Commit 823a5662 authored by Thomas Hellström's avatar Thomas Hellström Committed by Peter Zijlstra
Browse files

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: default avatarThomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241009092031.6356-1-thomas.hellstrom@linux.intel.com
parent afc256e1

include/linux/ww_mutex.h

+14 −0
Original line number Diff line number Diff line
@@ -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

+5 −3
Original line number Diff line number Diff line
@@ -62,6 +62,7 @@ static int __test_mutex(unsigned int flags) 
	int ret;



	ww_mutex_init(&mtx.mutex, &ww_class);

	if (flags & TEST_MTX_CTX)

		ww_acquire_init(&ctx, &ww_class);



	INIT_WORK_ONSTACK(&mtx.work, test_mutex_work);
@@ -90,6 +91,7 @@ static int __test_mutex(unsigned int flags) 
		ret = wait_for_completion_timeout(&mtx.done, TIMEOUT);

	}

	ww_mutex_unlock(&mtx.mutex);

	if (flags & TEST_MTX_CTX)

		ww_acquire_fini(&ctx);



	if (ret) {
@@ -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;