Re: [PATCH 2/3] virtio-blk: undo destructive iov_discard_*() operations

[Li Qiang <liq3ea@gmail.com>]

Stefan Hajnoczi <stefanha@redhat.com> 于2020年8月12日周三 下午6:51写道：
>
> Fuzzing discovered that virtqueue_unmap_sg() is being called on modified
> req->in/out_sg iovecs. This means dma_memory_map() and
> dma_memory_unmap() calls do not have matching memory addresses.
>
> Fuzzing discovered that non-RAM addresses trigger a bug:
>
>   void address_space_unmap(AddressSpace *as, void *buffer, hwaddr len,
>                            bool is_write, hwaddr access_len)
>   {
>       if (buffer != bounce.buffer) {
>           ^^^^^^^^^^^^^^^^^^^^^^^
>
> A modified iov->iov_base is no longer recognized as a bounce buffer and
> the wrong branch is taken.
>
> There are more potential bugs: dirty memory is not tracked correctly and
> MemoryRegion refcounts can be leaked.
>
> Use the new iov_discard_undo() API to restore elem->in/out_sg before
> virtqueue_push() is called.
>
> Reported-by: Alexander Bulekov <alxndr@bu.edu>
> Buglink: https://bugs.launchpad.net/qemu/+bug/1890360
> Fixes: 827805a2492c1bbf1c0712ed18ee069b4ebf3dd6 ("virtio-blk: Convert VirtIOBlockReq.out to structrue")
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/hw/virtio/virtio-blk.h | 2 ++
>  hw/block/virtio-blk.c          | 9 +++++++--
>  2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index b1334c3904..0af654cace 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -68,6 +68,8 @@ typedef struct VirtIOBlockReq {
>      int64_t sector_num;
>      VirtIOBlock *dev;
>      VirtQueue *vq;
> +    IOVDiscardUndo inhdr_undo;
> +    IOVDiscardUndo outhdr_undo;
>      struct virtio_blk_inhdr *in;
>      struct virtio_blk_outhdr out;
>      QEMUIOVector qiov;
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 413783693c..2b7cc3e1c8 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -80,6 +80,8 @@ static void virtio_blk_req_complete(VirtIOBlockReq *req, unsigned char status)
>      trace_virtio_blk_req_complete(vdev, req, status);
>
>      stb_p(&req->in->status, status);
> +    iov_discard_undo(&req->inhdr_undo);
> +    iov_discard_undo(&req->outhdr_undo);
>      virtqueue_push(req->vq, &req->elem, req->in_len);
>      if (s->dataplane_started && !s->dataplane_disabled) {
>          virtio_blk_data_plane_notify(s->dataplane, req->vq);
> @@ -632,10 +634,12 @@ static int virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
>          return -1;
>      }
>
> -    iov_discard_front(&out_iov, &out_num, sizeof(req->out));
> +    iov_discard_front_undoable(&out_iov, &out_num, sizeof(req->out),
> +                               &req->outhdr_undo);
>
>      if (in_iov[in_num - 1].iov_len < sizeof(struct virtio_blk_inhdr)) {
>          virtio_error(vdev, "virtio-blk request inhdr too short");
> +        iov_discard_undo(&req->outhdr_undo);
>          return -1;
>      }
>
> @@ -644,7 +648,8 @@ static int virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
>      req->in = (void *)in_iov[in_num - 1].iov_base
>                + in_iov[in_num - 1].iov_len
>                - sizeof(struct virtio_blk_inhdr);
> -    iov_discard_back(in_iov, &in_num, sizeof(struct virtio_blk_inhdr));
> +    iov_discard_back_undoable(in_iov, &in_num, sizeof(struct virtio_blk_inhdr),
> +                              &req->inhdr_undo);
>
>      type = virtio_ldl_p(vdev, &req->out.type);
>

It seems there is another error path need to do the undo operations.
case VIRTIO_BLK_T_WRITE_ZEROS & ~VIRTIO_BLK_T_OUT
handler has an error path.

Thanks,
Li Qiang

> --
> 2.26.2
>