CVE-2026-98818

In 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---