On 17/11/2015 11:20, Fam Zheng wrote: > When a request R is absorbed by request M, it is appended to the > "mr_next" queue led by M, and is completed together with the completion > of M, in virtio_blk_rw_complete. > > During DMA restart in virtio_blk_dma_restart_bh, requests in s->rq are > parsed and submitted again, possibly with a stale req->mr_next. It could > be a problem if the request merging in virtio_blk_handle_request hasn't > refreshed every mr_next pointer, in which case, virtio_blk_rw_complete > could walk through unexpected requests following the stale pointers. > > Fix this by unsetting the pointer in virtio_blk_rw_complete. It is safe > because this req is either completed and freed right away, or it will be > restarted and parsed from scratch out of the vq later. > > Signed-off-by: Fam Zheng <f...@redhat.com> > > --- > > v3: Fix as Stefan suggested. > --- > hw/block/virtio-blk.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c > index e70fccf..848f3fe 100644 > --- a/hw/block/virtio-blk.c > +++ b/hw/block/virtio-blk.c > @@ -112,6 +112,10 @@ static void virtio_blk_rw_complete(void *opaque, int ret) > * happen on the other side of the migration). > */ > if (virtio_blk_handle_rw_error(req, -ret, is_read)) { > + /* Break the link in case the next request is added to the > + * restart queue and is going to be parsed from the ring > again. > + */ > + req->mr_next = NULL; > continue; > } > } >
This is now a write-after-free for rerror/werror=stop. The right place to set req->mr_next is inside virtio_blk_handle_rw_error, I think. Paolo