[PATCH v2 2/2] block/null: Enable 'read-zeroes' mode by default

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

The null-co driver is meant for (performance) testing.
By default, read operation does nothing, the provided buffer
is not filled with zero values and its content is unchanged.

This performance 'feature' becomes an issue from a security
perspective.  For example, using the default null-co driver,
buf[] is uninitialized, the blk_pread() call succeeds and we
then access uninitialized memory:

  static int guess_disk_lchs(BlockBackend *blk,
                             int *pcylinders, int *pheads,
                             int *psectors)
  {
      uint8_t buf[BDRV_SECTOR_SIZE];
      ...

      if (blk_pread(blk, 0, buf, BDRV_SECTOR_SIZE) < 0) {
          return -1;
      }
      /* test msdos magic */
      if (buf[510] != 0x55 || buf[511] != 0xaa) {
          return -1;
      }

We could audit all the uninitialized buffers and the
bdrv_co_preadv() handlers, but it is simpler to change the
default of this testing driver. Performance tests will have
to adapt and use 'null-co,read-zeroes=off'.

Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 block/null.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/null.c b/block/null.c
index cc9b1d4ea72..f9658fd70ac 100644
--- a/block/null.c
+++ b/block/null.c
@@ -93,7 +93,7 @@ static int null_file_open(BlockDriverState *bs, QDict *options, int flags,
         error_setg(errp, "latency-ns is invalid");
         ret = -EINVAL;
     }
-    s->read_zeroes = qemu_opt_get_bool(opts, NULL_OPT_ZEROES, false);
+    s->read_zeroes = qemu_opt_get_bool(opts, NULL_OPT_ZEROES, true);
     qemu_opts_del(opts);
     bs->supported_write_flags = BDRV_REQ_FUA;
     return ret;
-- 
2.26.2