On 2026/1/28 12:42, Waiman Long wrote:
> The current cpuset partition code is able to dynamically update
> the sched domains of a running system and the corresponding
> HK_TYPE_DOMAIN housekeeping cpumask to perform what is essentally the
> "isolcpus=domain,..." boot command line feature at run time.
>
> The housekeeping cpumask update requires flushing a number of different
> workqueues which may not be safe with cpus_read_lock() held as the
> workqueue flushing code may acquire cpus_read_lock() or acquiring locks
> which have locking dependency with cpus_read_lock() down the chain. Below
> is an example of such circular locking problem.
>
> ======================================================
> WARNING: possible circular locking dependency detected
> 6.18.0-test+ #2 Tainted: G S
> ------------------------------------------------------
> test_cpuset_prs/10971 is trying to acquire lock:
> ffff888112ba4958 ((wq_completion)sync_wq){+.+.}-{0:0}, at:
> touch_wq_lockdep_map+0x7a/0x180
>
> but task is already holding lock:
> ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at:
> cpuset_partition_write+0x85/0x130
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
> -> #4 (cpuset_mutex){+.+.}-{4:4}:
> -> #3 (cpu_hotplug_lock){++++}-{0:0}:
> -> #2 (rtnl_mutex){+.+.}-{4:4}:
> -> #1 ((work_completion)(&arg.work)){+.+.}-{0:0}:
> -> #0 ((wq_completion)sync_wq){+.+.}-{0:0}:
>
> Chain exists of:
> (wq_completion)sync_wq --> cpu_hotplug_lock --> cpuset_mutex
>
> 5 locks held by test_cpuset_prs/10971:
> #0: ffff88816810e440 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0xf9/0x1d0
> #1: ffff8891ab620890 (&of->mutex#2){+.+.}-{4:4}, at:
> kernfs_fop_write_iter+0x260/0x5f0
> #2: ffff8890a78b83e8 (kn->active#187){.+.+}-{0:0}, at:
> kernfs_fop_write_iter+0x2b6/0x5f0
> #3: ffffffffadf32900 (cpu_hotplug_lock){++++}-{0:0}, at:
> cpuset_partition_write+0x77/0x130
> #4: ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at:
> cpuset_partition_write+0x85/0x130
>
> Call Trace:
> <TASK>
> :
> touch_wq_lockdep_map+0x93/0x180
> __flush_workqueue+0x111/0x10b0
> housekeeping_update+0x12d/0x2d0
> update_parent_effective_cpumask+0x595/0x2440
> update_prstate+0x89d/0xce0
> cpuset_partition_write+0xc5/0x130
> cgroup_file_write+0x1a5/0x680
> kernfs_fop_write_iter+0x3df/0x5f0
> vfs_write+0x525/0xfd0
> ksys_write+0xf9/0x1d0
> do_syscall_64+0x95/0x520
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> To avoid such a circular locking dependency problem, we have to
> call housekeeping_update() without holding the cpus_read_lock()
> and cpuset_mutex. One way to do that is to introduce a new top level
> isolcpus_update_mutex which will be acquired first if the set of isolated
> CPUs may have to be updated. This new isolcpus_update_mutex will provide
> the need mutual exclusion without the need to hold cpus_read_lock().
>
> As cpus_read_lock() is now no longer held when
> tmigr_isolated_exclude_cpumask() is called, it needs to acquire it
> directly.
>
> The lockdep_is_cpuset_held() is also updated to check the new
> isolcpus_update_mutex.
>
I worry about the issue:
CPU1 CPU2
rmdir
css->ss->css_killed(css);
cpuset_css_killed
__update_isolation_cpumasks
cpuset_full_unlock
css->flags |= CSS_DYING;
css_clear_dir(css);
...
// offline and free do not
// get isolcpus_update_mutex
cpuset_css_offline
cpuset_css_free
cpuset_full_lock
...
// UAF?
> Signed-off-by: Waiman Long <[email protected]>
> ---
> kernel/cgroup/cpuset.c | 79 ++++++++++++++++++++++++-----------
> kernel/sched/isolation.c | 4 +-
> kernel/time/timer_migration.c | 3 +-
> 3 files changed, 57 insertions(+), 29 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 98c7cb732206..96390ceb5122 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -78,7 +78,7 @@ static cpumask_var_t subpartitions_cpus;
> static cpumask_var_t isolated_cpus;
>
> /*
> - * isolated_cpus updating flag (protected by cpuset_mutex)
> + * isolated_cpus updating flag (protected by isolcpus_update_mutex)
> * Set if isolated_cpus is going to be updated in the current
> * cpuset_mutex crtical section.
> */
> @@ -223,29 +223,46 @@ struct cpuset top_cpuset = {
> };
>
> /*
> - * There are two global locks guarding cpuset structures - cpuset_mutex and
> - * callback_lock. The cpuset code uses only cpuset_mutex. Other kernel
> - * subsystems can use cpuset_lock()/cpuset_unlock() to prevent change to
> cpuset
> - * structures. Note that cpuset_mutex needs to be a mutex as it is used in
> - * paths that rely on priority inheritance (e.g. scheduler - on RT) for
> - * correctness.
> + * CPUSET Locking Convention
> + * -------------------------
> *
> - * A task must hold both locks to modify cpusets. If a task holds
> - * cpuset_mutex, it blocks others, ensuring that it is the only task able to
> - * also acquire callback_lock and be able to modify cpusets. It can perform
> - * various checks on the cpuset structure first, knowing nothing will change.
> - * It can also allocate memory while just holding cpuset_mutex. While it is
> - * performing these checks, various callback routines can briefly acquire
> - * callback_lock to query cpusets. Once it is ready to make the changes, it
> - * takes callback_lock, blocking everyone else.
> + * Below are the three global locks guarding cpuset structures in lock
> + * acquisition order:
> + * - isolcpus_update_mutex (optional)
> + * - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock)
> + * - cpuset_mutex
> + * - callback_lock (raw spinlock)
> *
> - * Calls to the kernel memory allocator can not be made while holding
> - * callback_lock, as that would risk double tripping on callback_lock
> - * from one of the callbacks into the cpuset code from within
> - * __alloc_pages().
> + * The first isolcpus_update_mutex should only be held if the existing set of
> + * isolated CPUs (in isolated partition) or any of the partition states may
> be
> + * changed when some cpuset control files are being written into. Otherwise,
> + * it can be skipped. Holding isolcpus_update_mutex/cpus_read_lock or
> + * cpus_write_lock will ensure mutual exclusion of isolated_cpus update.
> *
> - * If a task is only holding callback_lock, then it has read-only
> - * access to cpusets.
> + * As cpuset will now indirectly flush a number of different workqueues in
> + * housekeeping_update() when the set of isolated CPUs is going to be
> changed,
> + * it may not be safe from the circular locking perspective to hold the
> + * cpus_read_lock. So cpuset_full_lock() will be released before calling
> + * housekeeping_update() and re-acquired afterward.
> + *
> + * A task must hold all the remaining three locks to modify externally
> visible
> + * or used fields of cpusets, though some of the internally used cpuset
> fields
> + * can be modified by holding cpu_hotplug_lock and cpuset_mutex only. If only
> + * reliable read access of the externally used fields are needed, a task can
> + * hold either cpuset_mutex or callback_lock.
> + *
> + * If a task holds cpu_hotplug_lock and cpuset_mutex, it blocks others,
> + * ensuring that it is the only task able to also acquire callback_lock and
> + * be able to modify cpusets. It can perform various checks on the cpuset
> + * structure first, knowing nothing will change. It can also allocate memory
> + * without holding callback_lock. While it is performing these checks,
> various
> + * callback routines can briefly acquire callback_lock to query cpusets.
> Once
> + * it is ready to make the changes, it takes callback_lock, blocking everyone
> + * else.
> + *
> + * Calls to the kernel memory allocator cannot be made while holding
> + * callback_lock which is a spinlock, as the memory allocator may sleep or
> + * call back into cpuset code and acquire callback_lock.
> *
> * Now, the task_struct fields mems_allowed and mempolicy may be changed
> * by other task, we use alloc_lock in the task_struct fields to protect
> @@ -256,6 +273,7 @@ struct cpuset top_cpuset = {
> * cpumasks and nodemasks.
> */
>
> +static DEFINE_MUTEX(isolcpus_update_mutex);
> static DEFINE_MUTEX(cpuset_mutex);
>
> /**
> @@ -302,7 +320,7 @@ void cpuset_full_unlock(void)
> #ifdef CONFIG_LOCKDEP
> bool lockdep_is_cpuset_held(void)
> {
> - return lockdep_is_held(&cpuset_mutex);
> + return lockdep_is_held(&isolcpus_update_mutex);
> }
> #endif
>
> @@ -1294,9 +1312,8 @@ static bool prstate_housekeeping_conflict(int prstate,
> struct cpumask *new_cpus)
> static void __update_isolation_cpumasks(bool twork);
> static void isolation_task_work_fn(struct callback_head *cb)
> {
> - cpuset_full_lock();
> + guard(mutex)(&isolcpus_update_mutex);
> __update_isolation_cpumasks(true);
> - cpuset_full_lock();
> }
>
> /*
> @@ -1338,8 +1355,18 @@ static void __update_isolation_cpumasks(bool twork)
> return;
> }
>
> + lockdep_assert_held(&isolcpus_update_mutex);
> + /*
> + * Release cpus_read_lock & cpuset_mutex before calling
> + * housekeeping_update() and re-acquiring them afterward if not
> + * calling from task_work.
> + */
> + if (!twork)
> + cpuset_full_unlock();
> ret = housekeeping_update(isolated_cpus);
> WARN_ON_ONCE(ret < 0);
> + if (!twork)
> + cpuset_full_lock();
>
> isolated_cpus_updating = false;
> }
> @@ -3196,6 +3223,7 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file
> *of,
> return -EACCES;
>
> buf = strstrip(buf);
> + mutex_lock(&isolcpus_update_mutex);
> cpuset_full_lock();
> if (!is_cpuset_online(cs))
> goto out_unlock;
> @@ -3226,6 +3254,7 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file
> *of,
> rebuild_sched_domains_locked();
> out_unlock:
> cpuset_full_unlock();
> + mutex_unlock(&isolcpus_update_mutex);
> if (of_cft(of)->private == FILE_MEMLIST)
> schedule_flush_migrate_mm();
> return retval ?: nbytes;
> @@ -3329,6 +3358,7 @@ static ssize_t cpuset_partition_write(struct
> kernfs_open_file *of, char *buf,
> else
> return -EINVAL;
>
> + guard(mutex)(&isolcpus_update_mutex);
> cpuset_full_lock();
> if (is_cpuset_online(cs))
> retval = update_prstate(cs, val);
> @@ -3502,6 +3532,7 @@ static void cpuset_css_killed(struct
> cgroup_subsys_state *css)
> {
> struct cpuset *cs = css_cs(css);
>
> + guard(mutex)(&isolcpus_update_mutex);
> cpuset_full_lock();
> /* Reset valid partition back to member */
> if (is_partition_valid(cs))
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 3b725d39c06e..ef152d401fe2 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -123,8 +123,6 @@ int housekeeping_update(struct cpumask *isol_mask)
> struct cpumask *trial, *old = NULL;
> int err;
>
> - lockdep_assert_cpus_held();
> -
> trial = kmalloc(cpumask_size(), GFP_KERNEL);
> if (!trial)
> return -ENOMEM;
> @@ -136,7 +134,7 @@ int housekeeping_update(struct cpumask *isol_mask)
> }
>
> if (!housekeeping.flags)
> - static_branch_enable_cpuslocked(&housekeeping_overridden);
> + static_branch_enable(&housekeeping_overridden);
>
> if (housekeeping.flags & HK_FLAG_DOMAIN)
> old = housekeeping_cpumask_dereference(HK_TYPE_DOMAIN);
> diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
> index 6da9cd562b20..244a8d025e78 100644
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -1559,8 +1559,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask
> *exclude_cpumask)
> cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
> int cpu;
>
> - lockdep_assert_cpus_held();
> -
> if (!works)
> return -ENOMEM;
> if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> @@ -1570,6 +1568,7 @@ int tmigr_isolated_exclude_cpumask(struct cpumask
> *exclude_cpumask)
> * First set previously isolated CPUs as available (unisolate).
> * This cpumask contains only CPUs that switched to available now.
> */
> + guard(cpus_read_lock)();
> cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
> cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
>
--
Best regards,
Ridong
