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 <[email protected]> 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 <[email protected]> > Cc: Ingo Molnar <[email protected]> > Cc: Will Deacon <[email protected]> > Cc: Waiman Long <[email protected]> > Cc: Boqun Feng <[email protected]> > Cc: Maarten Lankhorst <[email protected]> > Cc: Christian König <[email protected]> > Cc: [email protected] > Cc: [email protected] > Signed-off-by: Thomas Hellström <[email protected]> > --- > 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",
