Documentation/module-signing.txt: Note need for version info if reusing a key

Signing a module should only make it trusted by the specific kernel it
was built for, not anything else.  If a module signing key is used for
multiple ABI-incompatible kernels, the modules need to include enough
version information to distinguish them.
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Documentation/module-signing.txt

@@ -271,3 +271,9 @@ Since the private key is used to sign modules, viruses and malware could use
 the private key to sign modules and compromise the operating system.  The
 private key must be either destroyed or moved to a secure location and not kept
 in the root node of the kernel source tree.
+
+If you use the same private key to sign modules for multiple kernel
+configurations, you must ensure that the module version information is
+sufficient to prevent loading a module into a different kernel.  Either
+set CONFIG_MODVERSIONS=y or ensure that each configuration has a different
+kernel release string by changing EXTRAVERSION or CONFIG_LOCALVERSION.