scsi: protect req->aiocb with AioContext lock

If requests are being processed in the IOThread when a SCSIDevice is
unplugged, scsi_device_purge_requests() -> scsi_req_cancel_async() races
with I/O completion callbacks. Both threads load and store req->aiocb.
This can lead to assert(r->req.aiocb == NULL) failures and undefined
behavior.

Protect r->req.aiocb with the AioContext lock to prevent the race.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230221212218.1378734-2-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/scsi/scsi-disk.c

@@ -273,9 +273,11 @@ static void scsi_aio_complete(void *opaque, int ret)
     SCSIDiskReq *r = (SCSIDiskReq *)opaque;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
+    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     if (scsi_disk_req_check_error(r, ret, true)) {
         goto done;
     }
@@ -357,10 +359,11 @@ static void scsi_dma_complete(void *opaque, int ret)
     SCSIDiskReq *r = (SCSIDiskReq *)opaque;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
+    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
     if (ret < 0) {
         block_acct_failed(blk_get_stats(s->qdev.conf.blk), &r->acct);
     } else {
@@ -393,10 +396,11 @@ static void scsi_read_complete(void *opaque, int ret)
     SCSIDiskReq *r = (SCSIDiskReq *)opaque;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
+    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
     if (ret < 0) {
         block_acct_failed(blk_get_stats(s->qdev.conf.blk), &r->acct);
     } else {
@@ -446,10 +450,11 @@ static void scsi_do_read_cb(void *opaque, int ret)
     SCSIDiskReq *r = (SCSIDiskReq *)opaque;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
+    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     assert (r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
     if (ret < 0) {
         block_acct_failed(blk_get_stats(s->qdev.conf.blk), &r->acct);
     } else {
@@ -530,10 +535,11 @@ static void scsi_write_complete(void * opaque, int ret)
     SCSIDiskReq *r = (SCSIDiskReq *)opaque;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
+    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     assert (r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
     if (ret < 0) {
         block_acct_failed(blk_get_stats(s->qdev.conf.blk), &r->acct);
     } else {
@@ -1737,10 +1743,11 @@ static void scsi_unmap_complete(void *opaque, int ret)
     SCSIDiskReq *r = data->r;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
+    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
     if (scsi_disk_req_check_error(r, ret, true)) {
         scsi_req_unref(&r->req);
         g_free(data);
@@ -1816,9 +1823,11 @@ static void scsi_write_same_complete(void *opaque, int ret)
     SCSIDiskReq *r = data->r;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
+    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));
+
     if (scsi_disk_req_check_error(r, ret, true)) {
         goto done;
     }

hw/scsi/scsi-generic.c

@@ -111,10 +111,11 @@ static void scsi_command_complete(void *opaque, int ret)
     SCSIGenericReq *r = (SCSIGenericReq *)opaque;
     SCSIDevice *s = r->req.dev;
 
+    aio_context_acquire(blk_get_aio_context(s->conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->conf.blk));
     scsi_command_complete_noio(r, ret);
     aio_context_release(blk_get_aio_context(s->conf.blk));
 }
@@ -269,11 +270,11 @@ static void scsi_read_complete(void * opaque, int ret)
     SCSIDevice *s = r->req.dev;
     int len;
 
+    aio_context_acquire(blk_get_aio_context(s->conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->conf.blk));
-
     if (ret || r->req.io_canceled) {
         scsi_command_complete_noio(r, ret);
         goto done;
@@ -386,11 +387,11 @@ static void scsi_write_complete(void * opaque, int ret)
 
     trace_scsi_generic_write_complete(ret);
 
+    aio_context_acquire(blk_get_aio_context(s->conf.blk));
+
     assert(r->req.aiocb != NULL);
     r->req.aiocb = NULL;
 
-    aio_context_acquire(blk_get_aio_context(s->conf.blk));
-
     if (ret || r->req.io_canceled) {
         scsi_command_complete_noio(r, ret);
         goto done;