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

[T.J. Mercier]

On Mon, Oct 26, 2026 at 7:42 PM Muchun Song <muchun.song@xxxxxxxxx> wrote:
>
>
>
> > On Oct 24, 2026, at 06:23, T.J. Mercier <tjmercier@xxxxxxxxxx> wrote:
> >
> > When memory.high is exceeded in non-task context, try_charge_memcg()
> > schedules memcg->high_work without taking a CSS reference on the memcg,
> > and high_work_func() invokes reclaim_high() without pinning the memcg.
> >
> > Before commit f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of
> > dying memory cgroup for LRU folios"), memory usage above memory.high
> > kept the memcg's refcount non-zero. With reparenting, folios and objcgs
> > charged to a child memcg are reparented to its parent in
> > mem_cgroup_css_offline(), leaving the offlined child's memory usage
> > permanently above its memory.high even after its last reference is
> > dropped.
> >
> > 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. This
> > causes lru_gen_rotate_memcg() to dereference a NULL pprev pointer.
> >
> > 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) <- Taint from lru_gen_rotate_memcg() VM_WARN_ON_ONCE
> > Tainted: [B]=BAD_PAGE, [W]=WARN
> > 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
> > Code: fd 7f 09 45 84 ed 0f 85 64 0c 00 00 48 89 cf c6 83 6a 07 00 00 00 48 c1 ef 03 88 83 69 07 00 00 48 b8 00 00 00 00 00 fc ff df <80> 3c 07 00 0f 85 ec 0b 00 00 48 89 31 40 f6 c6 01 0f 85 4e 00
> > RSP: 0018:ffa00000001ef4c8 EFLAGS: 00010046
> > RAX: dffffc0000000000 RBX: ff1100000779e440 RCX: 0000000000000000
> > RDX: 0000000000000001 RSI: ff1100000779b9b0 RDI: 0000000000000000
> > RBP: ff110000bffd8e40 R08: 0000000000000000 R09: ff1100000779ebb8
> > R10: ff1100000779ebb0 R11: 0000000000000246 R12: 0000000000000000
> > R13: 0000000000000000 R14: ffa00000001efb30 R15: 0000000000000002
> > FS:  0000000000000000(0000) GS:ff1100011c0e7000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 000000000251ac88 CR3: 0000000009d6c000 CR4: 0000000000751ef0
> > PKRU: 55555554
> > 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: f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios")
>
> Hi T.J.,
>
> Thanks for fixing this.
>
> I have one question about the Fixes tag.
>
> The changelog says that, before f1cf8d2f36dc, memory usage above
> memory.high kept the memcg reference count non-zero. This is true for
> LRU folios: before that commit, each charged LRU folio held a CSS
> reference until it was uncharged.
>
> However, I wonder whether objcg-based kmem already provided a reachable
> path before f1cf8d2f36dc.
>
> In particular, BPF alloc_bulk() mostly runs from irq_work. It obtains a
> temporary memcg reference from a persistent objcg, uses set_active_memcg()
> while performing __GFP_ACCOUNT allocations, and drops the temporary
> reference after the allocation. The charge can therefore queue
> memcg->high_work in non-task context without transferring a CSS
> reference to the work item.
>
> If this races with memcg offlining, objcg reparenting can redirect the
> objcg to the parent while the old child's page-counter usage remains
> above memory.high. Once the temporary CSS reference is dropped,
> mem_cgroup_css_released() can call lru_gen_release_memcg() before the
> queued high_work runs. The worker can then enter reclaim_high() and
> rotate an already removed lrugen.list.
>
> This path appears to have all the required pieces after:
>
>   e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
>
> Commit 6867c7a33206 later changed the release operation to
> hlist_nulls_del_init_rcu(), which explains the exact NULL pprev seen in
> the reported crash. Before that, the same invalid second deletion would
> instead encounter LIST_POISON2.
>
> So f1cf8d2f36dc clearly broadens the problem to ordinary LRU folios, but
> I am not very sure it is the first commit after which the underlying race is
> reachable.
>
> Is there an additional lifetime guarantee that prevents the pre-f1cf
> objcg/kmem path above? If no, would the following be more appropriate?
>
> Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
>
> Thanks,
> Muchun

Hi Muchun,

Everything you said sounds correct to me. I don't think there's
anything preventing kmem from keeping the memcg above high while the
refcount hits 0, and hitting the same problem in
lru_gen_rotate_memcg() from lru_gen_shrink_lruvec().

So using Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node
lru_gen_folio lists") sounds good to me.

Thanks,
T.J.