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Date:   Thu, 6 Aug 2020 15:17:02 +0200
From:   Marco Elver <elver@google.com>
To:     peterz@infradead.org
Cc:     Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        fenghua.yu@intel.com, "H. Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Luck, Tony" <tony.luck@intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        yu-cheng.yu@intel.com, jgross@suse.com, sdeep@vmware.com,
        virtualization@lists.linux-foundation.org,
        kasan-dev <kasan-dev@googlegroups.com>,
        syzbot <syzbot+8db9e1ecde74e590a657@syzkaller.appspotmail.com>
Subject: Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*()
 helpers
Message-ID: <20200806131702.GA3029162@elver.google.com>
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On Thu, Aug 06, 2020 at 01:32PM +0200, peterz@infradead.org wrote:
> On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver wrote:
> > Testing my hypothesis that raw then nested non-raw
> > local_irq_save/restore() breaks IRQ state tracking -- see the reproducer
> > below. This is at least 1 case I can think of that we're bound to hit.
...
> 
> /me goes ponder things...
> 
> How's something like this then?
> 
> ---
>  include/linux/sched.h |  3 ---
>  kernel/kcsan/core.c   | 62 ++++++++++++++++++++++++++++++++++++---------------
>  2 files changed, 44 insertions(+), 21 deletions(-)

Thank you! That approach seems to pass syzbot (also with
CONFIG_PARAVIRT) and kcsan-test tests.

I had to modify it some, so that report.c's use of the restore logic
works and not mess up the IRQ trace printed on KCSAN reports (with
CONFIG_KCSAN_VERBOSE).

I still need to fully convince myself all is well now and we don't end
up with more fixes. :-) If it passes further testing, I'll send it as a
real patch (I want to add you as Co-developed-by, but would need your
Signed-off-by for the code you pasted, I think.)

Thanks,
-- Marco

------ >8 ------

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 9147ff6a12e5..b1d5dca10aa5 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -4,6 +4,7 @@
 #include <linux/bug.h>
 #include <linux/delay.h>
 #include <linux/export.h>
+#include <linux/ftrace.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/list.h>
@@ -291,13 +292,28 @@ static inline unsigned int get_delay(void)
 				0);
 }
 
-void kcsan_save_irqtrace(struct task_struct *task)
-{
+/*
+ * KCSAN instrumentation is everywhere, which means we must treat the hooks
+ * NMI-like for interrupt tracing. In order to present a 'normal' as possible
+ * context to the code called by KCSAN when reporting errors we need to update
+ * the IRQ-tracing state.
+ *
+ * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
+ * runtime is entered for every memory access, and potentially useful
+ * information is lost if dirtied by KCSAN.
+ */
+
+struct kcsan_irq_state {
+	unsigned long		flags;
 #ifdef CONFIG_TRACE_IRQFLAGS
-	task->kcsan_save_irqtrace = task->irqtrace;
+	int			hardirqs;
 #endif
-}
+};
 
+/*
+ * This is also called by the reporting task for the other task, to generate the
+ * right report with CONFIG_KCSAN_VERBOSE. No harm in restoring more than once.
+ */
 void kcsan_restore_irqtrace(struct task_struct *task)
 {
 #ifdef CONFIG_TRACE_IRQFLAGS
@@ -305,6 +321,34 @@ void kcsan_restore_irqtrace(struct task_struct *task)
 #endif
 }
 
+static void kcsan_irq_save(struct kcsan_irq_state *irq_state) {
+#ifdef CONFIG_TRACE_IRQFLAGS
+	current->kcsan_save_irqtrace = current->irqtrace;
+	irq_state->hardirqs = lockdep_hardirqs_enabled();
+#endif
+	if (!kcsan_interrupt_watcher) {
+		raw_local_irq_save(irq_state->flags);
+		kcsan_disable_current(); /* Lockdep might WARN. */
+		lockdep_hardirqs_off(CALLER_ADDR0);
+		kcsan_enable_current();
+	}
+}
+
+static void kcsan_irq_restore(struct kcsan_irq_state *irq_state) {
+	if (!kcsan_interrupt_watcher) {
+#ifdef CONFIG_TRACE_IRQFLAGS
+		if (irq_state->hardirqs) {
+			kcsan_disable_current(); /* Lockdep might WARN. */
+			lockdep_hardirqs_on_prepare(CALLER_ADDR0);
+			lockdep_hardirqs_on(CALLER_ADDR0);
+			kcsan_enable_current();
+		}
+#endif
+		raw_local_irq_restore(irq_state->flags);
+	}
+	kcsan_restore_irqtrace(current);
+}
+
 /*
  * Pull everything together: check_access() below contains the performance
  * critical operations; the fast-path (including check_access) functions should
@@ -350,11 +394,13 @@ static noinline void kcsan_found_watchpoint(const volatile void *ptr,
 	flags = user_access_save();
 
 	if (consumed) {
-		kcsan_save_irqtrace(current);
+		struct kcsan_irq_state irqstate;
+
+		kcsan_irq_save(&irqstate);
 		kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_MAYBE,
 			     KCSAN_REPORT_CONSUMED_WATCHPOINT,
 			     watchpoint - watchpoints);
-		kcsan_restore_irqtrace(current);
+		kcsan_irq_restore(&irqstate);
 	} else {
 		/*
 		 * The other thread may not print any diagnostics, as it has
@@ -387,7 +433,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	unsigned long access_mask;
 	enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 	unsigned long ua_flags = user_access_save();
-	unsigned long irq_flags = 0;
+	struct kcsan_irq_state irqstate;
 
 	/*
 	 * Always reset kcsan_skip counter in slow-path to avoid underflow; see
@@ -412,14 +458,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 		goto out;
 	}
 
-	/*
-	 * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
-	 * runtime is entered for every memory access, and potentially useful
-	 * information is lost if dirtied by KCSAN.
-	 */
-	kcsan_save_irqtrace(current);
-	if (!kcsan_interrupt_watcher)
-		local_irq_save(irq_flags);
+	kcsan_irq_save(&irqstate);
 
 	watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
 	if (watchpoint == NULL) {
@@ -559,9 +598,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	remove_watchpoint(watchpoint);
 	kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS);
 out_unlock:
-	if (!kcsan_interrupt_watcher)
-		local_irq_restore(irq_flags);
-	kcsan_restore_irqtrace(current);
+	kcsan_irq_restore(&irqstate);
 out:
 	user_access_restore(ua_flags);
 }
diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
index 29480010dc30..6eb35a9514d8 100644
--- a/kernel/kcsan/kcsan.h
+++ b/kernel/kcsan/kcsan.h
@@ -24,9 +24,8 @@ extern unsigned int kcsan_udelay_interrupt;
 extern bool kcsan_enabled;
 
 /*
- * Save/restore IRQ flags state trace dirtied by KCSAN.
+ * Restore IRQ flags state trace dirtied by KCSAN.
  */
-void kcsan_save_irqtrace(struct task_struct *task);
 void kcsan_restore_irqtrace(struct task_struct *task);
 
 /*