[Qemu-devel] [PATCH] 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>
---
 hw/block/virtio-blk.c          | 9 +++++++--
 include/hw/virtio/virtio-blk.h | 1 +
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index e70fccf..4871f2a 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;
 }
 
@@ -72,8 +73,11 @@ static int virtio_blk_handle_rw_error(VirtIOBlockReq *req, int error,
     VirtIOBlock *s = req->dev;
 
     if (action == BLOCK_ERROR_ACTION_STOP) {
-        req->next = s->rq;
-        s->rq = req;
+        /* Merged requests don't need to be restarted. */
+        if (!req->merged) {
+            req->next = s->rq;
+            s->rq = req;
+        }
     } else if (action == BLOCK_ERROR_ACTION_REPORT) {
         virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
         block_acct_failed(blk_get_stats(s->blk), &req->acct);
@@ -344,6 +348,7 @@ 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;
         }
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