[PATCH v3 3/4] arm64: mte: Enable async tag check fault

[Vincenzo Frascino]

MTE provides a mode that asynchronously updates the TFSR_EL1 register
when a tag check exception is detected.

To take advantage of this mode the kernel has to verify the status of
the register at:
  1. Context switching
  2. Return to user/EL0 (Not required in entry from EL0 since the kernel
  did not run)
  3. Kernel entry from EL1
  4. Kernel exit to EL1

If the register is non-zero a trace is reported.

Add the required features for EL1 detection and reporting.

Note: ITFSB bit is set in the SCTLR_EL1 register hence it guaranties that
the indirect writes to TFSR_EL1 are synchronized at exception entry to
EL1. On the context switch path the synchronization is guarantied by the
dsb() in __switch_to().

Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will at kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
---
 arch/arm64/include/asm/mte.h     | 21 +++++++++++++++++++
 arch/arm64/kernel/entry-common.c | 11 ++++++++++
 arch/arm64/kernel/mte.c          | 35 ++++++++++++++++++++++++++++++++
 3 files changed, 67 insertions(+)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index d02aff9f493d..1a715963d909 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -92,5 +92,26 @@ static inline void mte_assign_mem_tag_range(void *addr, size_t size)
 
 #endif /* CONFIG_ARM64_MTE */
 
+#ifdef CONFIG_KASAN_HW_TAGS
+void mte_check_tfsr_el1_no_sync(void);
+static inline void mte_check_tfsr_el1(void)
+{
+	mte_check_tfsr_el1_no_sync();
+	/*
+	 * The asynchronous faults are synch'ed automatically with
+	 * TFSR_EL1 on kernel entry but for exit an explicit dsb()
+	 * is required.
+	 */
+	dsb(ish);
+}
+#else
+static inline void mte_check_tfsr_el1_no_sync(void)
+{
+}
+static inline void mte_check_tfsr_el1(void)
+{
+}
+#endif /* CONFIG_KASAN_HW_TAGS */
+
 #endif /* __ASSEMBLY__ */
 #endif /* __ASM_MTE_H  */
diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 5346953e4382..c6dfe8a525b0 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -37,6 +37,8 @@ static void noinstr enter_from_kernel_mode(struct pt_regs *regs)
 	lockdep_hardirqs_off(CALLER_ADDR0);
 	rcu_irq_enter_check_tick();
 	trace_hardirqs_off_finish();
+
+	mte_check_tfsr_el1_no_sync();
 }
 
 /*
@@ -47,6 +49,13 @@ static void noinstr exit_to_kernel_mode(struct pt_regs *regs)
 {
 	lockdep_assert_irqs_disabled();
 
+	/*
+	 * The dsb() in mte_check_tfsr_el1() is required to relate
+	 * the asynchronous tag check fault to the context in which
+	 * it happens.
+	 */
+	mte_check_tfsr_el1();
+
 	if (interrupts_enabled(regs)) {
 		if (regs->exit_rcu) {
 			trace_hardirqs_on_prepare();
@@ -243,6 +252,8 @@ asmlinkage void noinstr enter_from_user_mode(void)
 
 asmlinkage void noinstr exit_to_user_mode(void)
 {
+	mte_check_tfsr_el1();
+
 	trace_hardirqs_on_prepare();
 	lockdep_hardirqs_on_prepare(CALLER_ADDR0);
 	user_enter_irqoff();
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index df7a1ae26d7c..6cb92e9d6ad1 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -180,6 +180,32 @@ void mte_enable_kernel(enum kasan_hw_tags_mode mode)
 	isb();
 }
 
+#ifdef CONFIG_KASAN_HW_TAGS
+void mte_check_tfsr_el1_no_sync(void)
+{
+	u64 tfsr_el1;
+
+	if (!system_supports_mte())
+		return;
+
+	tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
+
+	/*
+	 * The kernel should never hit the condition TF0 == 1
+	 * at this point because for the futex code we set
+	 * PSTATE.TCO.
+	 */
+	WARN_ON(tfsr_el1 & SYS_TFSR_EL1_TF0);
+
+	if (tfsr_el1 & SYS_TFSR_EL1_TF1) {
+		write_sysreg_s(0, SYS_TFSR_EL1);
+		isb();
+
+		pr_err("MTE: Asynchronous tag exception detected!");
+	}
+}
+#endif
+
 static void update_sctlr_el1_tcf0(u64 tcf0)
 {
 	/* ISB required for the kernel uaccess routines */
@@ -245,6 +271,15 @@ void mte_thread_switch(struct task_struct *next)
 	/* avoid expensive SCTLR_EL1 accesses if no change */
 	if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
 		update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
+
+	/*
+	 * Check if an async tag exception occurred at EL1.
+	 *
+	 * Note: On the context switch path we rely on the dsb() present
+	 * in __switch_to() to guarantee that the indirect writes to TFSR_EL1
+	 * are synchronized before this point.
+	 */
+	mte_check_tfsr_el1_no_sync();
 }
 
 void mte_suspend_exit(void)
-- 
2.30.0