From 653186d8bee3ff669349cd37885a81cf01af6edd Mon Sep 17 00:00:00 2001 From: Keenan Dong Date: Wed, 8 Apr 2026 16:46:00 +0800 Subject: [PATCH 1/2] rtmutex: Use waiter::task instead of current in remove_waiter() cve CVE-2026-43499 commit-author Keenan Dong commit 3bfdc63936dd4773109b7b8c280c0f3b5ae7d349 upstream-diff | This tree builds with -std=gnu89, which rejects upstream's scoped_guard() form (cleanup.h expands it to a for-loop that declares a variable in the initializer), so an explicit raw_spin_lock()/raw_spin_unlock() pair on waiter_task->pi_lock is used instead. The locking is identical; the comment, the waiter_task local, and the rt_mutex_adjust_prio_chain() argument match upstream. remove_waiter() is used by the slowlock paths, but it is also used for proxy-lock rollback in rt_mutex_start_proxy_lock() when invoked from futex_requeue(). In the latter case waiter::task is not current, but remove_waiter() operates on current for the dequeue operation. That results in several problems: 1) the rbtree dequeue happens without waiter::task::pi_lock being held 2) the waiter task's pi_blocked_on state is not cleared, which leaves a dangling pointer primed for UAF around. 3) rt_mutex_adjust_prio_chain() operates on the wrong top priority waiter task Use waiter::task instead of current in all related operations in remove_waiter() to cure those problems. [ tglx: Fixup rt_mutex_adjust_prio_chain(), add a comment and amend the changelog ] Fixes: 8161239a8bcc ("rtmutex: Simplify PI algorithm and make highest prio task get lock") Reported-by: Yuan Tan Reported-by: Yifan Wu Reported-by: Juefei Pu Reported-by: Xin Liu Signed-off-by: Keenan Dong Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org (cherry picked from commit 3bfdc63936dd4773109b7b8c280c0f3b5ae7d349) Signed-off-by: Sultan Alsawaf --- kernel/locking/rtmutex.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 95db3bb1a2c5e..cb855e31e5b52 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1501,20 +1501,23 @@ static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock, * * Must be called with lock->wait_lock held and interrupts disabled. It must * have just failed to try_to_take_rt_mutex(). + * + * When invoked from rt_mutex_start_proxy_lock() waiter::task != current ! */ static void __sched remove_waiter(struct rt_mutex_base *lock, struct rt_mutex_waiter *waiter) { bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock)); struct task_struct *owner = rt_mutex_owner(lock); + struct task_struct *waiter_task = waiter->task; struct rt_mutex_base *next_lock; lockdep_assert_held(&lock->wait_lock); - raw_spin_lock(¤t->pi_lock); + raw_spin_lock(&waiter_task->pi_lock); rt_mutex_dequeue(lock, waiter); - current->pi_blocked_on = NULL; - raw_spin_unlock(¤t->pi_lock); + waiter_task->pi_blocked_on = NULL; + raw_spin_unlock(&waiter_task->pi_lock); /* * Only update priority if the waiter was the highest priority @@ -1550,7 +1553,7 @@ static void __sched remove_waiter(struct rt_mutex_base *lock, raw_spin_unlock_irq(&lock->wait_lock); rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock, - next_lock, NULL, current); + next_lock, NULL, waiter_task); raw_spin_lock_irq(&lock->wait_lock); } From 23804269a978c98b122eeff9a27b97be226be5e6 Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Thu, 7 May 2026 04:29:13 -0700 Subject: [PATCH 2/2] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued cve CVE-2026-53163 commit-author Davidlohr Bueso commit 40a25d59e85b3c8709ac2424d44f65610467871e upstream-diff | Functionally identical. In rtmutex_api.c the remove_waiter() call is gated on ret < 0 (skipped on the ret == 1 acquired path); this tree's __rt_mutex_start_proxy_lock() has no @wake_q and no preempt_disable() around the wait_lock release (later reworks), so only the context was adjusted. In rtmutex.c the !waiter_task guard is verbatim, atop the explicit-lock form from the previous commit. syzbot triggered the following splat in remove_waiter() via FUTEX_CMP_REQUEUE_PI: KASAN: null-ptr-deref in range [0x0000000000000a88-0x0000000000000a8f] class_raw_spinlock_constructor remove_waiter+0x159/0x1200 kernel/locking/rtmutex.c:1561 rt_mutex_start_proxy_lock+0x103/0x120 futex_requeue+0x10e4/0x20d0 __x64_sys_futex+0x34f/0x4d0 task_blocks_on_rt_mutex() does not arm the waiter upon deadlock detection, leaving waiter->task nil, where 3bfdc63936dd ("rtmutex: Use waiter::task instead of current in remove_waiter()") made this fatal. Furthermore, rt_mutex_start_proxy_lock() should not be calling into remove_waiter() upon a successfully grabbing the rtmutex. 1a1fb985f2e2 ("futex: Handle early deadlock return correctly"), moved the remove_waiter() out of __rt_mutex_start_proxy_lock() (where 'ret' was only ever 0 or < 0) into the wrapper. Tighten this check to account for try_to_take_rt_mutex(). Fixes: 3bfdc63936dd ("rtmutex: Use waiter::task instead of current in remove_waiter()") Reported-by: syzbot+78147abe6c524f183ee9@syzkaller.appspotmail.com Signed-off-by: Davidlohr Bueso Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Closes: https://lore.kernel.org/all/69f114ac.050a0220.ac8b.0003.GAE@google.com/ Link: https://patch.msgid.link/20260507112913.1019537-1-dave@stgolabs.net (cherry picked from commit 40a25d59e85b3c8709ac2424d44f65610467871e) Signed-off-by: Sultan Alsawaf --- kernel/locking/rtmutex.c | 3 +++ kernel/locking/rtmutex_api.c | 2 +- 2 files changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index cb855e31e5b52..469d43de1d387 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1514,6 +1514,9 @@ static void __sched remove_waiter(struct rt_mutex_base *lock, lockdep_assert_held(&lock->wait_lock); + if (!waiter_task) /* never enqueued */ + return; + raw_spin_lock(&waiter_task->pi_lock); rt_mutex_dequeue(lock, waiter); waiter_task->pi_blocked_on = NULL; diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c index 56d1938cb52a1..c4e191340c59b 100644 --- a/kernel/locking/rtmutex_api.c +++ b/kernel/locking/rtmutex_api.c @@ -322,7 +322,7 @@ int __sched rt_mutex_start_proxy_lock(struct rt_mutex_base *lock, raw_spin_lock_irq(&lock->wait_lock); ret = __rt_mutex_start_proxy_lock(lock, waiter, task); - if (unlikely(ret)) + if (unlikely(ret < 0)) remove_waiter(lock, waiter); raw_spin_unlock_irq(&lock->wait_lock);