Re: [PATCH v2] mm: memcontrol: fix NULL pointer deref from high_work_func() on dying memcg

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Oct 28, 2026 at 11:35:37AM -0700, T.J. Mercier wrote:
> When memory.high is exceeded in non-task context, try_charge_memcg()
> schedules memcg->high_work without taking a reference on the memcg, and
> high_work_func() invokes reclaim_high() without pinning the memcg.
> 
> cancel_work_sync(&memcg->high_work) only runs at the end of teardown in
> mem_cgroup_css_free(), so high_work_func() can start or still be running
> reclaim_high() after mem_cgroup_css_released() or shrink_many() has
> called lru_gen_release_memcg() and unhashed lruvec->lrugen.list. If
> reclaim tries to rotate the memcg, lru_gen_rotate_memcg() dereferences a
> NULL pprev pointer.
> 
> With obj_cgroup reparenting, introduced in commit bf4f059954dc ("mm:
> memcg/slab: obj_cgroup API"), objects charged to a child memcg are
> reparented to its parent in mem_cgroup_css_offline() instead of holding
> a reference on the child until uncharged. Subsequent uncharges of those
> objects go to the parent, which can leave the child's memory use
> permanently above its high limit, so any later charge on the offlined
> memcg in non-task context schedules high_work.
> 
> This problem was first exposed when commit e4dde56cd208 ("mm: multi-gen
> LRU: per-node lru_gen_folio lists") introduced the memcg LRU, and was
> later widened from kernel memory to LRU folios when
> commit f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying
> memory cgroup for LRU folios") converted LRU folios to use obj_cgroup
> reparenting.
> 
> Fix this by acquiring a reference with css_tryget_online() in
> high_work_func() before calling reclaim_high(), and by resetting
> memory.high and swap.high to PAGE_COUNTER_MAX in
> mem_cgroup_css_offline() so that charges via references on an offlined
> memcg do not trigger high_work on the dead child or skip checking live
> ancestors in try_charge_memcg().
> 
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> CPU: 1 UID: 0 PID: 27 Comm: kworker/1:0 Tainted: G    B   W           7.3.0-rc5-g2bba28e98578 #284 PREEMPT(full)
> Tainted: [B]=BAD_PAGE, [W]=WARN <- Taint from lru_gen_rotate_memcg() VM_WARN_ON_ONCE
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
> Workqueue: events high_work_func
> RIP: 0010:lru_gen_rotate_memcg+0x1a3/0xe90
> Call Trace:
>  <TASK>
>  ? read_ctrl_pos+0x22f/0x3f0
>  shrink_lruvec+0x16cb/0x1b80
>  ? ret_from_fork_asm+0x1a/0x30
>  ? lapic_next_event+0x15/0x20
>  ? clockevents_program_event+0x2bb/0x6f0
>  ? lapic_next_event+0x15/0x20
>  ? __pfx_shrink_lruvec+0x10/0x10
>  ? __pfx_clockevents_program_event+0x10/0x10
>  ? _raw_spin_lock+0x84/0xe0
>  ? __pfx__raw_spin_lock+0x10/0x10
>  ? kasan_save_stack+0x30/0x50
>  ? psi_group_change+0x381/0x740
>  ? __pfx_sched_clock_cpu+0x10/0x10
>  ? save_fpregs_to_fpstate+0xc4/0x2c0
>  ? finish_task_switch.isra.0+0x1de/0xac0
>  ? __schedule+0xf62/0x42c0
>  ? __pfx___schedule+0x10/0x10
>  ? __pfx___irq_work_queue_local+0x10/0x10
>  ? shrink_node+0x626/0x31e0
>  shrink_node+0x626/0x31e0
>  ? ring_buffer_unlock_commit+0x11d/0x5c0
>  ? kswapd_clear_hopeless+0xa0/0x150
>  ? __pfx_shrink_node+0x10/0x10
>  ? 0xffffffffc0000095
>  do_try_to_free_pages+0x255/0x13b0
>  ? ring_buffer_unlock_commit+0x11d/0x5c0
>  ? reclaim_high+0x10a/0x1b0
>  try_to_free_mem_cgroup_pages+0x1f9/0x5a0
>  ? __pfx_try_to_free_mem_cgroup_pages+0x10/0x10
>  ? irqentry_exit+0xc6/0x670
>  reclaim_high+0x10a/0x1b0
>  ? __pfx_reclaim_high+0x10/0x10
>  ? process_one_work+0x5ff/0x1030
>  ? __pfx_high_work_func+0x10/0x10
>  process_one_work+0x6e2/0x1030
>  ? __pfx___schedule+0x10/0x10
>  ? __pfx_process_one_work+0x10/0x10
>  worker_thread+0x422/0xbb0
>  ? __pfx_worker_thread+0x10/0x10
>  kthread+0x2cd/0x3a0
>  ? recalc_sigpending+0x15c/0x200
>  ? __pfx_kthread+0x10/0x10
>  ret_from_fork+0x368/0x590
>  ? __pfx_ret_from_fork+0x10/0x10
>  ? __switch_to+0x588/0xf50
>  ? __pfx_kthread+0x10/0x10
>  ret_from_fork_asm+0x1a/0x30
>  </TASK>
> 
> Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
> Cc: stable@vger.kernel.org
> Signed-off-by: T.J. Mercier <tjmercier@google.com>
> ---
> Changes since V1 (https://lore.kernel.org/all/20261023222313.3979455-1-tjmercier@google.com/)
> - Switch to Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
>   from Fixes: f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios")
>   per Muchun Song.
> - Reorder and update commit message content to describe reparenting
>   separately from the commits that exposed and then widened the problem.
> - Trim the backtrace a bit.
> 
>  mm/memcontrol.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a05d9a8baa1b..84f4a8fd4d16 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2457,7 +2457,10 @@ static void high_work_func(struct work_struct *work)
>  	struct mem_cgroup *memcg;
>  
>  	memcg = container_of(work, struct mem_cgroup, high_work);
> +	if (!css_tryget_online(&memcg->css))
> +		return;
>  	reclaim_high(memcg, MEMCG_CHARGE_BATCH, GFP_KERNEL);

The patch is correct but I think there are two very specific subtleties which
the commit message is not very explicit about.

First, I was wondering shouldn't we take reference before schedule_work() and
why this is safe. This is safe due to cancel_work_sync() in the
mem_cgroup_css_free() which will not free memcg until this there is no high
worker.

Second, cgroup core calls css_free() after percpu_ref_exit(). So, is it safe to
call css_tryget_online() on such css->refcnt. Yes css_tryget_online() is safe
but css_tryget() is not safe.