Re: [PATCH v2 3/4] arm64: mte: Enable async tag check fault

On 1/13/21 6:11 PM, Catalin Marinas wrote:

On Thu, Jan 07, 2021 at 05:29:07PM +0000, Vincenzo Frascino wrote:

quoted

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index d02aff9f493d..a60d3718baae 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h

@@ -39,6 +39,7 @@ void mte_free_tag_storage(char *storage);
 /* track which pages have valid allocation tags */
 #define PG_mte_tagged	PG_arch_2
 
+void mte_check_tfsr_el1(void);
 void mte_sync_tags(pte_t *ptep, pte_t pte);
 void mte_copy_page_tags(void *kto, const void *kfrom);
 void flush_mte_state(void);

@@ -56,6 +57,9 @@ void mte_assign_mem_tag_range(void *addr, size_t size);
 /* unused if !CONFIG_ARM64_MTE, silence the compiler */
 #define PG_mte_tagged	0
 
+static inline void mte_check_tfsr_el1(void)
+{
+}

I think we should enable this dummy function when !CONFIG_KASAN_HW_TAGS.
It saves us an unnecessary function call in a few places.

Ok, I will add it in v3.

quoted

 static inline void mte_sync_tags(pte_t *ptep, pte_t pte)
 {
 }

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 5346953e4382..74b020ce72d7 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();
 }
 
 /*

@@ -47,6 +49,8 @@ static void noinstr exit_to_kernel_mode(struct pt_regs *regs)
 {
 	lockdep_assert_irqs_disabled();
 
+	mte_check_tfsr_el1();
+
 	if (interrupts_enabled(regs)) {
 		if (regs->exit_rcu) {
 			trace_hardirqs_on_prepare();

@@ -243,6 +247,8 @@ asmlinkage void noinstr enter_from_user_mode(void)
 
 asmlinkage void noinstr exit_to_user_mode(void)
 {
+	mte_check_tfsr_el1();

While for kernel entry the asynchronous faults are sync'ed automatically
with TFSR_EL1, we don't have this for exit, so we'd need an explicit
DSB. But rather than placing it here, it's better if we add a bool sync
argument to mte_check_tfsr_el1() which issues a dsb() before checking
the register. I think that's the only place where such argument would be
true (for now).

Good point, I will add the dsb() in mte_check_tfsr_el1() but instead of a bool
parameter I will add something more explicit.

quoted

+
 	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 5d992e16b420..26030f0b79fe 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -185,6 +185,34 @@ void mte_enable_kernel(enum kasan_arg_mode mode)
 	isb();
 }
 
+void mte_check_tfsr_el1(void)
+{
+	u64 tfsr_el1;
+
+	if (!IS_ENABLED(CONFIG_KASAN_HW_TAGS))
+		return;

If we define the static inline when !CONFIG_KASAN_HW_TAGS, we could add
the #ifdef here around the whole function.

Ok. I will add it in v3.

quoted

+	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!");
+	}
+}
+NOKPROBE_SYMBOL(mte_check_tfsr_el1);

Do we need this to be NOKPROBE_SYMBOL? It's not that low level.

It is an inheritance from when I had this code called very early. I will remove
it in the next version.

quoted

+
 static void update_sctlr_el1_tcf0(u64 tcf0)
 {
 	/* ISB required for the kernel uaccess routines */

@@ -250,6 +278,15 @@ 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 patch we rely on the dsb() present

s/patch/path/

quoted

+	 * in __switch_to() to guaranty that the indirect writes to TFSR_EL1

s/guaranty/guarantee/ (well, still valid though I think rarely used).

quoted

+	 * are synchronized before this point.
+	 */
+	mte_check_tfsr_el1();
 }
 
 void mte_suspend_exit(void)
-- 
2.30.0

-- 
Regards,
Vincenzo

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