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>

3 files changed, 496 insertions, 0 deletions

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index 07bec09d1dad..a3d7afb86ae4 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -80,6 +80,14 @@ config XEN_GNTDEV
         help
           Allows userspace processes to use grants.
 
+config XEN_GRANT_DEV_ALLOC
+        tristate "User-space grant reference allocator driver"
+        depends on XEN
+        help
+          Allows userspace processes to create pages with access granted
+          to other domains. This can be used to implement frontend drivers
+          or as part of an inter-domain shared memory channel.
+
 config XEN_PLATFORM_PCI
         tristate "xen platform pci device driver"
         depends on XEN_PVHVM && PCI
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 5088cc2e6fe2..9585a1da52c6 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -10,6 +10,7 @@ obj-$(CONFIG_XEN_XENCOMM)	+= xencomm.o
 obj-$(CONFIG_XEN_BALLOON)       += balloon.o
 obj-$(CONFIG_XEN_DEV_EVTCHN)    += xen-evtchn.o
 obj-$(CONFIG_XEN_GNTDEV)        += xen-gntdev.o
+obj-$(CONFIG_XEN_GRANT_DEV_ALLOC)       += xen-gntalloc.o
 obj-$(CONFIG_XENFS)             += xenfs/
 obj-$(CONFIG_XEN_SYS_HYPERVISOR)        += sys-hypervisor.o
 obj-$(CONFIG_XEN_PLATFORM_PCI)  += xen-platform-pci.o
@@ -18,5 +19,6 @@ obj-$(CONFIG_XEN_DOM0)		+= pci.o
 
 xen-evtchn-y                    := evtchn.o
 xen-gntdev-y                            := gntdev.o
+xen-gntalloc-y                          := gntalloc.o
 
 xen-platform-pci-y              := platform-pci.o
diff --git a/drivers/xen/gntalloc.c b/drivers/xen/gntalloc.c
new file mode 100644
index 000000000000..d06bf2b4cd07
--- /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");