Re: [PATCH 4/5] arm64: mte: Add asymmetric mode support

MTE provides an asymmetric mode for detecting tag exceptions. In
particular, when such a mode is present, the CPU triggers a fault
on a tag mismatch during a load operation and asynchronously updates
a register when a tag mismatch is detected during a store operation.

Add support for MTE asymmetric mode.

Note: If the CPU does not support MTE asymmetric mode the kernel falls
back on synchronous mode which is the default for kasan=on.

Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
 arch/arm64/include/asm/memory.h    |  1 +
 arch/arm64/include/asm/mte-kasan.h |  5 +++++
 arch/arm64/kernel/mte.c            | 26 ++++++++++++++++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index f1745a843414..1b9a1e242612 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h

@@ -243,6 +243,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
 #ifdef CONFIG_KASAN_HW_TAGS
 #define arch_enable_tagging_sync()             mte_enable_kernel_sync()
 #define arch_enable_tagging_async()            mte_enable_kernel_async()
+#define arch_enable_tagging_asymm()            mte_enable_kernel_asymm()
 #define arch_force_async_tag_fault()           mte_check_tfsr_exit()
 #define arch_get_random_tag()                  mte_get_random_tag()
 #define arch_get_mem_tag(addr)                 mte_get_mem_tag(addr)

diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
index 22420e1f8c03..478b9bcf69ad 100644
--- a/arch/arm64/include/asm/mte-kasan.h
+++ b/arch/arm64/include/asm/mte-kasan.h

@@ -130,6 +130,7 @@ static inline void mte_set_mem_tag_range(void *addr, size_t size, u8 tag,

 void mte_enable_kernel_sync(void);
 void mte_enable_kernel_async(void);
+void mte_enable_kernel_asymm(void);

 #else /* CONFIG_ARM64_MTE */

@@ -161,6 +162,10 @@ static inline void mte_enable_kernel_async(void)
 {
 }

+static inline void mte_enable_kernel_asymm(void)
+{
+}
+
 #endif /* CONFIG_ARM64_MTE */

 #endif /* __ASSEMBLY__ */

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 9d314a3bad3b..ef5484ecb2da 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -137,6 +137,32 @@ void mte_enable_kernel_async(void)
        if (!system_uses_mte_async_mode())
                static_branch_enable(&mte_async_mode);
 }
+
+void mte_enable_kernel_asymm(void)
+{
+       if (cpus_have_cap(ARM64_MTE_ASYMM)) {
+               __mte_enable_kernel("asymmetric", SCTLR_ELx_TCF_ASYMM);
+
+               /*
+                * MTE asymm mode behaves as async mode for store
+                * operations. The mode is set system wide by the
+                * first PE that executes this function.
+                *
+                * Note: If in future KASAN acquires a runtime switching
+                * mode in between sync and async, this strategy needs
+                * to be reviewed.
+                */
+               if (!system_uses_mte_async_mode())
+                       static_branch_enable(&mte_async_mode);