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

[Maarten Lankhorst]

Hey,

I'm pretty sure I've seen this use-after-free in the wild, just never found the root cause since it's so unlikely to trigger on demand.

Acked-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>

Den 2024-10-02 kl. 14:56, skrev Thomas Hellström:
> 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.
> 
> 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 |  6 ++++--
>  2 files changed, 18 insertions(+), 2 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..4c2b8b567de5 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",