Re: [Qemu-devel] [PATCH v2] virtio-blk: Fix double completion for werror=stop

[Fam Zheng <famz@redhat.com>]

On Tue, 11/17 14:58, Stefan Hajnoczi wrote:
> On Mon, Nov 16, 2015 at 02:10:36PM +0800, 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.
> > 
> > With error policy equals stop, if M has an I/O error, now R also gets
> > prepended to the per device DMA restart queue, which will be retried
> > when VM resumes.  It leads to a double completion (in symptoms of memory
> > corruption or use after free).
> > 
> > Adding R to the queue is superfluous, only M needs to be in the queue.
> > 
> > Fix this by marking request R as "merged" and skipping it in
> > virtio_blk_handle_rw_error.

The commit message is outdated. "merged" requests are actually skipped in
virtio_blk_handle_request.

> > 
> > Cc: qemu-stable@nongnu.org
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > 
> > ---
> > 
> > v2: Don't lose the request in migration. [Paolo]
> > ---
> >  hw/block/virtio-blk.c          | 7 +++++++
> >  include/hw/virtio/virtio-blk.h | 1 +
> >  2 files changed, 8 insertions(+)
> > 
> > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> > index e70fccf..5cdb06f 100644
> > --- a/hw/block/virtio-blk.c
> > +++ b/hw/block/virtio-blk.c
> > @@ -36,6 +36,7 @@ VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
> >      req->in_len = 0;
> >      req->next = NULL;
> >      req->mr_next = NULL;
> > +    req->merged = false;
> >      return req;
> >  }
> >  
> > @@ -344,6 +345,7 @@ static inline void submit_requests(BlockBackend *blk, MultiReqBuffer *mrb,
> >          for (i = start + 1; i < start + num_reqs; i++) {
> >              qemu_iovec_concat(qiov, &mrb->reqs[i]->qiov, 0,
> >                                mrb->reqs[i]->qiov.size);
> > +            mrb->reqs[i]->merged = true;
> >              mrb->reqs[i - 1]->mr_next = mrb->reqs[i];
> >              nb_sectors += mrb->reqs[i]->qiov.size / BDRV_SECTOR_SIZE;
> >          }
> > @@ -511,6 +513,11 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
> >                - sizeof(struct virtio_blk_inhdr);
> >      iov_discard_back(in_iov, &in_num, sizeof(struct virtio_blk_inhdr));
> >  
> > +    if (req->merged) {
> > +        /* Enough for restarting a (migrated) merged request, no need to
> > +         * actually submit I/O. */
> > +        return;
> > +    }

This is not enough.

There is a risk that the coalesced requests being restarted here are also
merged, which will leak the requests originally merged into it.

> >      type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
> >  
> >      /* VIRTIO_BLK_T_OUT defines the command direction. VIRTIO_BLK_T_BARRIER
> > diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> > index 6bf5905..db4adf4 100644
> > --- a/include/hw/virtio/virtio-blk.h
> > +++ b/include/hw/virtio/virtio-blk.h
> > @@ -70,6 +70,7 @@ typedef struct VirtIOBlockReq {
> >      size_t in_len;
> >      struct VirtIOBlockReq *next;
> >      struct VirtIOBlockReq *mr_next;
> > +    bool merged;
> >      BlockAcctCookie acct;
> >  } VirtIOBlockReq;
> 
> I'm not sure if this patch truly fixes the bug:
> 
> virtio_blk_rw_complete() doesn't do req->mr_next = NULL.  There is a
> potential double-free if resubmitting failed requests doesn't overwrite
> themr_next field.
> 
> This can be fixed by adding req->mr_next = NULL to the loop in
> virtio_blk_rw_complete().
> 
> Is that enough to solve the bug?  I don't think adding a new field is
> necessary.
> 
> If not, please explain the double-free.

The first free is the expected one when the coalesced request is completed.
The second free is because virtio_blk_rw_complete was also called on requests
who have "merged == true", which is a mistake in virtio_blk_dma_restart_bh.

I don't think adding req->mr_next in virtio_blk_rw_complete in the I/O error
path is right or helpful.

Fam