[PATCH 02/11] kasan: clarify HW_TAGS impact on TBI

From: Andrey Konovalov
Date: Tue Jan 05 2021 - 13:29:00 EST


Mention in the documentation that enabling CONFIG_KASAN_HW_TAGS
always results in in-kernel TBI (Top Byte Ignore) being enabled.

Also do a few minor documentation cleanups.

Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Link: https://linux-review.googlesource.com/id/Iba2a6697e3c6304cb53f89ec61dedc77fa29e3ae
---
Documentation/dev-tools/kasan.rst | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 0fc3fb1860c4..26c99852a852 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -147,15 +147,14 @@ negative values to distinguish between different kinds of inaccessible memory
like redzones or freed memory (see mm/kasan/kasan.h).

In the report above the arrows point to the shadow byte 03, which means that
-the accessed address is partially accessible.
-
-For tag-based KASAN this last report section shows the memory tags around the
-accessed address (see `Implementation details`_ section).
+the accessed address is partially accessible. For tag-based KASAN modes this
+last report section shows the memory tags around the accessed address
+(see the `Implementation details`_ section).

Boot parameters
~~~~~~~~~~~~~~~

-Hardware tag-based KASAN mode (see the section about different mode below) is
+Hardware tag-based KASAN mode (see the section about various modes below) is
intended for use in production as a security mitigation. Therefore it supports
boot parameters that allow to disable KASAN competely or otherwise control
particular KASAN features.
@@ -305,6 +304,13 @@ reserved to tag freed memory regions.
Hardware tag-based KASAN currently only supports tagging of
kmem_cache_alloc/kmalloc and page_alloc memory.

+If the hardware doesn't support MTE (pre ARMv8.5), hardware tag-based KASAN
+won't be enabled. In this case all boot parameters are ignored.
+
+Note, that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
+enabled. Even when kasan.mode=off is provided, or when the hardware doesn't
+support MTE (but supports TBI).
+
What memory accesses are sanitised by KASAN?
--------------------------------------------

--
2.29.2.729.g45daf8777d-goog