[PATCH 24/35] arm64: mte: Switch GCR_EL1 in kernel entry and exit

[Andrey Konovalov]

From: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>

When MTE is present, the GCR_EL1 register contains the tags mask that
allows to exclude tags from the random generation via the IRG instruction.

With the introduction of the new Tag-Based KASAN API that provides a
mechanism to reserve tags for special reasons, the MTE implementation
has to make sure that the GCR_EL1 setting for the kernel does not affect
the userspace processes and viceversa.

Save and restore the kernel/user mask in GCR_EL1 in kernel entry and exit.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
---
 arch/arm64/include/asm/mte.h    |  8 ++++++++
 arch/arm64/kernel/asm-offsets.c |  3 +++
 arch/arm64/kernel/cpufeature.c  |  5 +++--
 arch/arm64/kernel/entry.S       | 28 ++++++++++++++++++++++++++++
 arch/arm64/kernel/mte.c         | 19 +++++++++++++++++--
 5 files changed, 59 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index 733be1cb5c95..4929f744d103 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -21,6 +21,8 @@
 
 #include <asm/pgtable-types.h>
 
+extern u64 gcr_kernel_excl;
+
 void mte_clear_page_tags(void *addr);
 unsigned long mte_copy_tags_from_user(void *to, const void __user *from,
 				      unsigned long n);
@@ -59,6 +61,8 @@ u8 mte_get_mem_tag(void *addr);
 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);
+
 #else /* CONFIG_ARM64_MTE */
 
 /* unused if !CONFIG_ARM64_MTE, silence the compiler */
@@ -120,6 +124,10 @@ static inline void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
 	return addr;
 }
 
+static inline void mte_init_tags(u64 max_tag)
+{
+}
+
 #endif /* CONFIG_ARM64_MTE */
 
 #endif /* __ASSEMBLY__ */
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 0577e2142284..a1ef256cad4f 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -47,6 +47,9 @@ int main(void)
 #ifdef CONFIG_ARM64_PTR_AUTH
   DEFINE(THREAD_KEYS_USER,	offsetof(struct task_struct, thread.keys_user));
   DEFINE(THREAD_KEYS_KERNEL,	offsetof(struct task_struct, thread.keys_kernel));
+#endif
+#ifdef CONFIG_ARM64_MTE
+  DEFINE(THREAD_GCR_EL1_USER,	offsetof(struct task_struct, thread.gcr_user_excl));
 #endif
   BLANK();
   DEFINE(S_X0,			offsetof(struct pt_regs, regs[0]));
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 4d94af19d8f6..54bc3b315063 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1665,14 +1665,15 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
 {
 	u64 mair;
 
-	/* all non-zero tags excluded by default */
-	write_sysreg_s(SYS_GCR_EL1_RRND | SYS_GCR_EL1_EXCL_MASK, SYS_GCR_EL1);
 	write_sysreg_s(0, SYS_TFSR_EL1);
 	write_sysreg_s(0, SYS_TFSRE0_EL1);
 
 	/* Enable Match-All at EL1 */
 	sysreg_clear_set(tcr_el1, 0, SYS_TCR_EL1_TCMA1);
 
+	/* Enable the kernel exclude mask for random tags generation */
+	write_sysreg_s((SYS_GCR_EL1_RRND | gcr_kernel_excl), SYS_GCR_EL1);
+
 	/*
 	 * CnP must be enabled only after the MAIR_EL1 register has been set
 	 * up. Inconsistent MAIR_EL1 between CPUs sharing the same TLB may
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index cde127508e38..a17fefb0571b 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -172,6 +172,29 @@ alternative_else_nop_endif
 #endif
 	.endm
 
+	/* Note: tmp should always be a callee-saved register */
+	.macro mte_restore_gcr, el, tsk, tmp, tmp2
+#ifdef CONFIG_ARM64_MTE
+alternative_if_not ARM64_MTE
+	b	1f
+alternative_else_nop_endif
+	.if	\el == 0
+	ldr	\tmp, [\tsk, #THREAD_GCR_EL1_USER]
+	.else
+	ldr_l	\tmp, gcr_kernel_excl
+	.endif
+	/*
+	 * Calculate and set the exclude mask preserving
+	 * the RRND (bit[16]) setting.
+	 */
+	mrs_s	\tmp2, SYS_GCR_EL1
+	bfi	\tmp2, \tmp, #0, #16
+	msr_s	SYS_GCR_EL1, \tmp2
+	isb
+1:
+#endif
+	.endm
+
 	.macro	kernel_entry, el, regsize = 64
 	.if	\regsize == 32
 	mov	w0, w0				// zero upper 32 bits of x0
@@ -209,6 +232,8 @@ alternative_else_nop_endif
 
 	ptrauth_keys_install_kernel tsk, x20, x22, x23
 
+	mte_restore_gcr 1, tsk, x22, x23
+
 	scs_load tsk, x20
 	.else
 	add	x21, sp, #S_FRAME_SIZE
@@ -386,6 +411,8 @@ alternative_else_nop_endif
 	/* No kernel C function calls after this as user keys are set. */
 	ptrauth_keys_install_user tsk, x0, x1, x2
 
+	mte_restore_gcr 0, tsk, x0, x1
+
 	apply_ssbd 0, x0, x1
 	.endif
 
@@ -957,6 +984,7 @@ SYM_FUNC_START(cpu_switch_to)
 	mov	sp, x9
 	msr	sp_el0, x1
 	ptrauth_keys_install_kernel x1, x8, x9, x10
+	mte_restore_gcr 1, x1, x8, x9
 	scs_save x0, x8
 	scs_load x1, x8
 	ret
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 7717ea9bc2a7..cfac7d02f032 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -18,10 +18,14 @@
 
 #include <asm/barrier.h>
 #include <asm/cpufeature.h>
+#include <asm/kasan.h>
+#include <asm/kprobes.h>
 #include <asm/mte.h>
 #include <asm/ptrace.h>
 #include <asm/sysreg.h>
 
+u64 gcr_kernel_excl __read_mostly;
+
 static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 {
 	pte_t old_pte = READ_ONCE(*ptep);
@@ -115,6 +119,13 @@ void * __must_check mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
 	return ptr;
 }
 
+void mte_init_tags(u64 max_tag)
+{
+	u64 incl = ((1ULL << ((max_tag & MTE_TAG_MAX) + 1)) - 1);
+
+	gcr_kernel_excl = ~incl & SYS_GCR_EL1_EXCL_MASK;
+}
+
 static void update_sctlr_el1_tcf0(u64 tcf0)
 {
 	/* ISB required for the kernel uaccess routines */
@@ -150,7 +161,11 @@ static void update_gcr_el1_excl(u64 excl)
 static void set_gcr_el1_excl(u64 excl)
 {
 	current->thread.gcr_user_excl = excl;
-	update_gcr_el1_excl(excl);
+
+	/*
+	 * SYS_GCR_EL1 will be set to current->thread.gcr_user_incl value
+	 * by mte_restore_gcr() in kernel_exit,
+	 */
 }
 
 void flush_mte_state(void)
@@ -184,7 +199,7 @@ void mte_suspend_exit(void)
 	if (!system_supports_mte())
 		return;
 
-	update_gcr_el1_excl(current->thread.gcr_user_excl);
+	update_gcr_el1_excl(gcr_kernel_excl);
 }
 
 long set_mte_ctrl(struct task_struct *task, unsigned long arg)
-- 
2.28.0.220.ged08abb693-goog