Re: [PATCH v2] locking/ww_mutex: Adjust to lockdep nest_lock requirements

[Thomas Hellström]

On Mon, 2024-10-14 at 13:23 -0700, Boqun Feng wrote:
> Hi Thomas,
> 
> On Wed, Oct 09, 2024 at 11:20:31AM +0200, Thomas Hellström wrote:
> > When using mutex_acquire_nest() with a nest_lock, lockdep refcounts
> > the
> > number of acquired lockdep_maps of mutexes of the same class, and
> > also
> > keeps a pointer to the first acquired lockdep_map of a class. That
> > pointer
> > is then used for various comparison-, printing- and checking
> > purposes,
> > but there is no mechanism to actively ensure that lockdep_map stays
> > in
> > memory. Instead, a warning is printed if the lockdep_map is freed
> > and
> > there are still held locks of the same lock class, even if the
> > lockdep_map
> > itself has been released.
> > 
> > In the context of WW/WD transactions that means that if a user
> > unlocks
> > and frees a ww_mutex from within an ongoing ww transaction, and
> > that
> > mutex happens to be the first ww_mutex grabbed in the transaction,
> > such a warning is printed and there might be a risk of a UAF.
> > 
> > Note that this is only problem when lockdep is enabled and affects
> > only
> > dereferences of struct lockdep_map.
> > 
> > Adjust to this by adding a fake lockdep_map to the acquired context
> > and
> > make sure it is the first acquired lockdep map of the associated
> > ww_mutex class. Then hold it for the duration of the WW/WD
> > transaction.
> > 
> > This has the side effect that trying to lock a ww mutex *without* a
> > ww_acquire_context but where a such context has been acquire, we'd
> > see
> > a lockdep splat. The test-ww_mutex.c selftest attempts to do that,
> > so
> > modify that particular test to not acquire a ww_acquire_context if
> > it
> > is not going to be used.
> > 
> > v2:
> > - Lower the number of locks in the test-ww_mutex
> >   stress(STRESS_ALL) test to accommodate the dummy lock
> >   introduced in this patch without overflowing lockdep held lock
> >   references.
> > 
> 
> Have you tested your patch with lib/locking-selftests.c? It reported
> two
> errors for me:
> 
> 	[..]   | Wound/wait tests |
> 	[..]   ---------------------
> 	[..]                   ww api failures:  ok  |FAILED|  ok  |
> 	[..]                ww contexts mixing:  ok  |  ok  |
> 	[..]              finishing ww context:  ok  |  ok  |  ok 
> |  ok  |
> 	[..]                locking mismatches:  ok  |  ok  |  ok  |
> 	[..]                  EDEADLK handling:  ok  |  ok  |  ok 
> |  ok  |  ok  |  ok  |  ok  |  ok  |  ok  |  ok  |
> 	[..]            spinlock nest unlocked:  ok  |
> 	[..]                spinlock nest test:  ok  |
> 	[..]   -----------------------------------------------------
> 	[..]                                  |block | try 
> |context|
> 	[..]   -----------------------------------------------------
> 	[..]                           context:  ok  |  ok  |  ok  |
> 	[..]                               try:  ok  |  ok  |  ok  |
> 	[..]                             block:  ok  |  ok  |  ok  |
> 	[..]                          spinlock:  ok  |  ok  |FAILED|
> 
> The first one is a use case issue, I think and can be fixed similar
> to
> your changes in test-ww_mutex.c:
> 
> diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
> index 6f6a5fc85b42..6750321e3e9a 100644
> --- a/lib/locking-selftest.c
> +++ b/lib/locking-selftest.c
> @@ -1720,8 +1720,6 @@ static void ww_test_normal(void)
>  {
>         int ret;
> 
> -       WWAI(&t);
> -
>         /*
>          * None of the ww_mutex codepaths should be taken in the
> 'normal'
>          * mutex calls. The easiest way to verify this is by using
> the
> @@ -1770,6 +1768,8 @@ static void ww_test_normal(void)
>         ww_mutex_base_unlock(&o.base);
>         WARN_ON(o.ctx != (void *)~0UL);
> 
> +       WWAI(&t);
> +
>         /* nest_lock */
>         o.ctx = (void *)~0UL;
>         ww_mutex_base_lock_nest_lock(&o.base, &t);
> 
> Please confirm whether this change is intended.

This fix looks correct and while this change was not intended, I think
it makes sense and if this locking order is present in existing code
apart from this selftest, it's probably easily fixable.

> 
> The second is a case as follow:
> 
> 	ww_acquire_init(...);
> 	spin_lock(...);
> 	ww_mutex_lock(...); // this should trigger a context
> 			    // invalidation. But the mutex was
> 			    // initialized by ww_acquire_init() as a
> 			    // LD_WAIT_INV lock.
> 
> The following could fix this:
> 
> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> index a401a2f31a77..45ff6f7a872b 100644
> --- a/include/linux/ww_mutex.h
> +++ b/include/linux/ww_mutex.h
> @@ -156,8 +156,8 @@ static inline void ww_acquire_init(struct
> ww_acquire_ctx *ctx,
>         debug_check_no_locks_freed((void *)ctx, sizeof(*ctx));
>         lockdep_init_map(&ctx->dep_map, ww_class->acquire_name,
>                          &ww_class->acquire_key, 0);
> -       lockdep_init_map(&ctx->first_lock_dep_map, ww_class-
> >mutex_name,
> -                        &ww_class->mutex_key, 0);
> +       lockdep_init_map_wait(&ctx->first_lock_dep_map, ww_class-
> >mutex_name,
> +                             &ww_class->mutex_key, 0,
> LD_WAIT_SLEEP);
>         mutex_acquire(&ctx->dep_map, 0, 0, _RET_IP_);
>         mutex_acquire_nest(&ctx->first_lock_dep_map, 0, 0, &ctx-
> >dep_map, _RET_IP_);
>  #endif
> 
> A v3 with all these fixed would look good to me, and I can add a
> Tested-by tag to it. Thanks!

The fix here is a bit confusing. It looks like this test is crafted to
fail because we take a sleeping ww_mutex inside a spinlock. But the
ww_mutex lockdep map is already initialized as LD_WAIT_SLEEP. How come
the first_lock_dep_map locking mode LD_WAIT_INV is used in the
ww_mutex_lock()? Is that because of the lockdep hlock refcounting?

Thanks,
Thomas



> 
> Regards,
> Boqun
> 
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Waiman Long <longman@xxxxxxxxxx>
> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Cc: Maarten Lankhorst <maarten@xxxxxxxxxxxx>
> > Cc: Christian König <christian.koenig@xxxxxxx>
> > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > ---
> >  include/linux/ww_mutex.h       | 14 ++++++++++++++
> >  kernel/locking/test-ww_mutex.c |  8 +++++---
> >  2 files changed, 19 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> > index bb763085479a..a401a2f31a77 100644
> > --- a/include/linux/ww_mutex.h
> > +++ b/include/linux/ww_mutex.h
> > @@ -65,6 +65,16 @@ struct ww_acquire_ctx {
> >  #endif
> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >  	struct lockdep_map dep_map;
> > +	/**
> > +	 * @first_lock_dep_map: fake lockdep_map for first locked
> > ww_mutex.
> > +	 *
> > +	 * lockdep requires the lockdep_map for the first locked
> > ww_mutex
> > +	 * in a ww transaction to remain in memory until all
> > ww_mutexes of
> > +	 * the transaction have been unlocked. Ensure this by
> > keeping a
> > +	 * fake locked ww_mutex lockdep map between
> > ww_acquire_init() and
> > +	 * ww_acquire_fini().
> > +	 */
> > +	struct lockdep_map first_lock_dep_map;
> >  #endif
> >  #ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
> >  	unsigned int deadlock_inject_interval;
> > @@ -146,7 +156,10 @@ static inline void ww_acquire_init(struct
> > ww_acquire_ctx *ctx,
> >  	debug_check_no_locks_freed((void *)ctx, sizeof(*ctx));
> >  	lockdep_init_map(&ctx->dep_map, ww_class->acquire_name,
> >  			 &ww_class->acquire_key, 0);
> > +	lockdep_init_map(&ctx->first_lock_dep_map, ww_class-
> > >mutex_name,
> > +			 &ww_class->mutex_key, 0);
> >  	mutex_acquire(&ctx->dep_map, 0, 0, _RET_IP_);
> > +	mutex_acquire_nest(&ctx->first_lock_dep_map, 0, 0, &ctx-
> > >dep_map, _RET_IP_);
> >  #endif
> >  #ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
> >  	ctx->deadlock_inject_interval = 1;
> > @@ -185,6 +198,7 @@ static inline void ww_acquire_done(struct
> > ww_acquire_ctx *ctx)
> >  static inline void ww_acquire_fini(struct ww_acquire_ctx *ctx)
> >  {
> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > +	mutex_release(&ctx->first_lock_dep_map, _THIS_IP_);
> >  	mutex_release(&ctx->dep_map, _THIS_IP_);
> >  #endif
> >  #ifdef DEBUG_WW_MUTEXES
> > diff --git a/kernel/locking/test-ww_mutex.c b/kernel/locking/test-
> > ww_mutex.c
> > index 10a5736a21c2..5d58b2c0ef98 100644
> > --- a/kernel/locking/test-ww_mutex.c
> > +++ b/kernel/locking/test-ww_mutex.c
> > @@ -62,7 +62,8 @@ static int __test_mutex(unsigned int flags)
> >  	int ret;
> >  
> >  	ww_mutex_init(&mtx.mutex, &ww_class);
> > -	ww_acquire_init(&ctx, &ww_class);
> > +	if (flags & TEST_MTX_CTX)
> > +		ww_acquire_init(&ctx, &ww_class);
> >  
> >  	INIT_WORK_ONSTACK(&mtx.work, test_mutex_work);
> >  	init_completion(&mtx.ready);
> > @@ -90,7 +91,8 @@ static int __test_mutex(unsigned int flags)
> >  		ret = wait_for_completion_timeout(&mtx.done,
> > TIMEOUT);
> >  	}
> >  	ww_mutex_unlock(&mtx.mutex);
> > -	ww_acquire_fini(&ctx);
> > +	if (flags & TEST_MTX_CTX)
> > +		ww_acquire_fini(&ctx);
> >  
> >  	if (ret) {
> >  		pr_err("%s(flags=%x): mutual exclusion failure\n",
> > @@ -679,7 +681,7 @@ static int __init test_ww_mutex_init(void)
> >  	if (ret)
> >  		return ret;
> >  
> > -	ret = stress(2047, hweight32(STRESS_ALL)*ncpus,
> > STRESS_ALL);
> > +	ret = stress(2046, hweight32(STRESS_ALL)*ncpus,
> > STRESS_ALL);
> >  	if (ret)
> >  		return ret;
> >  
> > -- 
> > 2.46.0
> >