linux-kernel - Re: [PATCH v4 4/4] locking/lockdep: Add kasan_check_byte() check in lock

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Message-ID: <CANpmjNNLn9=UA+cai=rL+6zsEQyppf6-4_YL4GAFi+dLt+4oSA@mail.gmail.com>
Date: Fri, 14 Feb 2025 15:44:26 +0100
From: Marco Elver <elver@...gle.com>
To: Waiman Long <llong@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Will Deacon <will.deacon@....com>, Boqun Feng <boqun.feng@...il.com>, 
	Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, linux-kernel@...r.kernel.org, 
	kasan-dev@...glegroups.com
Subject: Re: [PATCH v4 4/4] locking/lockdep: Add kasan_check_byte() check in lock_acquire()

On Fri, 14 Feb 2025 at 15:09, Waiman Long <llong@...hat.com> wrote:
>
> On 2/14/25 5:44 AM, Marco Elver wrote:
> > On Thu, 13 Feb 2025 at 21:02, Waiman Long <longman@...hat.com> wrote:
> >> KASAN instrumentation of lockdep has been disabled as we don't need
> >> KASAN to check the validity of lockdep internal data structures and
> >> incur unnecessary performance overhead. However, the lockdep_map pointer
> >> passed in externally may not be valid (e.g. use-after-free) and we run
> >> the risk of using garbage data resulting in false lockdep reports. Add
> >> kasan_check_byte() call in lock_acquire() for non kernel core data
> >> object to catch invalid lockdep_map and abort lockdep processing if
> >> input data isn't valid.
> >>
> >> Suggested-by: Marco Elver <elver@...gle.com>
> >> Signed-off-by: Waiman Long <longman@...hat.com>
> > Reviewed-by: Marco Elver <elver@...gle.com>
> >
> > but double-check if the below can be simplified.
> >
> >> ---
> >>   kernel/locking/lock_events_list.h |  1 +
> >>   kernel/locking/lockdep.c          | 14 ++++++++++++++
> >>   2 files changed, 15 insertions(+)
> >>
> >> diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
> >> index 9ef9850aeebe..bed59b2195c7 100644
> >> --- a/kernel/locking/lock_events_list.h
> >> +++ b/kernel/locking/lock_events_list.h
> >> @@ -95,3 +95,4 @@ LOCK_EVENT(rtmutex_deadlock)  /* # of rt_mutex_handle_deadlock()'s    */
> >>   LOCK_EVENT(lockdep_acquire)
> >>   LOCK_EVENT(lockdep_lock)
> >>   LOCK_EVENT(lockdep_nocheck)
> >> +LOCK_EVENT(lockdep_kasan_fail)
> >> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> >> index 8436f017c74d..98dd0455d4be 100644
> >> --- a/kernel/locking/lockdep.c
> >> +++ b/kernel/locking/lockdep.c
> >> @@ -57,6 +57,7 @@
> >>   #include <linux/lockdep.h>
> >>   #include <linux/context_tracking.h>
> >>   #include <linux/console.h>
> >> +#include <linux/kasan.h>
> >>
> >>   #include <asm/sections.h>
> >>
> >> @@ -5830,6 +5831,19 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
> >>          if (!debug_locks)
> >>                  return;
> >>
> >> +       /*
> >> +        * As KASAN instrumentation is disabled and lock_acquire() is usually
> >> +        * the first lockdep call when a task tries to acquire a lock, add
> >> +        * kasan_check_byte() here to check for use-after-free of non kernel
> >> +        * core lockdep_map data to avoid referencing garbage data.
> >> +        */
> >> +       if (unlikely(IS_ENABLED(CONFIG_KASAN) &&
> > This is not needed - kasan_check_byte() will always return true if
> > KASAN is disabled or not compiled in.
> I added this check because of the is_kernel_core_data() call.
> >
> >> +                    !is_kernel_core_data((unsigned long)lock) &&
> > Why use !is_kernel_core_data()? Is it to improve performance?
>
> Not exactly. In my testing, just using kasan_check_byte() doesn't quite
> work out. It seems to return false positive in some cases causing
> lockdep splat. I didn't look into exactly why this happens and I added
> the is_kernel_core_data() call to work around that.

Globals should have their shadow memory unpoisoned by default, so
that's definitely odd.

Out of curiosity, do you have such a false positive splat? Wondering
which data it's accessing. Maybe that'll tell us more about what's
wrong.