[PATCH v3] docs/secure-coding-practices: Describe how to use 'null-co' block driver

[Philippe Mathieu-Daudé]

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).

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
v3: Simplified using Vladimir suggestion.
---
 docs/devel/secure-coding-practices.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/docs/devel/secure-coding-practices.rst 
b/docs/devel/secure-coding-practices.rst
index cbfc8af67e6..79a3dcd09a3 100644
--- a/docs/devel/secure-coding-practices.rst
+++ b/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 it 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.
-- 
2.26.3