[RFC PATCH 30/30] vfio: Allow to bind foreign task

Alex Williamson
2017-02-28T03:54:11Z

On Mon, 27 Feb 2017 19:54:41 +0000
Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:

> Let the process that owns the device create an address space bond on
> behalf of another process. We add a pid argument to the BIND_TASK ioctl,
> allowing the caller to bind a foreign task. The expected program flow in
> this case is:
>
> * Process A creates the VFIO context and initializes the device.
> * Process B asks A to bind its address space.
> * Process A issues an ioctl to the VFIO device fd with BIND_TASK(pid).
> It may communicate the given PASID back to process B or keep track of it
> internally.
> * Process B asks A to perform transactions on its virtual address.
> * Process A launches transaction tagged with the given PASID.
>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> ---
> drivers/vfio/vfio.c | 35 +++++++++++++++++++++++++++++++++--
> include/uapi/linux/vfio.h | 15 +++++++++++++++
> 2 files changed, 48 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index c4505d8f4c61..ecc5d07e3dbb 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -26,6 +26,7 @@
> #include <linux/module.h>
> #include <linux/mutex.h>
> #include <linux/pci.h>
> +#include <linux/ptrace.h>
> #include <linux/rwsem.h>
> #include <linux/sched.h>
> #include <linux/slab.h>
> @@ -1660,7 +1661,7 @@ static long vfio_svm_ioctl(struct vfio_device *device, unsigned int cmd,
> struct vfio_device_svm svm;
> struct vfio_task *vfio_task;
>
> - minsz = offsetofend(struct vfio_device_svm, pasid);
> + minsz = offsetofend(struct vfio_device_svm, pid);

This is only the minsz if flags includes VFIO_SVM_PID, right?
Otherwise this isn't a backward compatible change (granted you're
proposing both in the same series), userspace built against 29/30
won't work against 30/30.

>
> if (copy_from_user(&svm, (void __user *)arg, minsz))
> return -EFAULT;
> @@ -1669,9 +1670,39 @@ static long vfio_svm_ioctl(struct vfio_device *device, unsigned int cmd,
> return -EINVAL;
>
> if (cmd == VFIO_DEVICE_BIND_TASK) {
> - struct task_struct *task = current;
> + struct mm_struct *mm;
> + struct task_struct *task;
> +
> + if (svm.flags & ~VFIO_SVM_PID)
> + return -EINVAL;

29/30 never validated flags, so theoretically userspace compiled
against 29/30 could have put anything in flags and it would have
worked, no longer the case here.

> +
> + if (svm.flags & VFIO_SVM_PID) {
> + rcu_read_lock();
> + task = find_task_by_vpid(svm.pid);
> + if (task)
> + get_task_struct(task);
> + rcu_read_unlock();
> + if (!task)
> + return -ESRCH;
> +
> + /*
> + * Ensure process has RW access on the task's mm
> + * FIXME:
> + * - I think this ought to be in the IOMMU API
> + * - I'm assuming permission is never revoked during the
> + * task's lifetime. Might be mistaken.
> + */
> + mm = mm_access(task, PTRACE_MODE_ATTACH_REALCREDS);
> + if (!mm || IS_ERR(mm))
> + return IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
> + mmput(mm);
> + } else {
> + get_task_struct(current);
> + task = current;
> + }
>
> ret = iommu_bind_task(device->dev, task, &svm.pasid, 0, NULL);
> + put_task_struct(task);
> if (ret)
> return ret;
>
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index 3fe4197a5ea0..41ae8a231d42 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -415,7 +415,9 @@ struct vfio_device_svm {
> __u32 flags;
> #define VFIO_SVM_PASID_RELEASE_FLUSHED (1 << 0)
> #define VFIO_SVM_PASID_RELEASE_CLEAN (1 << 1)
> +#define VFIO_SVM_PID (1 << 2)
> __u32 pasid;
> + __u32 pid;
> };
> /*
> * VFIO_DEVICE_BIND_TASK - _IOWR(VFIO_TYPE, VFIO_BASE + 22,
> @@ -432,6 +434,19 @@ struct vfio_device_svm {
> * On success, VFIO writes a Process Address Space ID (PASID) into @pasid. This
> * ID is unique to a device.
> *
> + * VFIO_SVM_PID: bind task @pid instead of current task. The shared address
> + * space identified by @pasid is that of task identified by @pid.
> + *
> + * Given that the caller owns the device, setting this flag grants the
> + * caller read and write permissions on the entire address space of
> + * foreign task described by @pid. Therefore, permission to perform the
> + * bind operation on a foreign process is governed by the ptrace access
> + * mode PTRACE_MODE_ATTACH_REALCREDS check. See man ptrace(2) for more
> + * information.
> + *
> + * If the VFIO_SVM_PID flag is not set, @pid is unused and it is the
> + * current task that is bound to the device.
> + *
> * The bond between device and process must be removed with
> * VFIO_DEVICE_UNBIND_TASK before exiting.
> *

BTW, nice commit logs throughout this series, I probably need to read
through them a few more times to really digest it all. AIUI, the VFIO
support here is really only useful for basic userspace drivers, I don't
see how we could take advantage of it for a VM use case where the guest
manages the PASID space for a domain. Perhaps it hasn't spent enough
cycles bouncing around in my head yet. Thanks,

Alex