GHSA-HQWF-4R5G-Q2H4
Vulnerability from github – Published: 2026-10-18 18:32 – Updated: 2026-10-18 18:32In the Linux kernel, the following vulnerability has been resolved:
mm/slab: do not wake up kswapd in __kfree_rcu_sheaf()
Since kfree_rcu() can be called under pi_lock (a raw spinlock in the scheduler), kfree_rcu() itself should never allocate memory with __GFP_KSWAPD_RECLAIM as waking up kswapd ends up acquiring pi_lock, which leads to a deadlock.
Reproducing the issue even intentionally was not straightforward. The set_cpus_allowed_force() path that is called under pi_lock is exercised very rarely, and competing tasks that allocate memory will most likely wake kswapd up.
Therefore the existence of the deadlock was verified with a modified kernel that has a lockdep map for waking up kswapd:
====================================================== WARNING: possible circular locking dependency detected 7.2.0-rc1-slab-for-next+ #10 Not tainted
git/6660 is trying to acquire lock: ffff8e524533c550 (&p->pi_lock){-.-.}-{2:2}, at: _raw_spin_lock_irqsave+0x12/0x20
but task is already holding lock: ffff8e57bffff1a0 (&pgdat->kswapd_wait){....}-{3:3}, at: _raw_spin_lock_irqsave+0x12/0x20
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&pgdat->kswapd_wait){....}-{3:3}: __lock_acquire+0x5a4/0xc50 lock_acquire.part.0+0xb7/0x240 lock_acquire+0x70/0x170 __raw_spin_lock_irqsave+0x44/0x80 _raw_spin_lock_irqsave+0x12/0x20 __wake_up_common_lock+0x31/0xa0 __wake_up+0x20/0x40 kswapd_wakeup_wake+0x7e/0x110 wakeup_kswapd+0x295/0x320 wake_all_kswapds+0xac/0x1a0 __alloc_pages_slowpath.constprop.0+0x282/0xf80 __alloc_frozen_pages_noprof+0x32a/0x360 alloc_slab_page+0x2e/0x160 allocate_slab+0x82/0x420 new_slab+0x52/0xb0 refill_objects+0x13d/0x190 refill_sheaf+0x5d/0xd0 __pcs_replace_empty_main+0x230/0xb10 [...]
-> #1 (kswapd_wakeup){-.-.}-{0:0}: __lock_acquire+0x5a4/0xc50 lock_sync.part.0+0x75/0x100 lock_sync+0x36/0x70 might_wakeup_kswapd+0x61/0xa0 __kfree_rcu_sheaf+0x33/0xd90 kvfree_call_rcu+0x1d4/0x3b0 set_cpus_allowed_force+0x163/0x220 cpuset_cpus_allowed_fallback+0x18b/0x240 select_fallback_rq+0x1e6/0x250 [...]
-> #0 (&p->pi_lock){-.-.}-{2:2}: check_prev_add+0xe6/0xe00 validate_chain+0x51e/0x6e0 __lock_acquire+0x5a4/0xc50 lock_acquire.part.0+0xb7/0x240 lock_acquire+0x70/0x170 __raw_spin_lock_irqsave+0x44/0x80 _raw_spin_lock_irqsave+0x12/0x20 try_to_wake_up+0x77/0xa90 default_wake_function+0x27/0x60 autoremove_wake_function+0x23/0xb0 __wake_up_common+0xb8/0x170 __wake_up_common_lock+0x51/0xa0 __wake_up+0x20/0x40 kswapd_wakeup_wake+0x7e/0x110 wakeup_kswapd+0x295/0x320 wake_all_kswapds+0xac/0x1a0 __alloc_pages_slowpath.constprop.0+0x282/0xf80 __alloc_frozen_pages_noprof+0x32a/0x360 alloc_slab_page+0x2e/0x160 allocate_slab+0x82/0x420 new_slab+0x52/0xb0 refill_objects+0x13d/0x190 refill_sheaf+0x5d/0xd0 __pcs_replace_empty_main+0x230/0xb10 [...]
other info that might help us debug this:
Chain exists of: &p->pi_lock --> kswapd_wakeup --> &pgdat->kswapd_wait
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&pgdat->kswapd_wait);
lock(kswapd_wakeup);
lock(&pgdat->kswapd_wait);
lock(&p->pi_lock);
*** DEADLOCK ***
3 locks held by git/6660: #0: ffff8e519f0e9e20 (&type->i_mutex_dir_key#6){++++}-{4:4}, at: lookup_slow+0x2d/0x60 #1: ffffffff8a31c460 (kswapd_wakeup){-.-.}- ---truncated---
{
"affected": [],
"aliases": [
"CVE-2026-98818"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-10-18T16:17:36Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm/slab: do not wake up kswapd in __kfree_rcu_sheaf()\n\nSince kfree_rcu() can be called under pi_lock (a raw spinlock in the\nscheduler), kfree_rcu() itself should never allocate memory with\n__GFP_KSWAPD_RECLAIM as waking up kswapd ends up acquiring pi_lock,\nwhich leads to a deadlock.\n\nReproducing the issue even intentionally was not straightforward.\nThe set_cpus_allowed_force() path that is called under pi_lock is\nexercised very rarely, and competing tasks that allocate memory will\nmost likely wake kswapd up.\n\nTherefore the existence of the deadlock was verified with a modified\nkernel that has a lockdep map for waking up kswapd:\n\n ======================================================\n WARNING: possible circular locking dependency detected\n 7.2.0-rc1-slab-for-next+ #10 Not tainted\n ------------------------------------------------------\n git/6660 is trying to acquire lock:\n ffff8e524533c550 (\u0026p-\u003epi_lock){-.-.}-{2:2}, at: _raw_spin_lock_irqsave+0x12/0x20\n\n but task is already holding lock:\n ffff8e57bffff1a0 (\u0026pgdat-\u003ekswapd_wait){....}-{3:3}, at: _raw_spin_lock_irqsave+0x12/0x20\n\n which lock already depends on the new lock.\n\n the existing dependency chain (in reverse order) is:\n\n -\u003e #2 (\u0026pgdat-\u003ekswapd_wait){....}-{3:3}:\n __lock_acquire+0x5a4/0xc50\n lock_acquire.part.0+0xb7/0x240\n lock_acquire+0x70/0x170\n __raw_spin_lock_irqsave+0x44/0x80\n _raw_spin_lock_irqsave+0x12/0x20\n __wake_up_common_lock+0x31/0xa0\n __wake_up+0x20/0x40\n kswapd_wakeup_wake+0x7e/0x110\n wakeup_kswapd+0x295/0x320\n wake_all_kswapds+0xac/0x1a0\n __alloc_pages_slowpath.constprop.0+0x282/0xf80\n __alloc_frozen_pages_noprof+0x32a/0x360\n alloc_slab_page+0x2e/0x160\n allocate_slab+0x82/0x420\n new_slab+0x52/0xb0\n refill_objects+0x13d/0x190\n refill_sheaf+0x5d/0xd0\n __pcs_replace_empty_main+0x230/0xb10\n [...]\n\n -\u003e #1 (kswapd_wakeup){-.-.}-{0:0}:\n __lock_acquire+0x5a4/0xc50\n lock_sync.part.0+0x75/0x100\n lock_sync+0x36/0x70\n might_wakeup_kswapd+0x61/0xa0\n __kfree_rcu_sheaf+0x33/0xd90\n kvfree_call_rcu+0x1d4/0x3b0\n set_cpus_allowed_force+0x163/0x220\n cpuset_cpus_allowed_fallback+0x18b/0x240\n select_fallback_rq+0x1e6/0x250\n\t [...]\n\n -\u003e #0 (\u0026p-\u003epi_lock){-.-.}-{2:2}:\n check_prev_add+0xe6/0xe00\n validate_chain+0x51e/0x6e0\n __lock_acquire+0x5a4/0xc50\n lock_acquire.part.0+0xb7/0x240\n lock_acquire+0x70/0x170\n __raw_spin_lock_irqsave+0x44/0x80\n _raw_spin_lock_irqsave+0x12/0x20\n try_to_wake_up+0x77/0xa90\n default_wake_function+0x27/0x60\n autoremove_wake_function+0x23/0xb0\n __wake_up_common+0xb8/0x170\n __wake_up_common_lock+0x51/0xa0\n __wake_up+0x20/0x40\n kswapd_wakeup_wake+0x7e/0x110\n wakeup_kswapd+0x295/0x320\n wake_all_kswapds+0xac/0x1a0\n __alloc_pages_slowpath.constprop.0+0x282/0xf80\n __alloc_frozen_pages_noprof+0x32a/0x360\n alloc_slab_page+0x2e/0x160\n allocate_slab+0x82/0x420\n new_slab+0x52/0xb0\n refill_objects+0x13d/0x190\n refill_sheaf+0x5d/0xd0\n __pcs_replace_empty_main+0x230/0xb10\n [...]\n\n other info that might help us debug this:\n\n Chain exists of:\n \u0026p-\u003epi_lock --\u003e kswapd_wakeup --\u003e \u0026pgdat-\u003ekswapd_wait\n\n Possible unsafe locking scenario:\n\n CPU0 CPU1\n ---- ----\n lock(\u0026pgdat-\u003ekswapd_wait);\n lock(kswapd_wakeup);\n lock(\u0026pgdat-\u003ekswapd_wait);\n lock(\u0026p-\u003epi_lock);\n\n *** DEADLOCK ***\n\n 3 locks held by git/6660:\n #0: ffff8e519f0e9e20 (\u0026type-\u003ei_mutex_dir_key#6){++++}-{4:4}, at: lookup_slow+0x2d/0x60\n #1: ffffffff8a31c460 (kswapd_wakeup){-.-.}-\n---truncated---",
"id": "GHSA-hqwf-4r5g-q2h4",
"modified": "2026-10-18T18:32:04Z",
"published": "2026-10-18T18:32:04Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98818"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/46545c86b54ffb969b6e3662942b4a6191619eaa"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/af2fb3ee6679b517e2fdbd3ba16ec276a8b74312"
}
],
"schema_version": "1.4.0",
"severity": []
}
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