xen-gntalloc: Userspace grant allocation driver
This allows a userspace application to allocate a shared page for
implementing inter-domain communication or device drivers. These
shared pages can be mapped using the gntdev device or by the kernel
in another domain.
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/drivers/xen/gntalloc.c b/drivers/xen/gntalloc.c
new file mode 100644
index 0000000..d06bf2b
--- /dev/null
+++ b/drivers/xen/gntalloc.c
@@ -0,0 +1,486 @@
+/******************************************************************************
+ * 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 <xen/xen.h>
+#include <xen/page.h>
+#include <xen/grant_table.h>
+#include <xen/gntalloc.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_SPINLOCK(gref_lock);
+static int gref_size;
+
+/* 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 gntalloc_file_private_data {
+ struct list_head list;
+ uint64_t index;
+};
+
+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 (gref->gref_id < 0) {
+ rc = gref->gref_id;
+ goto undo;
+ }
+ gref_ids[i] = gref->gref_id;
+ }
+
+ /* Add to gref lists. */
+ spin_lock(&gref_lock);
+ list_splice_tail(&queue_gref, &gref_list);
+ list_splice_tail(&queue_file, &priv->list);
+ spin_unlock(&gref_lock);
+
+ return 0;
+
+undo:
+ spin_lock(&gref_lock);
+ 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);
+ spin_unlock(&gref_lock);
+ return rc;
+}
+
+static void __del_gref(struct gntalloc_gref *gref)
+{
+ 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;
+ }
+
+ 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);
+
+ spin_lock(&gref_lock);
+ 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);
+ spin_unlock(&gref_lock);
+
+ 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 = kzalloc(sizeof(gref_ids[0]) * op.count, GFP_TEMPORARY);
+ if (!gref_ids) {
+ rc = -ENOMEM;
+ goto out;
+ }
+
+ spin_lock(&gref_lock);
+ /* 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) {
+ spin_unlock(&gref_lock);
+ rc = -ENOSPC;
+ goto out_free;
+ }
+ gref_size += op.count;
+ op.index = priv->index;
+ priv->index += op.count * PAGE_SIZE;
+ spin_unlock(&gref_lock);
+
+ 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;
+ }
+
+ spin_lock(&gref_lock);
+ 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();
+
+ spin_unlock(&gref_lock);
+dealloc_grant_out:
+ 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);
+
+ default:
+ return -ENOIOCTLCMD;
+ }
+
+ return 0;
+}
+
+static void gntalloc_vma_close(struct vm_area_struct *vma)
+{
+ struct gntalloc_gref *gref = vma->vm_private_data;
+ if (!gref)
+ return;
+
+ spin_lock(&gref_lock);
+ gref->users--;
+ if (gref->users == 0)
+ __del_gref(gref);
+ spin_unlock(&gref_lock);
+}
+
+static struct vm_operations_struct gntalloc_vmops = {
+ .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_gref *gref;
+ int count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+ int rv, i;
+
+ pr_debug("%s: priv %p, page %lu+%d\n", __func__,
+ priv, vma->vm_pgoff, count);
+
+ if (!(vma->vm_flags & VM_SHARED)) {
+ printk(KERN_ERR "%s: Mapping must be shared.\n", __func__);
+ return -EINVAL;
+ }
+
+ spin_lock(&gref_lock);
+ gref = find_grefs(priv, vma->vm_pgoff << PAGE_SHIFT, count);
+ if (gref == NULL) {
+ rv = -ENOENT;
+ pr_debug("%s: Could not find grant reference",
+ __func__);
+ goto out_unlock;
+ }
+
+ vma->vm_private_data = gref;
+
+ vma->vm_flags |= VM_RESERVED;
+ vma->vm_flags |= VM_DONTCOPY;
+ vma->vm_flags |= VM_PFNMAP | VM_PFN_AT_MMAP;
+
+ 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:
+ spin_unlock(&gref_lock);
+ 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 <carter.weatherly@jhuapl.edu>, "
+ "Daniel De Graaf <dgdegra@tycho.nsa.gov>");
+MODULE_DESCRIPTION("User-space grant reference allocator driver");