[Qemu-devel] [STABLE PATCH 6/6] qcow2: Fix corruption after error in update_refcount

[Kevin Wolf <kwolf@redhat.com>]

After it is done with updating refcounts in the cache, update_refcount writes
all changed entries to disk. If a refcount block allocation fails, however,
there was no change yet and therefore first_index = last_index = -1. Don't
treat -1 as a normal sector index (resulting in a 512 byte write!) but return
without updating anything in this case.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 86fa8da83771238de55dc44819a1a27bafef5353)
---
 block/qcow2-refcount.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index fa78e46..465d5d3 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -402,6 +402,10 @@ static int write_refcount_block_entries(BDRVQcowState *s,
         return 0;
     }
 
+    if (first_index < 0) {
+        return 0;
+    }
+
     first_index &= ~(REFCOUNTS_PER_SECTOR - 1);
     last_index = (last_index + REFCOUNTS_PER_SECTOR)
         & ~(REFCOUNTS_PER_SECTOR - 1);
-- 
1.6.6.1