virtio-blk: handle block_device_operations callbacks after hot unplug

A userspace process holding a file descriptor to a virtio_blk device can
still invoke block_device_operations after hot unplug.  This leads to a
use-after-free accessing vblk->vdev in virtblk_getgeo() when
ioctl(HDIO_GETGEO) is invoked:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
  IP: [<ffffffffc00e5450>] virtio_check_driver_offered_feature+0x10/0x90 [virtio]
  PGD 800000003a92f067 PUD 3a930067 PMD 0
  Oops: 0000 [#1] SMP
  CPU: 0 PID: 1310 Comm: hdio-getgeo Tainted: G           OE  ------------   3.10.0-1062.el7.x86_64 #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
  task: ffff9be5fbfb8000 ti: ffff9be5fa890000 task.ti: ffff9be5fa890000
  RIP: 0010:[<ffffffffc00e5450>]  [<ffffffffc00e5450>] virtio_check_driver_offered_feature+0x10/0x90 [virtio]
  RSP: 0018:ffff9be5fa893dc8  EFLAGS: 00010246
  RAX: ffff9be5fc3f3400 RBX: ffff9be5fa893e30 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffff9be5fbc10b40
  RBP: ffff9be5fa893dc8 R08: 0000000000000301 R09: 0000000000000301
  R10: 0000000000000000 R11: 0000000000000000 R12: ffff9be5fdc24680
  R13: ffff9be5fbc10b40 R14: ffff9be5fbc10480 R15: 0000000000000000
  FS:  00007f1bfb968740(0000) GS:ffff9be5ffc00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000090 CR3: 000000003a894000 CR4: 0000000000360ff0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   [<ffffffffc016ac37>] virtblk_getgeo+0x47/0x110 [virtio_blk]
   [<ffffffff8d3f200d>] ? handle_mm_fault+0x39d/0x9b0
   [<ffffffff8d561265>] blkdev_ioctl+0x1f5/0xa20
   [<ffffffff8d488771>] block_ioctl+0x41/0x50
   [<ffffffff8d45d9e0>] do_vfs_ioctl+0x3a0/0x5a0
   [<ffffffff8d45dc81>] SyS_ioctl+0xa1/0xc0

A related problem is that virtblk_remove() leaks the vd_index_ida index
when something still holds a reference to vblk->disk during hot unplug.
This causes virtio-blk device names to be lost (vda, vdb, etc).

Fix these issues by protecting vblk->vdev with a mutex and reference
counting vblk so the vd_index_ida index can be removed in all cases.

Fixes: 48e4043d4529 ("virtio: add virtio disk geometry feature")
Reported-by: Lance Digby <ldigby@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200430140442.171016-1-stefanha@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 93468b7c67019b3af6dd16cf639f11bb8dd77337..9d21bf0f155eed65fa150d71e6b92104fd701960 100644 (file)
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -33,6 +33,15 @@ struct virtio_blk_vq {
 } ____cacheline_aligned_in_smp;
 
 struct virtio_blk {
+       /*
+        * This mutex must be held by anything that may run after
+        * virtblk_remove() sets vblk->vdev to NULL.
+        *
+        * blk-mq, virtqueue processing, and sysfs attribute code paths are
+        * shut down before vblk->vdev is set to NULL and therefore do not need
+        * to hold this mutex.
+        */
+       struct mutex vdev_mutex;
        struct virtio_device *vdev;
 
        /* The disk structure for the kernel. */
@@ -44,6 +53,13 @@ struct virtio_blk {
        /* Process context for config space updates */
        struct work_struct config_work;
 
+       /*
+        * Tracks references from block_device_operations open/release and
+        * virtio_driver probe/remove so this object can be freed once no
+        * longer in use.
+        */
+       refcount_t refs;
+
        /* What host tells us, plus 2 for header & tailer. */
        unsigned int sg_elems;
 
@@ -295,10 +311,55 @@ out:
        return err;
 }
 
+static void virtblk_get(struct virtio_blk *vblk)
+{
+       refcount_inc(&vblk->refs);
+}
+
+static void virtblk_put(struct virtio_blk *vblk)
+{
+       if (refcount_dec_and_test(&vblk->refs)) {
+               ida_simple_remove(&vd_index_ida, vblk->index);
+               mutex_destroy(&vblk->vdev_mutex);
+               kfree(vblk);
+       }
+}
+
+static int virtblk_open(struct block_device *bd, fmode_t mode)
+{
+       struct virtio_blk *vblk = bd->bd_disk->private_data;
+       int ret = 0;
+
+       mutex_lock(&vblk->vdev_mutex);
+
+       if (vblk->vdev)
+               virtblk_get(vblk);
+       else
+               ret = -ENXIO;
+
+       mutex_unlock(&vblk->vdev_mutex);
+       return ret;
+}
+
+static void virtblk_release(struct gendisk *disk, fmode_t mode)
+{
+       struct virtio_blk *vblk = disk->private_data;
+
+       virtblk_put(vblk);
+}
+
 /* We provide getgeo only to please some old bootloader/partitioning tools */
 static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
 {
        struct virtio_blk *vblk = bd->bd_disk->private_data;
+       int ret = 0;
+
+       mutex_lock(&vblk->vdev_mutex);
+
+       if (!vblk->vdev) {
+               ret = -ENXIO;
+               goto out;
+       }
 
        /* see if the host passed in geometry config */
        if (virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_GEOMETRY)) {
@@ -314,11 +375,15 @@ static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
                geo->sectors = 1 << 5;
                geo->cylinders = get_capacity(bd->bd_disk) >> 11;
        }
-       return 0;
+out:
+       mutex_unlock(&vblk->vdev_mutex);
+       return ret;
 }
 
 static const struct block_device_operations virtblk_fops = {
        .owner  = THIS_MODULE,
+       .open = virtblk_open,
+       .release = virtblk_release,
        .getgeo = virtblk_getgeo,
 };
 
@@ -655,6 +720,10 @@ static int virtblk_probe(struct virtio_device *vdev)
                goto out_free_index;
        }
 
+       /* This reference is dropped in virtblk_remove(). */
+       refcount_set(&vblk->refs, 1);
+       mutex_init(&vblk->vdev_mutex);
+
        vblk->vdev = vdev;
        vblk->sg_elems = sg_elems;
 
@@ -820,8 +889,6 @@ out:
 static void virtblk_remove(struct virtio_device *vdev)
 {
        struct virtio_blk *vblk = vdev->priv;
-       int index = vblk->index;
-       int refc;
 
        /* Make sure no work handler is accessing the device. */
        flush_work(&vblk->config_work);
@@ -831,18 +898,21 @@ static void virtblk_remove(struct virtio_device *vdev)
 
        blk_mq_free_tag_set(&vblk->tag_set);
 
+       mutex_lock(&vblk->vdev_mutex);
+
        /* Stop all the virtqueues. */
        vdev->config->reset(vdev);
 
-       refc = kref_read(&disk_to_dev(vblk->disk)->kobj.kref);
+       /* Virtqueues are stopped, nothing can use vblk->vdev anymore. */
+       vblk->vdev = NULL;
+
        put_disk(vblk->disk);
        vdev->config->del_vqs(vdev);
        kfree(vblk->vqs);
-       kfree(vblk);
 
-       /* Only free device id if we don't have any users */
-       if (refc == 1)
-               ida_simple_remove(&vd_index_ida, index);
+       mutex_unlock(&vblk->vdev_mutex);
+
+       virtblk_put(vblk);
 }
 
 #ifdef CONFIG_PM_SLEEP