in
> > > > > question should this state ever occur? Thanks,
> > > >
> > > > ioasid_set_data() was called in one patch from Jacob's vSVA
> > > > patchset. [PATCH v6 08/10] iommu/vt-d: Add bind guest PASID
> > > > support https://lkml.org/lkml/2019/10/22/946
> > > >
> > > > The basic idea is to allocate pasid with private=NULL, and set
> > > > it when the pasid is actually bind to a device (bind_gpasid()).
> > > > Each bind_gpasid() will increase the ref_cnt in the private
> > > > data, and each unbind_gpasid() will decrease the ref_cnt. So if
> > > > bind/unbind_gpasid() is called in mirror, the private data
> > > > should be null when comes to free operation. If not, vfio can
> > > > believe that the pasid is still in use.
> > >
> > > So this is another opportunity to leak pasids. What's a user
> > > supposed to do when their attempt to free a pasid fails? It
> > > invites leaks to allow this path to fail. Thanks,
> >
> > Agreed, may no need to fail pasid free as it may leak pasid. How
> > about always let free successful? If the ref_cnt is non-zero,
> > notify the remaining users to release their reference.
>
> If a user frees an PASID, they've done their due diligence in
> indicating it's no longer used. The kernel should handle reclaiming
> it from that point. Thanks,

Yeah, I think we can add a atomic notifier for each PASID.
Consumers such as IOMMU driver and KVM gets notified when IOASID is
freed by VFIO. IOMMU driver can do the unbind and tear down.

In case of all the users already did unbind() before ioasid_free(), the
free will proceed as usual.

> Alex
>