Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

peterz
2020-08-06T17:02:59Z

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.

Aaargh!

> diff --git a/init/main.c b/init/main.c
> index 15bd0efff3df..0873319dcff4 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1041,6 +1041,22 @@ asmlinkage __visible void __init start_kernel(void)
> sfi_init_late();
> kcsan_init();
>
> + /* DEBUG CODE */
> + lockdep_assert_irqs_enabled(); /* Pass. */
> + {
> + unsigned long flags1;
> + raw_local_irq_save(flags1);

This disables IRQs but doesn't trace..

> + {
> + unsigned long flags2;
> + lockdep_assert_irqs_enabled(); /* Pass - expectedly blind. */

Indeed, we didn't trace the above disable, so software state is still
on.

> + local_irq_save(flags2);

So here we save IRQ state, and unconditionally disable IRQs and trace
them disabled.

> + lockdep_assert_irqs_disabled(); /* Pass. */
> + local_irq_restore(flags2);

But here, we restore IRQ state to 'disabled' and explicitly trace it
disabled *again* (which is a bit daft, but whatever).

> + }
> + raw_local_irq_restore(flags1);

This then restores the IRQ state to enable, but no tracing.

> + }
> + lockdep_assert_irqs_enabled(); /* FAIL! */

And we're out of sync... :/

/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(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 06ec60462af0..2f5aef57e687 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1193,9 +1193,6 @@ struct task_struct {

#ifdef CONFIG_KCSAN
struct kcsan_ctx kcsan_ctx;
-#ifdef CONFIG_TRACE_IRQFLAGS
- struct irqtrace_events kcsan_save_irqtrace;
-#endif
#endif

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 9147ff6a12e5..9c4436bf0561 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -291,17 +291,50 @@ static inline unsigned int get_delay(void)
0);
}

-void kcsan_save_irqtrace(struct task_struct *task)
+/*
+ * KCSAN hooks are everywhere, which means they're 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
+ int hardirqs;
+ struct irqtrace_events irqtrace;
+#endif
+};
+
+void kcsan_save_irqtrace(struct kcsan_irq_state *irq_state)
{
#ifdef CONFIG_TRACE_IRQFLAGS
- task->kcsan_save_irqtrace = task->irqtrace;
+ irq_state->irqtrace = task->irqtrace;
+ irq_state->hardirq = lockdep_hardirqs_enabled();
#endif
+ if (!kcsan_interrupt_watcher) {
+ raw_local_irq_save(irq_state->flags);
+ lockdep_hardirqs_off(CALLER_ADDR0);
+ }
}

-void kcsan_restore_irqtrace(struct task_struct *task)
+void kcsan_restore_irqtrace(struct kcsan_irq_state *irq_state)
{
+ if (!kcsan_interrupt_watcher) {
+#ifdef CONFIG_TRACE_IRQFLAGS
+ if (irq_state->hardirqs) {
+ lockdep_hardirqs_on_prepare(CALLER_ADDR0);
+ lockdep_hardirqs_on(CALLER_ADDR0);
+ }
+#endif
+ raw_local_irq_restore(irq_state->flags);
+ }
#ifdef CONFIG_TRACE_IRQFLAGS
- task->irqtrace = task->kcsan_save_irqtrace;
+ task->irqtrace = irq_state->irqtrace;
#endif
}

@@ -350,11 +383,13 @@ 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_save_irqtrace(&irqstate);
kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_MAYBE,
KCSAN_REPORT_CONSUMED_WATCHPOINT,
watchpoint - watchpoints);
- kcsan_restore_irqtrace(current);
+ kcsan_restore_irqtrace(&irqstate);
} else {
/*
* The other thread may not print any diagnostics, as it has
@@ -387,7 +422,7 @@ 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 +447,7 @@ 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_save_irqtrace(&irqstate);

watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
if (watchpoint == NULL) {
@@ -559,9 +587,7 @@ 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_restore_irqtrace(&irqstate);
out:
user_access_restore(ua_flags);
}