Re: [PATCH] sched/cache: Fix use-after-free of the mm replaced by exec

[Chen, Yu C]

Hi Hyunwoo,

On 8/30/2026 3:30 PM, Hyunwoo Kim wrote:
> When the waker cannot use the wakelist, ttwu_queue() takes the target rq
> lock and goes down into update_curr(). If the target rq belongs to another
> CPU, the task handed to account_mm_sched() is the one running on that CPU,
> not the task being woken.
>
> account_mm_sched() reads p->mm and updates mm->sc_stat. Nothing keeps that
> mm alive. The rq lock and rq->cpu_epoch_lock it holds have nothing to do
> with the lifetime of the mm.
>
> If that task happens to be in execve(), exec_mmap() points tsk->mm and
> tsk->active_mm at the new mm, and exec_mm_put_old() from setup_new_exec()
> drops the old one. free_bprm() does the same when exec fails. On the way
> from mmput() down to __mmdrop(), mm_destroy_sched() calls free_percpu() on
> sc_stat.pcpu_sched and free_mm() returns the mm_struct.
>
> Whoever already read the old pointer keeps using it. It adds to runtime in
> the freed per-cpu area and reads sc_stat in the freed mm_struct. Depending
> on the condition it also writes sc_stat.cpu. That is a use-after-free.
>
> Commit 9f23469401b0 ("sched/cache: Fix potential NULL mm pointer access")
> changed the remaining p->mm dereference to the local variable, and said the
> active_mm reference keeps the structure allocated. That holds for the other
> paths that detach an mm, since they take an mmgrab_lazy_tlb() reference.
> exec reassigns active_mm to the new mm as well, so that reference is gone.
> What is left is the mm_users reference in bprm->old_mm, and dropping it is
> the free.
>
>               CPU0                                CPU1
>
>                                        write(pipe)
>                                        try_to_wake_up()
>                                          ttwu_queue()      // takes rq0 lock
>                                            enqueue_task_fair()
>                                              update_curr()
>                                                update_se()
>                                                  account_mm_sched()
>                                                    mm = rq0->curr->mm
>                                                          // old mm
>    execve()
>      exec_mmap()                       // tsk->mm = new mm
>      setup_new_exec()
>        exec_mm_put_old()
>          mmput() -> ... -> __mmdrop()
>            mm_destroy_sched()          // free_percpu()
>            free_mm()
>                                                    read mm->sc_stat.epoch
>                                                          // use-after-free

Ah, thanks for catching this.

> ---
>   fs/exec.c             |  1 +
>   include/linux/sched.h |  4 ++++
>   kernel/events/core.c  |  2 ++
>   kernel/sched/fair.c   | 16 ++++++++++++++++
>   4 files changed, 23 insertions(+)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 745f6eb5279e6..6194c38807980 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -916,6 +916,7 @@ static void exec_mm_put_old(struct mm_struct *old_mm)
>   {
>   	setmax_mm_hiwater_rss(&current->signal->maxrss, old_mm);
>   	mm_update_next_owner(old_mm);
> +	sched_cache_exec_done();
>   	mmput(old_mm);
>   }
>   
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8b3d47a325cca..6ae31bffe049e 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2415,10 +2415,14 @@ struct sched_cache_stat {
>   	int cpu;
>   } ____cacheline_aligned_in_smp;
>   
> +void sched_cache_exec_done(void);
> +
>   #else
>   
>   struct sched_cache_stat { };
>   
> +static inline void sched_cache_exec_done(void) { }
> +
>   #endif
>   
>   #ifndef MODULE
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a6c8e38a31104..2f29cbccf03f1 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5427,6 +5427,8 @@ attach_task_ctx_data(struct task_struct *task, struct kmem_cache *ctx_cache,
>   	if (!cd)
>   		return -ENOMEM;
>   
> +	/* @old, loaded by the try_cmpxchg() below, is only stable under RCU. */
> +	guard(rcu)();

Is this change related to this UAF issue?

> +/* exec() has switched to the new mm and is about to drop the old one. */
> +void sched_cache_exec_done(void)
> +{
> +	struct rq_flags rf;
> +	struct rq *rq;
> +
> +	/*
> +	 * account_mm_sched() dereferences rq->curr->mm under this rq's lock,
> +	 * so a remote CPU can still be using the old mm. The lock cycle waits
> +	 * for it, and the store to tsk->mm cannot be reordered past the
> +	 * release, so later acquirers see the new mm.
> +	 */
> +	rq = this_rq_lock_irq(&rf);

A smart fix, learnt! It behaves like a synchronize_rcu() to protect against
the read in account_mm_sched(). Small open: since the context of invoking
account_mm_sched() is preemption-disabled, I wonder if we can simply use
synchronize_rcu() directly instead of this_rq_lock_irq() - just to avoid
contention for rq-lock in heavy system?

thanks,
Chenyu