kasan: clarify that only first bug is reported in HW_TAGS

Hwardware tag-based KASAN only reports the first found bug. After that MTE
tag checking gets disabled. Clarify this in comments and documentation.

Link: https://lkml.kernel.org/r/00383ba88a47c3f8342d12263c24bdf95527b07d.1612546384.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/kasan.rst

@@ -155,7 +155,7 @@ Boot parameters
 ~~~~~~~~~~~~~~~
 
 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
+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.
 
@@ -165,7 +165,8 @@ particular KASAN features.
   traces collection (default: ``on``).
 
 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
-  report or also panic the kernel (default: ``report``).
+  report or also panic the kernel (default: ``report``). Note, that tag
+  checking gets disabled after the first reported bug.
 
 For developers
 ~~~~~~~~~~~~~~
@@ -295,6 +296,9 @@ 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).
 
+Hardware tag-based KASAN only reports the first found bug. After that MTE tag
+checking gets disabled.
+
 What memory accesses are sanitised by KASAN?
 --------------------------------------------
 

mm/kasan/hw_tags.c

@@ -48,7 +48,7 @@ EXPORT_SYMBOL(kasan_flag_enabled);
 /* Whether to collect alloc/free stack traces. */
 DEFINE_STATIC_KEY_FALSE(kasan_flag_stacktrace);
 
-/* Whether panic or disable tag checking on fault. */
+/* Whether to panic or print a report and disable tag checking on fault. */
 bool kasan_flag_panic __ro_after_init;
 
 /* kasan=off/on */