From dd94d8c7660a60dbfcd425e0d10120de6d02a7bd Mon Sep 17 00:00:00 2001 From: "T.J. Mercier" Date: Thu, 29 Oct 2026 13:18:58 -0700 Subject: [PATCH] mm: memcontrol: fix NULL pointer deref and UAF from high_work_func() on dying memcg 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: ? 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 Link: https://lore.kernel.org/20261029201858.2932905-1-tjmercier@google.com Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists") Fixes: 5da3bfa029d6 ("cgroup: use separate rstat trees for each subsystem") Signed-off-by: T.J. Mercier Signed-off-by: Andrew Morton Reviewed-by: Johannes Weiner Acked-by: Muchun Song Acked-by: Shakeel Butt Acked-by: Michal Hocko Cc: Michal Hocko Cc: Roman Gushchin Cc: Qi Zheng Cc: JP Kobryn Cc: --- 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); + css_put(&memcg->css); } static void high_irq_work_func(struct irq_work *work) @@ -4463,6 +4466,8 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css) page_counter_set_min(&memcg->memory, 0); page_counter_set_low(&memcg->memory, 0); + page_counter_set_high(&memcg->memory, PAGE_COUNTER_MAX); + page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX); zswap_memcg_offline_cleanup(memcg);