linux-kernel - [PATCH v6 04/18] kasan: sw_tags: Support tag widths less than 8 bits

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Message-ID: <8319582016f3e433bf7cd1c88ce7858c4a3c60fa.1761763681.git.m.wieczorretman@pm.me>
Date: Wed, 29 Oct 2025 19:06:33 +0000
From: Maciej Wieczor-Retman <m.wieczorretman@...me>
To: xin@...or.com, peterz@...radead.org, kaleshsingh@...gle.com, kbingham@...nel.org, akpm@...ux-foundation.org, nathan@...nel.org, ryabinin.a.a@...il.com, dave.hansen@...ux.intel.com, bp@...en8.de, morbo@...gle.com, jeremy.linton@....com, smostafa@...gle.com, kees@...nel.org, baohua@...nel.org, vbabka@...e.cz, justinstitt@...gle.com, wangkefeng.wang@...wei.com, leitao@...ian.org, jan.kiszka@...mens.com, fujita.tomonori@...il.com, hpa@...or.com, urezki@...il.com, ubizjak@...il.com, ada.coupriediaz@....com, nick.desaulniers+lkml@...il.com, ojeda@...nel.org, brgerst@...il.com, elver@...gle.com, pankaj.gupta@....com, glider@...gle.com, mark.rutland@....com, trintaeoitogc@...il.com, jpoimboe@...nel.org, thuth@...hat.com, pasha.tatashin@...een.com, dvyukov@...gle.com, jhubbard@...dia.com, catalin.marinas@....com, yeoreum.yun@....com, mhocko@...e.com, lorenzo.stoakes@...cle.com, samuel.holland@...ive.com, vincenzo.frascino@....com, bigeasy@...utronix.de, surenb@...gle.com,
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Cc: kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, x86@...nel.org, linux-kbuild@...r.kernel.org, linux-mm@...ck.org, llvm@...ts.linux.dev, linux-doc@...r.kernel.org, m.wieczorretman@...me
Subject: [PATCH v6 04/18] kasan: sw_tags: Support tag widths less than 8 bits

From: Samuel Holland <samuel.holland@...ive.com>

Allow architectures to override KASAN_TAG_KERNEL in asm/kasan.h. This
is needed on RISC-V, which supports 57-bit virtual addresses and 7-bit
pointer tags. For consistency, move the arm64 MTE definition of
KASAN_TAG_MIN to asm/kasan.h, since it is also architecture-dependent;
RISC-V's equivalent extension is expected to support 7-bit hardware
memory tags.

Reviewed-by: Andrey Konovalov <andreyknvl@...il.com>
Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
---
 arch/arm64/include/asm/kasan.h   |  6 ++++--
 arch/arm64/include/asm/uaccess.h |  1 +
 include/linux/kasan-tags.h       | 13 ++++++++-----
 3 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
index e1b57c13f8a4..4ab419df8b93 100644
--- a/arch/arm64/include/asm/kasan.h
+++ b/arch/arm64/include/asm/kasan.h
@@ -6,8 +6,10 @@
 
 #include <linux/linkage.h>
 #include <asm/memory.h>
-#include <asm/mte-kasan.h>
-#include <asm/pgtable-types.h>
+
+#ifdef CONFIG_KASAN_HW_TAGS
+#define KASAN_TAG_MIN			0xF0 /* minimum value for random tags */
+#endif
 
 #define arch_kasan_set_tag(addr, tag)	__tag_set(addr, tag)
 #define arch_kasan_reset_tag(addr)	__tag_reset(addr)
diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 1aa4ecb73429..8f700a7dd2cd 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -22,6 +22,7 @@
 #include <asm/cpufeature.h>
 #include <asm/mmu.h>
 #include <asm/mte.h>
+#include <asm/mte-kasan.h>
 #include <asm/ptrace.h>
 #include <asm/memory.h>
 #include <asm/extable.h>
diff --git a/include/linux/kasan-tags.h b/include/linux/kasan-tags.h
index 4f85f562512c..e07c896f95d3 100644
--- a/include/linux/kasan-tags.h
+++ b/include/linux/kasan-tags.h
@@ -2,13 +2,16 @@
 #ifndef _LINUX_KASAN_TAGS_H
 #define _LINUX_KASAN_TAGS_H
 
+#include <asm/kasan.h>
+
+#ifndef KASAN_TAG_KERNEL
 #define KASAN_TAG_KERNEL	0xFF /* native kernel pointers tag */
-#define KASAN_TAG_INVALID	0xFE /* inaccessible memory tag */
-#define KASAN_TAG_MAX		0xFD /* maximum value for random tags */
+#endif
+
+#define KASAN_TAG_INVALID	(KASAN_TAG_KERNEL - 1) /* inaccessible memory tag */
+#define KASAN_TAG_MAX		(KASAN_TAG_KERNEL - 2) /* maximum value for random tags */
 
-#ifdef CONFIG_KASAN_HW_TAGS
-#define KASAN_TAG_MIN		0xF0 /* minimum value for random tags */
-#else
+#ifndef KASAN_TAG_MIN
 #define KASAN_TAG_MIN		0x00 /* minimum value for random tags */
 #endif
 
-- 
2.51.0