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

[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.

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: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: intel-xe@xxxxxxxxxxxxxxxxxxxxx
Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
 include/linux/ww_mutex.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

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
-- 
2.41.0