nvme_ctrl_reset() freed every SQ/CQ right after nvme_ns_drain(), which
only waits out requests on a per-namespace BlockBackend. That is safe
as long as the guest first tore down I/O queues gracefully (Delete
I/O SQ/CQ), since nvme_del_sq() already cancels and waits for
anything left on a queue before freeing it.

A reset that happens without that graceful sequence first (e.g. an
abrupt/asynchronous controller reset) can still have commands
inflight on blk_aio_*. Freeing sq/cq before those complete leaves
their completion callbacks (nvme_rw_cb() and friends) to run against
already-freed NvmeRequest/NvmeSQueue/NvmeCQueue memory via
nvme_enqueue_req_completion(), causing a use-after-free/segfault.

Factor the cancel-and-wait loop already used by nvme_del_sq() into
nvme_sq_cancel_inflight(), and run it over every queue in
nvme_ctrl_reset() before the free loops. A pending Async Event
Request has no aiocb (nvme_aer() parks it without issuing any block
I/O), so drop it directly instead of asserting.

Signed-off-by: Minwoo Im <minwoo.im@samsung.com>
---
 hw/nvme/ctrl.c | 47 +++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 39 insertions(+), 8 deletions(-)

diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c
index a67e1598891c..1482da57e4d9 100644
--- a/hw/nvme/ctrl.c
+++ b/hw/nvme/ctrl.c
@@ -4822,6 +4822,32 @@ static int nvme_init_sq_ioeventfd(NvmeSQueue *sq)
     return 0;
 }
 
+/*
+ * Cancel every command still inflight on sq and block until each one's
+ * completion callback has run. Safe to call regardless of how the queue is
+ * going away (explicit Delete SQ, or a controller reset that never went
+ * through the guest's graceful queue teardown) since it guarantees no
+ * in-flight blk_aio_* callback can fire after sq/req memory is freed.
+ *
+ * A pending Async Event Request has no aiocb (nvme_aer() parks it without
+ * issuing any block I/O), so there is nothing to cancel; just drop it.
+ */
+static void nvme_sq_cancel_inflight(NvmeSQueue *sq, uint16_t status)
+{
+    NvmeRequest *r;
+
+    while (!QTAILQ_EMPTY(&sq->out_req_list)) {
+        r = QTAILQ_FIRST(&sq->out_req_list);
+        r->status = status;
+
+        if (r->aiocb) {
+            blk_aio_cancel(r->aiocb);
+        } else {
+            QTAILQ_REMOVE(&sq->out_req_list, r, entry);
+        }
+    }
+}
+
 static void nvme_free_sq(NvmeSQueue *sq, NvmeCtrl *n)
 {
     uint16_t offset = sq->sqid << 3;
@@ -4856,14 +4882,7 @@ static uint16_t nvme_del_sq(NvmeCtrl *n, NvmeRequest *req)
     trace_pci_nvme_del_sq(qid);
 
     sq = n->sq[qid];
-    while (!QTAILQ_EMPTY(&sq->out_req_list)) {
-        r = QTAILQ_FIRST(&sq->out_req_list);
-        assert(r->aiocb);
-        r->status = NVME_CMD_ABORT_SQ_DEL;
-        blk_aio_cancel(r->aiocb);
-    }
-
-    assert(QTAILQ_EMPTY(&sq->out_req_list));
+    nvme_sq_cancel_inflight(sq, NVME_CMD_ABORT_SQ_DEL);
 
     if (!nvme_check_cqid(n, sq->cqid)) {
         cq = n->cq[sq->cqid];
@@ -8022,6 +8041,18 @@ static void nvme_ctrl_reset(NvmeCtrl *n, NvmeResetType rst)
         nvme_ns_drain(ns);
     }
 
+    /*
+     * Cancel and wait out every inflight command on every queue first. A
+     * reset is not required to be preceded by the guest's graceful
+     * Delete I/O SQ/CQ sequence, so sq/cq must not be freed below while a
+     * blk_aio_* completion for them could still be in flight.
+     */
+    for (i = 0; i < n->num_queues; i++) {
+        if (n->sq[i] != NULL) {
+            nvme_sq_cancel_inflight(n->sq[i], NVME_CMD_ABORT_SQ_DEL);
+        }
+    }
+
     for (i = 0; i < n->num_queues; i++) {
         if (n->sq[i] != NULL) {
             nvme_free_sq(n->sq[i], n);
-- 
2.34.1