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 06:12:45PM -0400, Johannes Weiner wrote:
> 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>
> 
> Hrm, yes I suppose the work itself keeps the memcg allocated, but not
> from going deep into the destruction sequence. I was briefly wondering
> whether mglru shouldn't move its callback from css_released into
> css_free, but I think your fix is more robust after all: don't send a
> barely allocated memcg down the reclaim stack.
> 
> The _online variant of the tryget seems somewhat unnecessary. There is
> no real harm in reclaiming an offline group - and it was the standard
> way, before reparenting, to garbage collect an offline group with
> lingering pages. With the updated Fixes, this would look odd in
> backports to kernels without reparenting. The PAGE_COUNTER_MAX reset
> would also cause reclaim_high to bail.
> 
> So I wonder if css_tryget() would be simpler and just as correct. But
> no strong feelings either way:

css_tryget() is not safe after percpu_ref_exit() which can happen here. Only
css_tryget_online() is safe.