[PATCH] ARM64: Provide the ARM64_TBI option

[Robin Murphy]

On 2021-07-14 19:06, Yuan Li wrote:
> 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.

This would have to be something that processes explicitly opt in to. See 
Documentation/arm64/tagged-pointers.rst - silently disabling TBI0 *will* 
break existing userspace software.

Robin.

> 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
>