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

[Muchun Song <muchun.song@linux.dev>]

> On Oct 28, 2026, at 07:11, T.J. Mercier <tjmercier@google.com> wrote:
> 
> On Tue, Oct 27, 2026 at 10:42 AM T.J. Mercier <tjmercier@google.com> wrote:
>> 
>> On Mon, Oct 26, 2026 at 7:42 PM Muchun Song <muchun.song@linux.dev> wrote:
>>> 
>>> 
>>> 
>>>> On Oct 24, 2026, at 06:23, T.J. Mercier <tjmercier@google.com> 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.
> 
> I was able to get a reproduction on e4dde56cd208. Fortunately this
> patch applies cleanly on e4dde56cd208, so I expect the stable
> backports to be straightforward.

Excellent job. Yeah, we need this backported.

Thanks,
Muchun

> 
> general protection fault, probably for non-canonical address
> 0xfbd59c0000000024: 0000 [#1] PREEMPT SMP KASAN NOPTI
> KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127]
> CPU: 1 PID: 37 Comm: kworker/1:1 Not tainted 6.2.0-rc4-00108-ge4dde56cd208 #1
> 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+0x147/0xef0
> Code: 49 c1 ea 03 41 80 3c 3a 00 0f 85 a5 09 00 00 48 89 f7 4c 8b 93
> 08 07 00 00 48 c1 ef 03 48 89 fa 48 bf 00 00 00 00 00 fc ff df <80> 3c
> 3a 00 0f 85 2a 07 00 00 4c 89 16 41 f6 c2 01 0f 85 72 00
> RSP: 0018:ffa00000002a75e0 EFLAGS: 00010206
> RAX: ff11000008d46701 RBX: ff11000008d46000 RCX: 0000000000000000
> RDX: 1bd5a00000000024 RSI: dead000000000122 RDI: dffffc0000000000
> RBP: ff110000bfe54000 R08: ff11000008d46708 R09: ff11000008d46710
> R10: ff11000009a5a708 R11: 0000000000000000 R12: 0000000000000006
> R13: 0000000000000004 R14: 1ff4000000054ee4 R15: 0000000000000001
> FS:  0000000000000000(0000) GS:ff110000a3480000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 0000000007d5c000 CR4: 0000000000751ee0
> PKRU: 55555554
> Call Trace:
> <TASK>
> ? reset_ctrl_pos+0x10f/0x460
> shrink_lruvec+0x19e0/0x2000
> ? kasan_set_track+0x25/0x30
> ? __kasan_slab_alloc+0x66/0x80
> ? kmem_cache_alloc+0x165/0x310
> ? __kernfs_new_node+0xd3/0x590
> ? __kernfs_create_file+0x2d/0x280
> ? sysfs_add_file_mode_ns+0x12b/0x3a0
> ? internal_create_group+0x286/0x940
> ? internal_create_groups+0x84/0x110
> ? really_probe+0x223/0x690
> ? unwind_next_frame+0xa9a/0x2040
> ? arch_stack_walk+0x7a/0x110
> ? unwind_next_frame+0xa9a/0x2040
> ? arch_stack_walk+0x7a/0x110
> ? __pfx_shrink_lruvec+0x10/0x10
> ? preempt_count_add+0x83/0x180
> ? unwind_next_frame+0xb2/0x2040
> ? unwind_next_frame+0x212/0x2040
> ? ret_from_fork+0x2c/0x50
> ? __pfx_stack_trace_consume_entry+0x10/0x10
> ? arch_stack_walk+0xa2/0x110
> ? sched_clock_local+0x132/0x210
> ? __pfx_sched_clock_local+0x10/0x10
> ? unwind_next_frame+0x212/0x2040
> ? __pfx_shrink_slab+0x10/0x10
> ? sched_clock_cpu+0x232/0x2c0
> ? __pfx_sched_clock_local+0x10/0x10
> ? __pfx_sched_clock_cpu+0x10/0x10
> ? mem_cgroup_iter+0x269/0x600
> ? vmpressure+0x30/0x3b0
> shrink_node+0xb39/0x26a0
> ? __pfx_shrink_node+0x10/0x10
> ? kasan_save_stack+0x42/0x60
> ? ring_buffer_lock_reserve+0x2fb/0xe30
> do_try_to_free_pages+0x31a/0x1880
> ? __pfx_do_try_to_free_pages+0x10/0x10
> try_to_free_mem_cgroup_pages+0x282/0x5f0
> ? try_to_free_mem_cgroup_pages+0x4/0x5f0
> ? reclaim_high.constprop.0+0x191/0x250
> ? __pfx_try_to_free_mem_cgroup_pages+0x10/0x10
> ? try_to_free_mem_cgroup_pages+0x9/0x5f0
> reclaim_high.constprop.0+0x191/0x250
> ? __pfx_reclaim_high.constprop.0+0x10/0x10
> ? __pfx_high_work_func+0x10/0x10
> process_one_work+0x641/0x1010
> worker_thread+0x424/0xec0
> ? __kthread_parkme+0x89/0x160
> ? __pfx_worker_thread+0x10/0x10
> kthread+0x26c/0x300
> ? __pfx_kthread+0x10/0x10
> ret_from_fork+0x2c/0x50
> </TASK>