Re: [PATCH] virtio_blk: NULL out vqs to avoid double free on failed resume

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 22, 2025 at 01:45:38AM -0400, Michael S. Tsirkin wrote:
> On Wed, Oct 22, 2025 at 12:19:19PM +0800, Jason Wang wrote:
> > On Tue, Oct 21, 2025 at 8:58 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Tue, Oct 21, 2025 at 07:07:56PM +0800, Cong Zhang wrote:
> > > > The vblk->vqs releases during freeze. If resume fails before vblk->vqs
> > > > is allocated, later freeze/remove may attempt to free vqs again.
> > > > Set vblk->vqs to NULL after freeing to avoid double free.
> > > >
> > > > Signed-off-by: Cong Zhang <cong.zhang@oss.qualcomm.com>
> > > > ---
> > > > The patch fixes a double free issue that occurs in virtio_blk during
> > > > freeze/resume.
> > > > The issue is caused by:
> > > > 1. During the first freeze, vblk->vqs is freed but pointer is not set to
> > > >    NULL.
> > > > 2. Virtio block device fails before vblk->vqs is allocated during resume.
> > > > 3. During the next freeze, vblk->vqs gets freed again, causing the
> > > >    double free crash.
> > >
> > > this part I don't get. if restore fails, how can freeze be called
> > > again?
> > 
> > For example, could it be triggered by the user?
> > 
> > Thanks
> 
> I don't know - were you able to trigger it? how?

Sorry I mean the submitter of course.

> 
> > >
> > > > ---
> > > >  drivers/block/virtio_blk.c | 13 ++++++++++++-
> > > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > > > index f061420dfb10c40b21765b173fab7046aa447506..746795066d7f56a01c9a9c0344d24f9fa06841eb 100644
> > > > --- a/drivers/block/virtio_blk.c
> > > > +++ b/drivers/block/virtio_blk.c
> > > > @@ -1026,8 +1026,13 @@ static int init_vq(struct virtio_blk *vblk)
> > > >  out:
> > > >       kfree(vqs);
> > > >       kfree(vqs_info);
> > > > -     if (err)
> > > > +     if (err) {
> > > >               kfree(vblk->vqs);
> > > > +             /*
> > > > +              * Set to NULL to prevent freeing vqs again during freezing.
> > > > +              */
> > > > +             vblk->vqs = NULL;
> > > > +     }
> > > >       return err;
> > > >  }
> > > >
> > >
> > > > @@ -1598,6 +1603,12 @@ static int virtblk_freeze_priv(struct virtio_device *vdev)
> > > >
> > > >       vdev->config->del_vqs(vdev);
> > > >       kfree(vblk->vqs);
> > > > +     /*
> > > > +      * Set to NULL to prevent freeing vqs again after a failed vqs
> > > > +      * allocation during resume. Note that kfree() already handles NULL
> > > > +      * pointers safely.
> > > > +      */
> > > > +     vblk->vqs = NULL;
> > > >
> > > >       return 0;
> > > >  }
> > > >
> > > > ---
> > > > base-commit: 8e2755d7779a95dd61d8997ebce33ff8b1efd3fb
> > > > change-id: 20250926-virtio_double_free-7ab880d82a17
> > > >
> > > > Best regards,
> > > > --
> > > > Cong Zhang <cong.zhang@oss.qualcomm.com>
> > >