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From 0fd571f772eb449da398b029705f4cadf0129f60 Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Wed, 10 Oct 2018 20:22:04 +0100
Subject: [PATCH 38/49] block-backend: Fix potential double blk_delete()

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20181010202213.7372-26-kwolf@redhat.com>
Patchwork-id: 82615
O-Subject: [RHEL-8 qemu-kvm PATCH 35/44] block-backend: Fix potential double blk_delete()
Bugzilla: 1637976
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: John Snow <jsnow@redhat.com>
RH-Acked-by: Thomas Huth <thuth@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>
(cherry picked from commit 5ca9d21bd1c8eeb578d0964e31bd03d47c25773d)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Danilo C. L. de Paula <ddepaula@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 dffc6f0..bfd0331 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