Re: [PATCH v2] locking/ww_mutex: Adjust to lockdep nest_lock requirements

From: Boqun Feng
Date: Mon Oct 14 2024 - 16:23:40 EST

Hi Thomas,

On Wed, Oct 09, 2024 at 11:20:31AM +0200, Thomas Hellström wrote:
> 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.
>
> v2:
> - Lower the number of locks in the test-ww_mutex
> stress(STRESS_ALL) test to accommodate the dummy lock
> introduced in this patch without overflowing lockdep held lock
> references.
>

Have you tested your patch with lib/locking-selftests.c? It reported two
errors for me:

[..] | Wound/wait tests |
[..] ---------------------
[..] ww api failures: ok |FAILED| ok |
[..] ww contexts mixing: ok | ok |
[..] finishing ww context: ok | ok | ok | ok |
[..] locking mismatches: ok | ok | ok |
[..] EDEADLK handling: ok | ok | ok | ok | ok | ok | ok | ok | ok | ok |
[..] spinlock nest unlocked: ok |
[..] spinlock nest test: ok |
[..] -----------------------------------------------------
[..] |block | try |context|
[..] -----------------------------------------------------
[..] context: ok | ok | ok |
[..] try: ok | ok | ok |
[..] block: ok | ok | ok |
[..] spinlock: ok | ok |FAILED|

The first one is a use case issue, I think and can be fixed similar to
your changes in test-ww_mutex.c:

diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
index 6f6a5fc85b42..6750321e3e9a 100644
--- a/lib/locking-selftest.c
+++ b/lib/locking-selftest.c
@@ -1720,8 +1720,6 @@ static void ww_test_normal(void)
{
int ret;

- WWAI(&t);
-
/*
* None of the ww_mutex codepaths should be taken in the 'normal'
* mutex calls. The easiest way to verify this is by using the
@@ -1770,6 +1768,8 @@ static void ww_test_normal(void)
ww_mutex_base_unlock(&o.base);
WARN_ON(o.ctx != (void *)~0UL);

+ WWAI(&t);
+
/* nest_lock */
o.ctx = (void *)~0UL;
ww_mutex_base_lock_nest_lock(&o.base, &t);

Please confirm whether this change is intended.

The second is a case as follow:

ww_acquire_init(...);
spin_lock(...);
ww_mutex_lock(...); // this should trigger a context
// invalidation. But the mutex was
// initialized by ww_acquire_init() as a
// LD_WAIT_INV lock.

The following could fix this:

diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
index a401a2f31a77..45ff6f7a872b 100644
--- a/include/linux/ww_mutex.h
+++ b/include/linux/ww_mutex.h
@@ -156,8 +156,8 @@ 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);
+ lockdep_init_map_wait(&ctx->first_lock_dep_map, ww_class->mutex_name,
+ &ww_class->mutex_key, 0, LD_WAIT_SLEEP);
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

A v3 with all these fixed would look good to me, and I can add a
Tested-by tag to it. Thanks!

Regards,
Boqun

> 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 | 8 +++++---
> 2 files changed, 19 insertions(+), 3 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..5d58b2c0ef98 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",
> @@ -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;
>
> --
> 2.46.0
>