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

[Fam Zheng <famz@redhat.com>]

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.

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;
+    }
     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;
 
-- 
2.4.3