docs/secure-coding-practices: Describe how to use 'null-co' block driver

Document that security reports must use 'null-co,read-zeroes=on'
because otherwise the memory is left uninitialized (which is an
on-purpose performance feature).

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210601162548.2076631-1-philmd@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/devel/secure-coding-practices.rst

@@ -104,3 +104,12 @@ structures and only process the local copy.  This prevents
 time-of-check-to-time-of-use (TOCTOU) race conditions that could cause QEMU to
 crash when a vCPU thread modifies guest RAM while device emulation is
 processing it.
+
+Use of null-co block drivers
+----------------------------
+
+The ``null-co`` block driver is designed for performance: its read accesses are
+not initialized by default. In case this driver has to be used for security
+research, it must be used with the ``read-zeroes=on`` option which fills read
+buffers with zeroes. Security issues reported with the default
+(``read-zeroes=off``) will be discarded.