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

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Message-ID: <CANpmjNPYFjv4TTCG+t0zyr2efCtjPKV7zQQu-ccsgX5XtGtDLg@mail.gmail.com>
Date: Fri, 14 Feb 2025 17:43:36 +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 17:18, Waiman Long <llong@...hat.com> wrote:
>
> On 2/14/25 9:44 AM, Marco Elver wrote:
> > 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.
>
> The kasan_check_byte() failure happens very early in the boot cycle.
> There is no KASAN report, but the API returns false. I inserted a
> WARN_ON(1) to dump out the stack.

I see - I suspect this is before ctors had a chance to run, which is
the way globals are registered with KASAN.

I think it'd be fair to just remove the lockdep_kasan_fail event,
given KASAN would produce its own report on a real error anyway.

I.e. just do the kasan_check_byte(), and don't bail even if it returns
false. The KASAN report would appear before everything else (incl. a
bad lockdep report due to possible corrupted memory) and I think
that's all we need to be able to debug a real bug.