[PATCH RFC v2 13/21] arm64: kasan: Add cpu_supports_tags helper

[Andrey Konovalov]

Add an arm64 helper called cpu_supports_mte() that exposes information
about whether the CPU supports memory tagging and that can be called
during early boot (unlike system_supports_mte()).

Use that helper to implement a generic cpu_supports_tags() helper, that
will be used by hardware tag-based KASAN.

Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
Link: https://linux-review.googlesource.com/id/Ib4b56a42c57c6293df29a0cdfee334c3ca7bdab4
---
 arch/arm64/include/asm/memory.h    |  1 +
 arch/arm64/include/asm/mte-kasan.h |  6 ++++++
 arch/arm64/kernel/mte.c            | 20 ++++++++++++++++++++
 mm/kasan/kasan.h                   |  4 ++++
 4 files changed, 31 insertions(+)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index b5d6b824c21c..f496abfcf7f5 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -232,6 +232,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
 }
 
 #ifdef CONFIG_KASAN_HW_TAGS
+#define arch_cpu_supports_tags()		cpu_supports_mte()
 #define arch_init_tags(max_tag)			mte_init_tags(max_tag)
 #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 a4c61b926d4a..4c3f2c6b4fe6 100644
--- a/arch/arm64/include/asm/mte-kasan.h
+++ b/arch/arm64/include/asm/mte-kasan.h
@@ -9,6 +9,7 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/init.h>
 #include <linux/types.h>
 
 /*
@@ -30,6 +31,7 @@ u8 mte_get_random_tag(void);
 void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag);
 
 void mte_init_tags(u64 max_tag);
+bool __init cpu_supports_mte(void);
 
 #else /* CONFIG_ARM64_MTE */
 
@@ -54,6 +56,10 @@ static inline void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
 static inline void mte_init_tags(u64 max_tag)
 {
 }
+static inline bool cpu_supports_mte(void)
+{
+	return false;
+}
 
 #endif /* CONFIG_ARM64_MTE */
 
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index ca8206b7f9a6..8fcd17408515 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -134,6 +134,26 @@ void mte_init_tags(u64 max_tag)
 	gcr_kernel_excl = ~incl & SYS_GCR_EL1_EXCL_MASK;
 }
 
+/*
+ * This function can be used during early boot to determine whether the CPU
+ * supports MTE. The alternative that must be used after boot is completed is
+ * system_supports_mte(), but it only works after the cpufeature framework
+ * learns about MTE.
+ */
+bool __init cpu_supports_mte(void)
+{
+	u64 pfr1;
+	u32 val;
+
+	if (!IS_ENABLED(CONFIG_ARM64_MTE))
+		return false;
+
+	pfr1 = read_cpuid(ID_AA64PFR1_EL1);
+	val = cpuid_feature_extract_unsigned_field(pfr1, ID_AA64PFR1_MTE_SHIFT);
+
+	return val >= ID_AA64PFR1_MTE;
+}
+
 static void update_sctlr_el1_tcf0(u64 tcf0)
 {
 	/* ISB required for the kernel uaccess routines */
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index da08b2533d73..f7ae0c23f023 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -240,6 +240,9 @@ static inline const void *arch_kasan_set_tag(const void *addr, u8 tag)
 #define set_tag(addr, tag)	((void *)arch_kasan_set_tag((addr), (tag)))
 #define get_tag(addr)		arch_kasan_get_tag(addr)
 
+#ifndef arch_cpu_supports_tags
+#define arch_cpu_supports_tags() (false)
+#endif
 #ifndef arch_init_tags
 #define arch_init_tags(max_tag)
 #endif
@@ -253,6 +256,7 @@ static inline const void *arch_kasan_set_tag(const void *addr, u8 tag)
 #define arch_set_mem_tag_range(addr, size, tag) ((void *)(addr))
 #endif
 
+#define cpu_supports_tags()			arch_cpu_supports_tags()
 #define init_tags(max_tag)			arch_init_tags(max_tag)
 #define get_random_tag()			arch_get_random_tag()
 #define get_mem_tag(addr)			arch_get_mem_tag(addr)
-- 
2.29.0.rc1.297.gfa9743e501-goog