[PATCH] ARM64: Provide the ARM64_TBI option

[Yuan Li]

The ARM64 provides the Top Byte Ignore (TBI) early on, so the kernel turns TBI
on by default, but, it does not provide any option to turn the feature off.

In ARMv8.3, the Pointer Authentication (PA) was introduced, and if TBI is
turned off, the PA will be able to use the top byte, resulting longer pointer
authentication codes, which is more secure.

This patch changes the default support for the TBI to an option that can be
turned off.

Signed-off-by: Yuan Li <lydorazoe at gmail.com>
---
 arch/arm64/Kconfig                     | 6 ++++++
 arch/arm64/include/asm/memory.h        | 6 +++++-
 arch/arm64/include/asm/pgtable-hwdef.h | 5 +++++
 lib/Kconfig.kasan                      | 2 ++
 4 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b5b13a932561..22be64358df2 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -357,6 +357,10 @@ source "arch/arm64/Kconfig.platforms"
 
 menu "Kernel Features"
 
+config ARM64_TBI
+	bool "Enable support for Top Byte Ignore (TBI)"
+	default y
+
 menu "ARM errata workarounds via the alternatives framework"
 
 config ARM64_WORKAROUND_CLEAN_CACHE
@@ -1212,6 +1216,7 @@ config ARM64_SW_TTBR0_PAN
 config ARM64_TAGGED_ADDR_ABI
 	bool "Enable the tagged user addresses syscall ABI"
 	default y
+	depends on ARM64_TBI
 	help
 	  When this option is enabled, user applications can opt in to a
 	  relaxed ABI via prctl() allowing tagged addresses to be passed
@@ -1649,6 +1654,7 @@ config ARM64_AS_HAS_MTE
 config ARM64_MTE
 	bool "Memory Tagging Extension support"
 	default y
+	depends on ARM64_TBI
 	depends on ARM64_AS_HAS_MTE && ARM64_TAGGED_ADDR_ABI
 	depends on AS_HAS_ARMV8_5
 	depends on AS_HAS_LSE_ATOMICS
diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 824a3655dd93..74323aa69811 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -215,8 +215,12 @@ static inline unsigned long kaslr_offset(void)
  * up with a tagged userland pointer. Clear the tag to get a sane pointer to
  * pass on to access_ok(), for instance.
  */
-#define __untagged_addr(addr)	\
+#ifdef CONFIG_ARM64_TBI
+#define __untagged_addr(addr) \
 	((__force __typeof__(addr))sign_extend64((__force u64)(addr), 55))
+#else /* CONFIG_ARM64_TBI */
+#define __untagged_addr(addr) (addr)
+#endif /* CONFIG_ARM64_TBI */
 
 #define untagged_addr(addr)	({					\
 	u64 __addr = (__force u64)(addr);					\
diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index 40085e53f573..827904470515 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -264,8 +264,13 @@
 #define TCR_IPS_MASK		(UL(7) << TCR_IPS_SHIFT)
 #define TCR_A1			(UL(1) << 22)
 #define TCR_ASID16		(UL(1) << 36)
+#ifdef CONFIG_ARM64_TBI
 #define TCR_TBI0		(UL(1) << 37)
 #define TCR_TBI1		(UL(1) << 38)
+#else /* CONFIG_ARM64_TBI */
+#define TCR_TBI0		(UL(0) << 37)
+#define TCR_TBI1		(UL(0) << 38)
+#endif /* CONFIG_ARM64_TBI */
 #define TCR_HA			(UL(1) << 39)
 #define TCR_HD			(UL(1) << 40)
 #define TCR_TBID1		(UL(1) << 52)
diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index 1e2d10f86011..df30ed3dac4d 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -85,6 +85,7 @@ config KASAN_GENERIC
 
 config KASAN_SW_TAGS
 	bool "Software tag-based mode"
+	depends on ARM64_TBI
 	depends on HAVE_ARCH_KASAN_SW_TAGS && CC_HAS_KASAN_SW_TAGS
 	select SLUB_DEBUG if SLUB
 	select CONSTRUCTORS
@@ -108,6 +109,7 @@ config KASAN_SW_TAGS
 
 config KASAN_HW_TAGS
 	bool "Hardware tag-based mode"
+	depends on ARM64_TBI
 	depends on HAVE_ARCH_KASAN_HW_TAGS
 	depends on SLUB
 	help
-- 
2.17.1