In case of multiple chunks, code in qxl_unpack_chunks() takes size of the
wrong (next in the chain) chunk, instead of using current chunk size.
This leads to wrong number of bytes being copied, and to crashes if next
chunk size is larger than the current one.

Based on the code by Gao Yong.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1628
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit b8882becd572d3afb888c836a6ffc7f92c17d1c5)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/hw/display/qxl-render.c b/hw/display/qxl-render.c
index fcfd40c3ac..6d1df16fa4 100644
--- a/hw/display/qxl-render.c
+++ b/hw/display/qxl-render.c
@@ -222,6 +222,7 @@ static void qxl_unpack_chunks(void *dest, size_t size, PCIQXLDevice *qxl,
     uint32_t max_chunks = 32;
     size_t offset = 0;
     size_t bytes;
+    QXLPHYSICAL next_chunk_phys = 0;
 
     for (;;) {
         bytes = MIN(size - offset, chunk->data_size);
@@ -230,7 +231,15 @@ static void qxl_unpack_chunks(void *dest, size_t size, PCIQXLDevice *qxl,
         if (offset == size) {
             return;
         }
-        chunk = qxl_phys2virt(qxl, chunk->next_chunk, group_id,
+        next_chunk_phys = chunk->next_chunk;
+        /* fist time, only get the next chunk's data size */
+        chunk = qxl_phys2virt(qxl, next_chunk_phys, group_id,
+                              sizeof(QXLDataChunk));
+        if (!chunk) {
+            return;
+        }
+        /* second time, check data size and get data */
+        chunk = qxl_phys2virt(qxl, next_chunk_phys, group_id,
                               sizeof(QXLDataChunk) + chunk->data_size);
         if (!chunk) {
             return;
-- 
2.47.2