[PATCH v3] locking/ww_mutex: Adjust to lockdep nest_lock requirements

[Boqun Feng]

On Thu, Oct 17, 2024 at 05:10:07PM +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.
> 
> v3:
> - Adjust the ww_test_normal locking-api selftest to avoid
>   recursive locking (Boqun Feng)
> - Initialize the dummy lock map with LD_WAIT_SLEEP to agree with
>   how the corresponding ww_mutex lockmaps are initialized
>   (Boqun Feng)
> 

Thanks!

> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Ingo Molnar <mingo at redhat.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Waiman Long <longman at redhat.com>
> Cc: Boqun Feng <boqun.feng at gmail.com>
> Cc: Maarten Lankhorst <maarten at lankhorst.se>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: dri-devel at lists.freedesktop.org
> Cc: linux-kernel at vger.kernel.org

Feel free to use these tags if you need.

Co-developed-by: Boqun Feng <boqun.feng at gmail.com>
Signed-off-by: Boqun Feng <boqun.feng at gmail.com>

> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Acked-by: maarten.lankhorst at linux.intel.com #v1

Tested-by: Boqun Feng <boqun.feng at gmail.com>

Peter, since the v2 of this is actually picked in tip/locking/core, I
assume you are going to drop that pick this v3? Let me know how you want
to proceed, since I have a PR based on tip/locking/core.

Regards,
Boqun

> ---
>  include/linux/ww_mutex.h       | 14 ++++++++++++++
>  kernel/locking/test-ww_mutex.c |  8 +++++---
>  lib/locking-selftest.c         |  4 ++--
>  3 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
> index bb763085479a..45ff6f7a872b 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_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
>  #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;
>  
> 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);
> -- 
> 2.46.0
>