GHSA-3j9f-4r3c-964g: 
In the Linux kernel, the following vulnerability has been resolved:powerpc/qspinlock: Fix deadlock in MCS queueIf an interrupt occurs in queued_spin_lock_slowpath() after we increment
qnodesp->count and before node->lock is initialized, another CPU might
see stale lock values in get_tail_qnode(). If the stale lock value happens
to match the lock on that CPU, then we write to the "next" pointer of
the wrong qnode. This causes a deadlock as the former CPU, once it becomes
the head of the MCS queue, will spin indefinitely until it's "next" pointer
is set by its successor in the queue.Running stress-ng on a 16 core (16EC/16VP) shared LPAR, results in
occasional lockups similar to the following:$ stress-ng --all 128 --vm-bytes 80% --aggressive \
               --maximize --oomable --verify  --syslog \
               --metrics  --times  --timeout 5mwatchdog: CPU 15 Hard LOCKUP
   ......
   NIP [c0000000000b78f4] queued_spin_lock_slowpath+0x1184/0x1490
   LR [c000000001037c5c] _raw_spin_lock+0x6c/0x90
   Call Trace:
    0xc000002cfffa3bf0 (unreliable)
    _raw_spin_lock+0x6c/0x90
    raw_spin_rq_lock_nested.part.135+0x4c/0xd0
    sched_ttwu_pending+0x60/0x1f0
    __flush_smp_call_function_queue+0x1dc/0x670
    smp_ipi_demux_relaxed+0xa4/0x100
    xive_muxed_ipi_action+0x20/0x40
    __handle_irq_event_percpu+0x80/0x240
    handle_irq_event_percpu+0x2c/0x80
    handle_percpu_irq+0x84/0xd0
    generic_handle_irq+0x54/0x80
    __do_irq+0xac/0x210
    __do_IRQ+0x74/0xd0
    0x0
    do_IRQ+0x8c/0x170
    hardware_interrupt_common_virt+0x29c/0x2a0
   --- interrupt: 500 at queued_spin_lock_slowpath+0x4b8/0x1490
   ......
   NIP [c0000000000b6c28] queued_spin_lock_slowpath+0x4b8/0x1490
   LR [c000000001037c5c] _raw_spin_lock+0x6c/0x90
   --- interrupt: 500
    0xc0000029c1a41d00 (unreliable)
    _raw_spin_lock+0x6c/0x90
    futex_wake+0x100/0x260
    do_futex+0x21c/0x2a0
    sys_futex+0x98/0x270
    system_call_exception+0x14c/0x2f0
    system_call_vectored_common+0x15c/0x2ecThe following code flow illustrates how the deadlock occurs.
For the sake of brevity, assume that both locks (A and B) are
contended and we call the queued_spin_lock_slowpath() function.spin_lock_irqsave(A)                          |
  spin_unlock_irqrestore(A)                     |
    spin_lock(B)                                |
         |                                      |
         ▼                                      |
   id = qnodesp->count++;                       |
  (Note that nodes[0].lock == A)                |
         |                                      |
         ▼                                      |
      Interrupt                                 |
  (happens before "nodes[0].lock = B")          |
         |                                      |
         ▼                                      |
  spin_lock_irqsave(A)                          |
         |                                      |
         ▼                                      |
   id = qnodesp->count++                        |
   nodes[1].lock = A                            |
         |                                      |
         ▼                                      |
  Tail of MCS queue                             |
         |                             spin_lock_irqsave(A)
         ▼                                      |
  Head of MCS queue                             ▼
         |                             CPU0 is previous tail
         ▼                                      |
   Spin indefinitely                            ▼
  (until "nodes[1].next != NULL")      prev = get_tail_qnode(A, CPU0)
                                                |
                                                ▼
                                       prev == &qnodes[CPU0].nodes[0]
                                     (as qnodes
---truncated---
CVSS v3.1: 5.5