nbd: safeguard against waking up invalid coroutine

The .reply_possible field of s->requests is never set to false.  This is
not a problem as it is only a safeguard to detect protocol errors,
but it's sloppy.  In fact, the field is actually not necessary at all,
because .coroutine is set to NULL in NBD_FOREACH_REPLY_CHUNK after
receiving the last chunk.  Thus, replace .reply_possible with .coroutine
and move the check before deciding the fate of this request.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414175756.671165-2-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 5 insertions, 7 deletions

diff --git a/block/nbd.c b/block/nbd.c
index 567872ac53..1f97160949 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -58,7 +58,6 @@ typedef struct {
     Coroutine *coroutine;
     uint64_t offset;        /* original offset of the request */
     bool receiving;         /* sleeping in the yield in nbd_receive_replies */
-    bool reply_possible;    /* reply header not yet received */
 } NBDClientRequest;
 
 typedef enum NBDClientState {
@@ -454,15 +453,15 @@ static coroutine_fn int nbd_receive_replies(BDRVNBDState *s, uint64_t handle)
             nbd_channel_error(s, -EINVAL);
             return -EINVAL;
         }
-        if (s->reply.handle == handle) {
-            /* We are done */
-            return 0;
-        }
         ind2 = HANDLE_TO_INDEX(s, s->reply.handle);
-        if (ind2 >= MAX_NBD_REQUESTS || !s->requests[ind2].reply_possible) {
+        if (ind2 >= MAX_NBD_REQUESTS || !s->requests[ind2].coroutine) {
             nbd_channel_error(s, -EINVAL);
             return -EINVAL;
         }
+        if (s->reply.handle == handle) {
+            /* We are done */
+            return 0;
+        }
         nbd_recv_coroutine_wake_one(&s->requests[ind2]);
     }
 }
@@ -505,7 +504,6 @@ static int nbd_co_send_request(BlockDriverState *bs,
     s->requests[i].coroutine = qemu_coroutine_self();
     s->requests[i].offset = request->from;
     s->requests[i].receiving = false;
-    s->requests[i].reply_possible = true;
 
     request->handle = INDEX_TO_HANDLE(s, i);