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

diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
index bb763085479affdb75dfa16886c7c89cba61b87b..a401a2f31a775197a100b7168431337fe60d3a48 100644 (file)
--- 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 10a5736a21c222df22fd11d96440c7ea0002cdd6..5d58b2c0ef98bca244345f708a77690b5c0e17e9 100644 (file)
--- 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;