From f006159bba058c873bf8c1a3684ec72f338f2a2c Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Fri, 14 Sep 2018 10:55:33 +0200
Subject: [PATCH 42/49] block-backend: Fix potential double blk_delete()

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20180914105540.18077-36-kwolf@redhat.com>
Patchwork-id: 82186
O-Subject: [RHV-7.6 qemu-kvm-rhev PATCH 35/42] block-backend: Fix potential double blk_delete()
Bugzilla: 1601212
RH-Acked-by: John Snow <jsnow@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: Fam Zheng <famz@redhat.com>

blk_unref() first decreases the refcount of the BlockBackend and calls
blk_delete() if the refcount reaches zero. Requests can still be in
flight at this point, they are only drained during blk_delete():

At this point, arbitrary callbacks can run. If any callback takes a
temporary BlockBackend reference, it will first increase the refcount to
1 and then decrease it to 0 again, triggering another blk_delete(). This
will cause a use-after-free crash in the outer blk_delete().

Fix it by draining the BlockBackend before decreasing to refcount to 0.
Assert in blk_ref() that it never takes the first refcount (which would
mean that the BlockBackend is already being deleted).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 block/block-backend.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index ea85770..1adf76b 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -436,6 +436,7 @@ int blk_get_refcnt(BlockBackend *blk)
  */
 void blk_ref(BlockBackend *blk)
 {
+    assert(blk->refcnt > 0);
     blk->refcnt++;
 }
 
@@ -448,7 +449,13 @@ void blk_unref(BlockBackend *blk)
 {
     if (blk) {
         assert(blk->refcnt > 0);
-        if (!--blk->refcnt) {
+        if (blk->refcnt > 1) {
+            blk->refcnt--;
+        } else {
+            blk_drain(blk);
+            /* blk_drain() cannot resurrect blk, nobody held a reference */
+            assert(blk->refcnt == 1);
+            blk->refcnt = 0;
             blk_delete(blk);
         }
     }
-- 
1.8.3.1