gntalloc.c

/******************************************************************************
 * gntalloc.c
 *
 * Device for creating grant references (in user-space) that may be shared
 * with other domains.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

/*
 * This driver exists to allow userspace programs in Linux to allocate kernel
 * memory that will later be shared with another domain.  Without this device,
 * Linux userspace programs cannot create grant references.
 *
 * How this stuff works:
 *   X -> granting a page to Y
 *   Y -> mapping the grant from X
 *
 *   1. X uses the gntalloc device to allocate a page of kernel memory, P.
 *   2. X creates an entry in the grant table that says domid(Y) can access P.
 *      This is done without a hypercall unless the grant table needs expansion.
 *   3. X gives the grant reference identifier, GREF, to Y.
 *   4. Y maps the page, either directly into kernel memory for use in a backend
 *      driver, or via a the gntdev device to map into the address space of an
 *      application running in Y. This is the first point at which Xen does any
 *      tracking of the page.
 *   5. A program in X mmap()s a segment of the gntalloc device that corresponds
 *      to the shared page, and can now communicate with Y over the shared page.
 *
 *
 * NOTE TO USERSPACE LIBRARIES:
 *   The grant allocation and mmap()ing are, naturally, two separate operations.
 *   You set up the sharing by calling the create ioctl() and then the mmap().
 *   Teardown requires munmap() and either close() or ioctl().
 *
 * WARNING: Since Xen does not allow a guest to forcibly end the use of a grant
 * reference, this device can be used to consume kernel memory by leaving grant
 * references mapped by another domain when an application exits. Therefore,
 * there is a global limit on the number of pages that can be allocated. When
 * all references to the page are unmapped, it will be freed during the next
 * grant operation.
 */

#define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt

#include <linux/atomic.h>
#include <linux/module.h>
#include <linux/miscdevice.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/mm.h>
#include <linux/uaccess.h>
#include <linux/types.h>
#include <linux/list.h>
#include <linux/highmem.h>

#include <xen/xen.h>
#include <xen/page.h>
#include <xen/grant_table.h>
#include <xen/gntalloc.h>
#include <xen/events.h>

static int limit = 1024;
module_param(limit, int, 0644);
MODULE_PARM_DESC(limit, "Maximum number of grants that may be allocated by "
		"the gntalloc device");

static LIST_HEAD(gref_list);
static DEFINE_MUTEX(gref_mutex);
static int gref_size;

struct notify_info {
	uint16_t pgoff:12;    /* Bits 0-11: Offset of the byte to clear */
	uint16_t flags:2;     /* Bits 12-13: Unmap notification flags */
	int event;            /* Port (event channel) to notify */
};

/* Metadata on a grant reference. */
struct gntalloc_gref {
	struct list_head next_gref;  /* list entry gref_list */
	struct list_head next_file;  /* list entry file->list, if open */
	struct page *page;	     /* The shared page */
	uint64_t file_index;         /* File offset for mmap() */
	unsigned int users;          /* Use count - when zero, waiting on Xen */
	grant_ref_t gref_id;         /* The grant reference number */
	struct notify_info notify;   /* Unmap notification */
};

struct gntalloc_file_private_data {
	struct list_head list;
	uint64_t index;
};

struct gntalloc_vma_private_data {
	struct gntalloc_gref *gref;
	int users;
	int count;
};

static void __del_gref(struct gntalloc_gref *gref);

static void do_cleanup(void)
{
	struct gntalloc_gref *gref, *n;
	list_for_each_entry_safe(gref, n, &gref_list, next_gref) {
		if (!gref->users)
			__del_gref(gref);
	}
}

static int add_grefs(struct ioctl_gntalloc_alloc_gref *op,
	uint32_t *gref_ids, struct gntalloc_file_private_data *priv)
{
	int i, rc, readonly;
	LIST_HEAD(queue_gref);
	LIST_HEAD(queue_file);
	struct gntalloc_gref *gref, *next;

	readonly = !(op->flags & GNTALLOC_FLAG_WRITABLE);
	rc = -ENOMEM;
	for (i = 0; i < op->count; i++) {
		gref = kzalloc(sizeof(*gref), GFP_KERNEL);
		if (!gref)
			goto undo;
		list_add_tail(&gref->next_gref, &queue_gref);
		list_add_tail(&gref->next_file, &queue_file);
		gref->users = 1;
		gref->file_index = op->index + i * PAGE_SIZE;
		gref->page = alloc_page(GFP_KERNEL|__GFP_ZERO);
		if (!gref->page)
			goto undo;

		/* Grant foreign access to the page. */
		rc = gnttab_grant_foreign_access(op->domid,
						 xen_page_to_gfn(gref->page),
						 readonly);
		if (rc < 0)
			goto undo;
		gref_ids[i] = gref->gref_id = rc;
	}

	/* Add to gref lists. */
	mutex_lock(&gref_mutex);
	list_splice_tail(&queue_gref, &gref_list);
	list_splice_tail(&queue_file, &priv->list);
	mutex_unlock(&gref_mutex);

	return 0;

undo:
	mutex_lock(&gref_mutex);
	gref_size -= (op->count - i);

	list_for_each_entry_safe(gref, next, &queue_file, next_file) {
		list_del(&gref->next_file);
		__del_gref(gref);
	}

	/* It's possible for the target domain to map the just-allocated grant
	 * references by blindly guessing their IDs; if this is done, then
	 * __del_gref will leave them in the queue_gref list. They need to be
	 * added to the global list so that we can free them when they are no
	 * longer referenced.
	 */
	if (unlikely(!list_empty(&queue_gref)))
		list_splice_tail(&queue_gref, &gref_list);
	mutex_unlock(&gref_mutex);
	return rc;
}

static void __del_gref(struct gntalloc_gref *gref)
{
	if (gref->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
		uint8_t *tmp = kmap(gref->page);
		tmp[gref->notify.pgoff] = 0;
		kunmap(gref->page);
	}
	if (gref->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
		notify_remote_via_evtchn(gref->notify.event);
		evtchn_put(gref->notify.event);
	}

	gref->notify.flags = 0;

	if (gref->gref_id) {
		if (gnttab_query_foreign_access(gref->gref_id))
			return;

		if (!gnttab_end_foreign_access_ref(gref->gref_id, 0))
			return;

		gnttab_free_grant_reference(gref->gref_id);
	}

	gref_size--;
	list_del(&gref->next_gref);

	if (gref->page)
		__free_page(gref->page);

	kfree(gref);
}

/* finds contiguous grant references in a file, returns the first */
static struct gntalloc_gref *find_grefs(struct gntalloc_file_private_data *priv,
		uint64_t index, uint32_t count)
{
	struct gntalloc_gref *rv = NULL, *gref;
	list_for_each_entry(gref, &priv->list, next_file) {
		if (gref->file_index == index && !rv)
			rv = gref;
		if (rv) {
			if (gref->file_index != index)
				return NULL;
			index += PAGE_SIZE;
			count--;
			if (count == 0)
				return rv;
		}
	}
	return NULL;
}

/*
 * -------------------------------------
 *  File operations.
 * -------------------------------------
 */
static int gntalloc_open(struct inode *inode, struct file *filp)
{
	struct gntalloc_file_private_data *priv;

	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
	if (!priv)
		goto out_nomem;
	INIT_LIST_HEAD(&priv->list);

	filp->private_data = priv;

	pr_debug("%s: priv %p\n", __func__, priv);

	return 0;

out_nomem:
	return -ENOMEM;
}

static int gntalloc_release(struct inode *inode, struct file *filp)
{
	struct gntalloc_file_private_data *priv = filp->private_data;
	struct gntalloc_gref *gref;

	pr_debug("%s: priv %p\n", __func__, priv);

	mutex_lock(&gref_mutex);
	while (!list_empty(&priv->list)) {
		gref = list_entry(priv->list.next,
			struct gntalloc_gref, next_file);
		list_del(&gref->next_file);
		gref->users--;
		if (gref->users == 0)
			__del_gref(gref);
	}
	kfree(priv);
	mutex_unlock(&gref_mutex);

	return 0;
}

static long gntalloc_ioctl_alloc(struct gntalloc_file_private_data *priv,
		struct ioctl_gntalloc_alloc_gref __user *arg)
{
	int rc = 0;
	struct ioctl_gntalloc_alloc_gref op;
	uint32_t *gref_ids;

	pr_debug("%s: priv %p\n", __func__, priv);

	if (copy_from_user(&op, arg, sizeof(op))) {
		rc = -EFAULT;
		goto out;
	}

	gref_ids = kcalloc(op.count, sizeof(gref_ids[0]), GFP_TEMPORARY);
	if (!gref_ids) {
		rc = -ENOMEM;
		goto out;
	}

	mutex_lock(&gref_mutex);
	/* Clean up pages that were at zero (local) users but were still mapped
	 * by remote domains. Since those pages count towards the limit that we
	 * are about to enforce, removing them here is a good idea.
	 */
	do_cleanup();
	if (gref_size + op.count > limit) {
		mutex_unlock(&gref_mutex);
		rc = -ENOSPC;
		goto out_free;
	}
	gref_size += op.count;
	op.index = priv->index;
	priv->index += op.count * PAGE_SIZE;
	mutex_unlock(&gref_mutex);

	rc = add_grefs(&op, gref_ids, priv);
	if (rc < 0)
		goto out_free;

	/* Once we finish add_grefs, it is unsafe to touch the new reference,
	 * since it is possible for a concurrent ioctl to remove it (by guessing
	 * its index). If the userspace application doesn't provide valid memory
	 * to write the IDs to, then it will need to close the file in order to
	 * release - which it will do by segfaulting when it tries to access the
	 * IDs to close them.
	 */
	if (copy_to_user(arg, &op, sizeof(op))) {
		rc = -EFAULT;
		goto out_free;
	}
	if (copy_to_user(arg->gref_ids, gref_ids,
			sizeof(gref_ids[0]) * op.count)) {
		rc = -EFAULT;
		goto out_free;
	}

out_free:
	kfree(gref_ids);
out:
	return rc;
}

static long gntalloc_ioctl_dealloc(struct gntalloc_file_private_data *priv,
		void __user *arg)
{
	int i, rc = 0;
	struct ioctl_gntalloc_dealloc_gref op;
	struct gntalloc_gref *gref, *n;

	pr_debug("%s: priv %p\n", __func__, priv);

	if (copy_from_user(&op, arg, sizeof(op))) {
		rc = -EFAULT;
		goto dealloc_grant_out;
	}

	mutex_lock(&gref_mutex);
	gref = find_grefs(priv, op.index, op.count);
	if (gref) {
		/* Remove from the file list only, and decrease reference count.
		 * The later call to do_cleanup() will remove from gref_list and
		 * free the memory if the pages aren't mapped anywhere.
		 */
		for (i = 0; i < op.count; i++) {
			n = list_entry(gref->next_file.next,
				struct gntalloc_gref, next_file);
			list_del(&gref->next_file);
			gref->users--;
			gref = n;
		}
	} else {
		rc = -EINVAL;
	}

	do_cleanup();

	mutex_unlock(&gref_mutex);
dealloc_grant_out:
	return rc;
}

static long gntalloc_ioctl_unmap_notify(struct gntalloc_file_private_data *priv,
		void __user *arg)
{
	struct ioctl_gntalloc_unmap_notify op;
	struct gntalloc_gref *gref;
	uint64_t index;
	int pgoff;
	int rc;

	if (copy_from_user(&op, arg, sizeof(op)))
		return -EFAULT;

	index = op.index & ~(PAGE_SIZE - 1);
	pgoff = op.index & (PAGE_SIZE - 1);

	mutex_lock(&gref_mutex);

	gref = find_grefs(priv, index, 1);
	if (!gref) {
		rc = -ENOENT;
		goto unlock_out;
	}

	if (op.action & ~(UNMAP_NOTIFY_CLEAR_BYTE|UNMAP_NOTIFY_SEND_EVENT)) {
		rc = -EINVAL;
		goto unlock_out;
	}

	/* We need to grab a reference to the event channel we are going to use
	 * to send the notify before releasing the reference we may already have
	 * (if someone has called this ioctl twice). This is required so that
	 * it is possible to change the clear_byte part of the notification
	 * without disturbing the event channel part, which may now be the last
	 * reference to that event channel.
	 */
	if (op.action & UNMAP_NOTIFY_SEND_EVENT) {
		if (evtchn_get(op.event_channel_port)) {
			rc = -EINVAL;
			goto unlock_out;
		}
	}

	if (gref->notify.flags & UNMAP_NOTIFY_SEND_EVENT)
		evtchn_put(gref->notify.event);

	gref->notify.flags = op.action;
	gref->notify.pgoff = pgoff;
	gref->notify.event = op.event_channel_port;
	rc = 0;

 unlock_out:
	mutex_unlock(&gref_mutex);
	return rc;
}

static long gntalloc_ioctl(struct file *filp, unsigned int cmd,
		unsigned long arg)
{
	struct gntalloc_file_private_data *priv = filp->private_data;

	switch (cmd) {
	case IOCTL_GNTALLOC_ALLOC_GREF:
		return gntalloc_ioctl_alloc(priv, (void __user *)arg);

	case IOCTL_GNTALLOC_DEALLOC_GREF:
		return gntalloc_ioctl_dealloc(priv, (void __user *)arg);

	case IOCTL_GNTALLOC_SET_UNMAP_NOTIFY:
		return gntalloc_ioctl_unmap_notify(priv, (void __user *)arg);

	default:
		return -ENOIOCTLCMD;
	}

	return 0;
}

static void gntalloc_vma_open(struct vm_area_struct *vma)
{
	struct gntalloc_vma_private_data *priv = vma->vm_private_data;

	if (!priv)
		return;

	mutex_lock(&gref_mutex);
	priv->users++;
	mutex_unlock(&gref_mutex);
}

static void gntalloc_vma_close(struct vm_area_struct *vma)
{
	struct gntalloc_vma_private_data *priv = vma->vm_private_data;
	struct gntalloc_gref *gref, *next;
	int i;

	if (!priv)
		return;

	mutex_lock(&gref_mutex);
	priv->users--;
	if (priv->users == 0) {
		gref = priv->gref;
		for (i = 0; i < priv->count; i++) {
			gref->users--;
			next = list_entry(gref->next_gref.next,
					  struct gntalloc_gref, next_gref);
			if (gref->users == 0)
				__del_gref(gref);
			gref = next;
		}
		kfree(priv);
	}
	mutex_unlock(&gref_mutex);
}

static struct vm_operations_struct gntalloc_vmops = {
	.open = gntalloc_vma_open,
	.close = gntalloc_vma_close,
};

static int gntalloc_mmap(struct file *filp, struct vm_area_struct *vma)
{
	struct gntalloc_file_private_data *priv = filp->private_data;
	struct gntalloc_vma_private_data *vm_priv;
	struct gntalloc_gref *gref;
	int count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
	int rv, i;

	if (!(vma->vm_flags & VM_SHARED)) {
		pr_err("%s: Mapping must be shared\n", __func__);
		return -EINVAL;
	}

	vm_priv = kmalloc(sizeof(*vm_priv), GFP_KERNEL);
	if (!vm_priv)
		return -ENOMEM;

	mutex_lock(&gref_mutex);

	pr_debug("%s: priv %p,%p, page %lu+%d\n", __func__,
		       priv, vm_priv, vma->vm_pgoff, count);

	gref = find_grefs(priv, vma->vm_pgoff << PAGE_SHIFT, count);
	if (gref == NULL) {
		rv = -ENOENT;
		pr_debug("%s: Could not find grant reference",
				__func__);
		kfree(vm_priv);
		goto out_unlock;
	}

	vm_priv->gref = gref;
	vm_priv->users = 1;
	vm_priv->count = count;

	vma->vm_private_data = vm_priv;

	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;

	vma->vm_ops = &gntalloc_vmops;

	for (i = 0; i < count; i++) {
		gref->users++;
		rv = vm_insert_page(vma, vma->vm_start + i * PAGE_SIZE,
				gref->page);
		if (rv)
			goto out_unlock;

		gref = list_entry(gref->next_file.next,
				struct gntalloc_gref, next_file);
	}
	rv = 0;

out_unlock:
	mutex_unlock(&gref_mutex);
	return rv;
}

static const struct file_operations gntalloc_fops = {
	.owner = THIS_MODULE,
	.open = gntalloc_open,
	.release = gntalloc_release,
	.unlocked_ioctl = gntalloc_ioctl,
	.mmap = gntalloc_mmap
};

/*
 * -------------------------------------
 * Module creation/destruction.
 * -------------------------------------
 */
static struct miscdevice gntalloc_miscdev = {
	.minor	= MISC_DYNAMIC_MINOR,
	.name	= "xen/gntalloc",
	.fops	= &gntalloc_fops,
};

static int __init gntalloc_init(void)
{
	int err;

	if (!xen_domain())
		return -ENODEV;

	err = misc_register(&gntalloc_miscdev);
	if (err != 0) {
		pr_err("Could not register misc gntalloc device\n");
		return err;
	}

	pr_debug("Created grant allocation device at %d,%d\n",
			MISC_MAJOR, gntalloc_miscdev.minor);

	return 0;
}

static void __exit gntalloc_exit(void)
{
	misc_deregister(&gntalloc_miscdev);
}

module_init(gntalloc_init);
module_exit(gntalloc_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Carter Weatherly <carter.weatherly@jhuapl.edu>, "
		"Daniel De Graaf <dgdegra@tycho.nsa.gov>");
MODULE_DESCRIPTION("User-space grant reference allocator driver");