Message-ID: <20200305111734.4025ce2f@w520.home>
Date:   Thu, 5 Mar 2020 11:17:34 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dev@...k.org" <dev@...k.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "thomas@...jalon.net" <thomas@...jalon.net>,
        "bluca@...ian.org" <bluca@...ian.org>,
        "jerinjacobk@...il.com" <jerinjacobk@...il.com>,
        "Richardson, Bruce" <bruce.richardson@...el.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>
Subject: Re: [PATCH v2 3/7] vfio/pci: Introduce VF token

On Tue, 25 Feb 2020 02:59:37 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Alex Williamson
> > Sent: Thursday, February 20, 2020 2:54 AM
> > 
> > If we enable SR-IOV on a vfio-pci owned PF, the resulting VFs are not
> > fully isolated from the PF.  The PF can always cause a denial of service
> > to the VF, even if by simply resetting itself.  The degree to which a PF
> > can access the data passed through a VF or interfere with its operation
> > is dependent on a given SR-IOV implementation.  Therefore we want to
> > avoid a scenario where an existing vfio-pci based userspace driver might
> > assume the PF driver is trusted, for example assigning a PF to one VM
> > and VF to another with some expectation of isolation.  IOMMU grouping
> > could be a solution to this, but imposes an unnecessarily strong
> > relationship between PF and VF drivers if they need to operate with the
> > same IOMMU context.  Instead we introduce a "VF token", which is
> > essentially just a shared secret between PF and VF drivers, implemented
> > as a UUID.
> > 
> > The VF token can be set by a vfio-pci based PF driver and must be known
> > by the vfio-pci based VF driver in order to gain access to the device.
> > This allows the degree to which this VF token is considered secret to be
> > determined by the applications and environment.  For example a VM might
> > generate a random UUID known only internally to the hypervisor while a
> > userspace networking appliance might use a shared, or even well know,
> > UUID among the application drivers.
> > 
> > To incorporate this VF token, the VFIO_GROUP_GET_DEVICE_FD interface is
> > extended to accept key=value pairs in addition to the device name.  This
> > allows us to most easily deny user access to the device without risk
> > that existing userspace drivers assume region offsets, IRQs, and other
> > device features, leading to more elaborate error paths.  The format of
> > these options are expected to take the form:
> > 
> > "$DEVICE_NAME $OPTION1=$VALUE1 $OPTION2=$VALUE2"
> > 
> > Where the device name is always provided first for compatibility and
> > additional options are specified in a space separated list.  The
> > relation between and requirements for the additional options will be
> > vfio bus driver dependent, however unknown or unused option within this
> > schema should return error.  This allow for future use of unknown
> > options as well as a positive indication to the user that an option is
> > used.
> > 
> > An example VF token option would take this form:
> > 
> > "0000:03:00.0 vf_token=2ab74924-c335-45f4-9b16-8569e5b08258"
> > 
> > When accessing a VF where the PF is making use of vfio-pci, the user
> > MUST provide the current vf_token.  When accessing a PF, the user MUST
> > provide the current vf_token IF there are active VF users or MAY provide
> > a vf_token in order to set the current VF token when no VF users are
> > active.  The former requirement assures VF users that an unassociated
> > driver cannot usurp the PF device.  These semantics also imply that a
> > VF token MUST be set by a PF driver before VF drivers can access their
> > device, the default token is random and mechanisms to read the token are
> > not provided in order to protect the VF token of previous users.  Use of
> > the vf_token option outside of these cases will return an error, as
> > discussed above.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > ---
> >  drivers/vfio/pci/vfio_pci.c         |  198
> > +++++++++++++++++++++++++++++++++++
> >  drivers/vfio/pci/vfio_pci_private.h |    8 +
> >  2 files changed, 205 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index 2ec6c31d0ab0..8dd6ef9543ca 100644
> > --- a/drivers/vfio/pci/vfio_pci.c
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -466,6 +466,44 @@ static void vfio_pci_disable(struct vfio_pci_device
> > *vdev)
> >  		vfio_pci_set_power_state(vdev, PCI_D3hot);
> >  }
> > 
> > +static struct pci_driver vfio_pci_driver;
> > +
> > +static struct vfio_pci_device *get_pf_vdev(struct vfio_pci_device *vdev,
> > +					   struct vfio_device **pf_dev)
> > +{
> > +	struct pci_dev *physfn = pci_physfn(vdev->pdev);
> > +
> > +	if (!vdev->pdev->is_virtfn)
> > +		return NULL;
> > +
> > +	*pf_dev = vfio_device_get_from_dev(&physfn->dev);
> > +	if (!*pf_dev)
> > +		return NULL;
> > +
> > +	if (pci_dev_driver(physfn) != &vfio_pci_driver) {
> > +		vfio_device_put(*pf_dev);
> > +		return NULL;
> > +	}
> > +
> > +	return vfio_device_data(*pf_dev);
> > +}
> > +
> > +static void vfio_pci_vf_token_user_add(struct vfio_pci_device *vdev, int val)
> > +{
> > +	struct vfio_device *pf_dev;
> > +	struct vfio_pci_device *pf_vdev = get_pf_vdev(vdev, &pf_dev);
> > +
> > +	if (!pf_vdev)
> > +		return;
> > +
> > +	mutex_lock(&pf_vdev->vf_token->lock);
> > +	pf_vdev->vf_token->users += val;
> > +	WARN_ON(pf_vdev->vf_token->users < 0);
> > +	mutex_unlock(&pf_vdev->vf_token->lock);
> > +
> > +	vfio_device_put(pf_dev);
> > +}
> > +
> >  static void vfio_pci_release(void *device_data)
> >  {
> >  	struct vfio_pci_device *vdev = device_data;
> > @@ -473,6 +511,7 @@ static void vfio_pci_release(void *device_data)
> >  	mutex_lock(&vdev->reflck->lock);
> > 
> >  	if (!(--vdev->refcnt)) {
> > +		vfio_pci_vf_token_user_add(vdev, -1);
> >  		vfio_spapr_pci_eeh_release(vdev->pdev);
> >  		vfio_pci_disable(vdev);
> >  	}
> > @@ -498,6 +537,7 @@ static int vfio_pci_open(void *device_data)
> >  			goto error;
> > 
> >  		vfio_spapr_pci_eeh_open(vdev->pdev);
> > +		vfio_pci_vf_token_user_add(vdev, 1);
> >  	}
> >  	vdev->refcnt++;
> >  error:
> > @@ -1278,11 +1318,148 @@ static void vfio_pci_request(void *device_data,
> > unsigned int count)
> >  	mutex_unlock(&vdev->igate);
> >  }
> > 
> > +static int vfio_pci_validate_vf_token(struct vfio_pci_device *vdev,
> > +				      bool vf_token, uuid_t *uuid)
> > +{
> > +	/*
> > +	 * There's always some degree of trust or collaboration between SR-
> > IOV
> > +	 * PF and VFs, even if just that the PF hosts the SR-IOV capability and
> > +	 * can disrupt VFs with a reset, but often the PF has more explicit
> > +	 * access to deny service to the VF or access data passed through the
> > +	 * VF.  We therefore require an opt-in via a shared VF token (UUID)
> > to
> > +	 * represent this trust.  This both prevents that a VF driver might
> > +	 * assume the PF driver is a trusted, in-kernel driver, and also that
> > +	 * a PF driver might be replaced with a rogue driver, unknown to in-
> > use
> > +	 * VF drivers.
> > +	 *
> > +	 * Therefore when presented with a VF, if the PF is a vfio device and
> > +	 * it is bound to the vfio-pci driver, the user needs to provide a VF
> > +	 * token to access the device, in the form of appending a vf_token to
> > +	 * the device name, for example:
> > +	 *
> > +	 * "0000:04:10.0 vf_token=bd8d9d2b-5a5f-4f5a-a211-f591514ba1f3"
> > +	 *
> > +	 * When presented with a PF which has VFs in use, the user must also
> > +	 * provide the current VF token to prove collaboration with existing
> > +	 * VF users.  If VFs are not in use, the VF token provided for the PF
> > +	 * device will act to set the VF token.
> > +	 *
> > +	 * If the VF token is provided but unused, a fault is generated.  
> 
> fault->error, otherwise it is easy to consider a CPU fault. 😊

Ok, I can make that change, but I think you might have a unique
background to make a leap that a userspace ioctl can trigger a CPU
fault ;)
 
> > +	 */
> > +	if (!vdev->pdev->is_virtfn && !vdev->vf_token && !vf_token)
> > +		return 0; /* No VF token provided or required */
> > +
> > +	if (vdev->pdev->is_virtfn) {
> > +		struct vfio_device *pf_dev;
> > +		struct vfio_pci_device *pf_vdev = get_pf_vdev(vdev,
> > &pf_dev);
> > +		bool match;
> > +
> > +		if (!pf_vdev) {
> > +			if (!vf_token)
> > +				return 0; /* PF is not vfio-pci, no VF token */
> > +
> > +			pci_info_ratelimited(vdev->pdev,
> > +				"VF token incorrectly provided, PF not bound
> > to vfio-pci\n");
> > +			return -EINVAL;
> > +		}
> > +
> > +		if (!vf_token) {
> > +			vfio_device_put(pf_dev);
> > +			pci_info_ratelimited(vdev->pdev,
> > +				"VF token required to access device\n");
> > +			return -EACCES;
> > +		}
> > +
> > +		mutex_lock(&pf_vdev->vf_token->lock);
> > +		match = uuid_equal(uuid, &pf_vdev->vf_token->uuid);
> > +		mutex_unlock(&pf_vdev->vf_token->lock);
> > +
> > +		vfio_device_put(pf_dev);
> > +
> > +		if (!match) {
> > +			pci_info_ratelimited(vdev->pdev,
> > +				"Incorrect VF token provided for device\n");
> > +			return -EACCES;
> > +		}
> > +	} else if (vdev->vf_token) {
> > +		mutex_lock(&vdev->vf_token->lock);
> > +		if (vdev->vf_token->users) {
> > +			if (!vf_token) {
> > +				mutex_unlock(&vdev->vf_token->lock);
> > +				pci_info_ratelimited(vdev->pdev,
> > +					"VF token required to access
> > device\n");
> > +				return -EACCES;
> > +			}
> > +
> > +			if (!uuid_equal(uuid, &vdev->vf_token->uuid)) {
> > +				mutex_unlock(&vdev->vf_token->lock);
> > +				pci_info_ratelimited(vdev->pdev,
> > +					"Incorrect VF token provided for
> > device\n");
> > +				return -EACCES;
> > +			}
> > +		} else if (vf_token) {
> > +			uuid_copy(&vdev->vf_token->uuid, uuid);
> > +		}  
> 
> It implies that we allow PF to be accessed w/o providing a VF token,
> as long as no VF is currently in-use, which further means no VF can
> be further assigned since no one knows the random uuid allocated
> by vfio. Just want to confirm whether it is the desired flavor. If an
> user really wants to use PF-only, possibly he should disable SR-IOV
> instead...

Yes, this is the behavior I'm intending.  Are you suggesting that we
should require a VF token in order to access a PF that has SR-IOV
already enabled?  This introduces an inconsistency that SR-IOV can be
enabled via sysfs asynchronous to the GET_DEVICE_FD ioctl, so we'd need
to secure the sysfs interface to only allow enabling SR-IOV when the PF
is already opened to cases where the VF token is already set?  Thus
SR-IOV could be pre-enabled, but the user must provide a vf_token
option on GET_DEVICE_FD, otherwise SR-IOV could only be enabled after
the user sets a VF token.  But then do we need to invalidate the token
at some point, or else it seems like we have the same scenario when the
next user comes along.  We believe there are PFs that require no
special VF support other than sriov_configure, so those driver could
theoretically close the PF after setting a VF token.  That makes it
difficult to determine the lifetime of a VF token and leads to the
interface proposed here of an initial random token, then the user set
token persisting indefinitely.

I've tended consider all of these to be mechanisms that a user can
shoot themselves in the foot.  Yes, the user and admin can do things
that will fail to work with this interface, for example my testing
involves QEMU, where we don't expose SR-IOV to the guest yet and the
igb driver for the PF will encounter problems running a device with
SR-IOV enabled that it doesn't know about.  Do we want to try to play
nanny and require specific semantics?  I've opt'd for the more simple
code here.

> > +
> > +		mutex_unlock(&vdev->vf_token->lock);
> > +	} else if (vf_token) {
> > +		pci_info_ratelimited(vdev->pdev,
> > +			"VF token incorrectly provided, not a PF or VF\n");
> > +		return -EINVAL;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +#define VF_TOKEN_ARG "vf_token="
> > +
> >  static int vfio_pci_match(void *device_data, char *buf)
> >  {
> >  	struct vfio_pci_device *vdev = device_data;
> > +	bool vf_token = false;
> > +	uuid_t uuid;
> > +	int ret;
> > +
> > +	if (strncmp(pci_name(vdev->pdev), buf, strlen(pci_name(vdev-  
> > >pdev))))  
> > +		return 0; /* No match */
> > +
> > +	if (strlen(buf) > strlen(pci_name(vdev->pdev))) {
> > +		buf += strlen(pci_name(vdev->pdev));
> > +
> > +		if (*buf != ' ')
> > +			return 0; /* No match: non-whitespace after name */
> > +
> > +		while (*buf) {
> > +			if (*buf == ' ') {
> > +				buf++;
> > +				continue;
> > +			}
> > +
> > +			if (!vf_token && !strncmp(buf, VF_TOKEN_ARG,
> > +						  strlen(VF_TOKEN_ARG))) {
> > +				buf += strlen(VF_TOKEN_ARG);
> > +
> > +				if (strlen(buf) < UUID_STRING_LEN)
> > +					return -EINVAL;
> > +
> > +				ret = uuid_parse(buf, &uuid);
> > +				if (ret)
> > +					return ret;
> > 
> > -	return !strcmp(pci_name(vdev->pdev), buf);
> > +				vf_token = true;
> > +				buf += UUID_STRING_LEN;
> > +			} else {
> > +				/* Unknown/duplicate option */
> > +				return -EINVAL;
> > +			}
> > +		}
> > +	}
> > +
> > +	ret = vfio_pci_validate_vf_token(vdev, vf_token, &uuid);
> > +	if (ret)
> > +		return ret;
> > +
> > +	return 1; /* Match */
> >  }
> > 
> >  static const struct vfio_device_ops vfio_pci_ops = {
> > @@ -1354,6 +1531,19 @@ static int vfio_pci_probe(struct pci_dev *pdev,
> > const struct pci_device_id *id)
> >  		return ret;
> >  	}
> > 
> > +	if (pdev->is_physfn) {
> > +		vdev->vf_token = kzalloc(sizeof(*vdev->vf_token),
> > GFP_KERNEL);
> > +		if (!vdev->vf_token) {
> > +			vfio_pci_reflck_put(vdev->reflck);
> > +			vfio_del_group_dev(&pdev->dev);
> > +			vfio_iommu_group_put(group, &pdev->dev);
> > +			kfree(vdev);
> > +			return -ENOMEM;
> > +		}
> > +		mutex_init(&vdev->vf_token->lock);
> > +		uuid_gen(&vdev->vf_token->uuid);  
> 
> should we also regenerate a random uuid somewhere when SR-IOV is
> disabled and then re-enabled on a PF? Although vfio disallows userspace
> to read uuid, it is always safer to avoid caching a secret from previous
> user.

What if our user is QEMU emulating SR-IOV to the guest.  Do we want to
force a new VF token is set every time we bounce the VFs?  Why?  As
above, the session lifetime of the VF token might be difficult to
determine and I'm not sure paranoia is a sufficient reason to try to
create boundaries for it.  Thanks,

Alex

> > +	}
> > +
> >  	if (vfio_pci_is_vga(pdev)) {
> >  		vga_client_register(pdev, vdev, NULL,
> > vfio_pci_set_vga_decode);
> >  		vga_set_legacy_decoding(pdev,
> > @@ -1387,6 +1577,12 @@ static void vfio_pci_remove(struct pci_dev *pdev)
> >  	if (!vdev)
> >  		return;
> > 
> > +	if (vdev->vf_token) {
> > +		WARN_ON(vdev->vf_token->users);
> > +		mutex_destroy(&vdev->vf_token->lock);
> > +		kfree(vdev->vf_token);
> > +	}
> > +
> >  	vfio_pci_reflck_put(vdev->reflck);
> > 
> >  	vfio_iommu_group_put(pdev->dev.iommu_group, &pdev->dev);
> > diff --git a/drivers/vfio/pci/vfio_pci_private.h
> > b/drivers/vfio/pci/vfio_pci_private.h
> > index 8a2c7607d513..76c11c915949 100644
> > --- a/drivers/vfio/pci/vfio_pci_private.h
> > +++ b/drivers/vfio/pci/vfio_pci_private.h
> > @@ -12,6 +12,7 @@
> >  #include <linux/pci.h>
> >  #include <linux/irqbypass.h>
> >  #include <linux/types.h>
> > +#include <linux/uuid.h>
> > 
> >  #ifndef VFIO_PCI_PRIVATE_H
> >  #define VFIO_PCI_PRIVATE_H
> > @@ -84,6 +85,12 @@ struct vfio_pci_reflck {
> >  	struct mutex		lock;
> >  };
> > 
> > +struct vfio_pci_vf_token {
> > +	struct mutex		lock;
> > +	uuid_t			uuid;
> > +	int			users;
> > +};
> > +
> >  struct vfio_pci_device {
> >  	struct pci_dev		*pdev;
> >  	void __iomem		*barmap[PCI_STD_NUM_BARS];
> > @@ -122,6 +129,7 @@ struct vfio_pci_device {
> >  	struct list_head	dummy_resources_list;
> >  	struct mutex		ioeventfds_lock;
> >  	struct list_head	ioeventfds_list;
> > +	struct vfio_pci_vf_token	*vf_token;
> >  };
> > 
> >  #define is_intx(vdev) (vdev->irq_type == VFIO_PCI_INTX_IRQ_INDEX)  
> 

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