locking/ww_mutex: Adjust to lockdep nest_lock requirements

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.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241009092031.6356-1-thomas.hellstrom@linux.intel.com

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

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;