Re: [PATCH v4] virtio-blk: handle block_device_operations callbacks after hot unplug

Michael S. Tsirkin
2020-05-04T12:07:52Z

On Mon, May 04, 2020 at 01:48:34PM +0200, Cornelia Huck wrote:
> On Thu, 30 Apr 2020 15:04:42 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> > 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: 48e4043d4529523cbc7fa8dd745bd8e2c45ce1d3
> > ("virtio: add virtio disk geometry feature")
>
> Should be
>
> Fixes: 48e4043d4529 ("virtio: add virtio disk geometry feature")

Yes - it was reported on linux-next so I fixed it up when applying.

> > Reported-by: Lance Digby <ldigby@redhat.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > v4:
> > * Clarify vdev_mutex usage [Stefano and Michael]
> >
> > drivers/block/virtio_blk.c | 86 ++++++++++++++++++++++++++++++++++----
> > 1 file changed, 78 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 93468b7c6701..9d21bf0f155e 100644
> > --- 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;
>
> Using a struct kref might be more idiomatic.
>
> > +
> > /* What host tells us, plus 2 for header & tailer. */
> > unsigned int sg_elems;
> >
> > @@ -295,10 +311,55 @@ static int virtblk_get_id(struct gendisk *disk, char *id_str)
> > return err;
> > }
> >
> > +static void virtblk_get(struct virtio_blk *vblk)
> > +{
> > + refcount_inc(&vblk->refs);
>
> Should the code even be able to grab a ref if !vblk->vdev?
>
> > +}
> > +
> > +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);
>
> I think that's where putting these cleanups into a release() funtion
> would be more idiomatic.
>
> > + }
> > +}
>
> (...)
>
> Looks sane to me.