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Message-ID: <927c0a1b7708500a4c0ba19cbee3353d8a83f093.camel@linux.intel.com>
Date: Tue, 15 Oct 2024 13:03:41 +0200
From: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: intel-xe@...ts.freedesktop.org, Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, Waiman Long
<longman@...hat.com>, Maarten Lankhorst <maarten@...khorst.se>, Christian
König <christian.koenig@....com>,
dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] locking/ww_mutex: Adjust to lockdep nest_lock
requirements
Hi!
On Mon, 2024-10-14 at 13:23 -0700, Boqun Feng wrote:
> 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:
Let me take a look at these. Thanks for the report.
/Thomas
>
> [..] | 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@...radead.org>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Waiman Long <longman@...hat.com>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Maarten Lankhorst <maarten@...khorst.se>
> > Cc: Christian König <christian.koenig@....com>
> > Cc: dri-devel@...ts.freedesktop.org
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
> > ---
> > 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
> >
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