gntalloc.c

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/******************************************************************************
 * 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.
 */

#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;

    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. */
        gref->gref_id = gnttab_grant_foreign_access(op->domid,
            pfn_to_mfn(page_to_pfn(gref->page)), readonly);
        if ((int)gref->gref_id < 0) {
            rc = gref->gref_id;
            goto undo;
        }
        gref_ids[i] = gref->gref_id;
    }

    /* 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(gref, &queue_file, next_file) {
        /* __del_gref does not remove from queue_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 > 0) {
        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)) {
        printk(KERN_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) {
        printk(KERN_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 <[email protected]>, "
        "Daniel De Graaf <[email protected]>");
MODULE_DESCRIPTION("User-space grant reference allocator driver");