[PATCH v4 02/15] kasan: clarify HW_TAGS impact on TBI

From: Andrey Konovalov <andreyknvl@google.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Alexander Potapenko <glider@google.com>,
	Marco Elver <elver@google.com>
Cc: Will Deacon <will.deacon@arm.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Peter Collingbourne <pcc@google.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Branislav Rankov <Branislav.Rankov@arm.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	kasan-dev@googlegroups.com, linux-arm-kernel@lists.infradead.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Andrey Konovalov <andreyknvl@google.com>
Subject: [PATCH v4 02/15] kasan: clarify HW_TAGS impact on TBI
Date: Fri, 15 Jan 2021 18:52:39 +0100	[thread overview]
Message-ID: <3b4ea6875bb14d312092ad14ac55cb456c83c08e.1610733117.git.andreyknvl@google.com> (raw)
In-Reply-To: <cover.1610733117.git.andreyknvl@google.com>

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.

Link: https://linux-review.googlesource.com/id/Iba2a6697e3c6304cb53f89ec61dedc77fa29e3ae
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 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?
 --------------------------------------------
 
-- 
2.30.0.284.gd98b1dd5eaa7-goog

