Re: [PATCH v3] mm: memcontrol: fix NULL pointer deref and UAF from high_work_func() on dying memcg

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

On Thu, Oct 29, 2026 at 01:18:58PM -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.
> 
> 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 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.
> 
> 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. This issue is specific to MGLRU and was introduced
> with the memcg LRU in commit e4dde56cd208 ("mm: multi-gen LRU: per-node
> lru_gen_folio lists").
> 
> With either traditional LRU or MGLRU, css_free_rwork_fn() calls
> css_rstat_exit() which frees per-CPU rstat data before calling
> mem_cgroup_css_free(). If reclaim_high() is still running, rstat updates
> via mod_lruvec_state() -> __css_rstat_updated() cause a use-after-free
> on the per-CPU rstat data. This was introduced when commit 5da3bfa029d6
> ("cgroup: use separate rstat trees for each subsystem") moved
> css_rstat_exit() ahead of ss->css_free().
> 
> Fix both problems 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 and skip checking live
> ancestors in try_charge_memcg().
> 
> Acquiring the reference in high_work_func() rather than before
> scheduling high_work is safe because mem_cgroup_css_free() calls
> irq_work_sync() and cancel_work_sync() which cancels pending work and
> waits for an already running high_work_func() to complete before
> mem_cgroup_free() frees the memcg. css_tryget_online() uses
> percpu_ref_tryget_live() and is safe to call after percpu_ref_exit()
> before the cgroup is freed unlike css_tryget().
> 
> 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")
> Fixes: 5da3bfa029d6 ("cgroup: use separate rstat trees for each subsystem")
> Cc: stable@vger.kernel.org
> Signed-off-by: T.J. Mercier <tjmercier@google.com>
> Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Muchun Song <muchun.song@linux.dev>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>