CVE-2024-56547

In the Linux kernel, the following vulnerability has been resolved:

rcu/nocb: Fix missed RCU barrier on deoffloading

Currently, running rcutorture test with torture_type=rcu fwd_progress=8 n_barrier_cbs=8 nocbs_nthreads=8 nocbs_toggle=100 onoff_interval=60 test_boost=2, will trigger the following warning:

WARNING: CPU: 19 PID: 100 at kernel/rcu/tree_nocb.h:1061 rcu_nocb_rdp_deoffload+0x292/0x2a0
RIP: 0010:rcu_nocb_rdp_deoffload+0x292/0x2a0
 Call Trace:
  <TASK>
  ? __warn+0x7e/0x120
  ? rcu_nocb_rdp_deoffload+0x292/0x2a0
  ? report_bug+0x18e/0x1a0
  ? handle_bug+0x3d/0x70
  ? exc_invalid_op+0x18/0x70
  ? asm_exc_invalid_op+0x1a/0x20
  ? rcu_nocb_rdp_deoffload+0x292/0x2a0
  rcu_nocb_cpu_deoffload+0x70/0xa0
  rcu_nocb_toggle+0x136/0x1c0
  ? __pfx_rcu_nocb_toggle+0x10/0x10
  kthread+0xd1/0x100
  ? __pfx_kthread+0x10/0x10
  ret_from_fork+0x2f/0x50
  ? __pfx_kthread+0x10/0x10
  ret_from_fork_asm+0x1a/0x30
  </TASK>

CPU0 CPU2 CPU3 //rcu_nocb_toggle //nocb_cb_wait //rcutorture

// deoffload CPU1 // process CPU1s rdp rcu_barrier() rcu_segcblist_entrain() rcu_segcblist_add_len(1); // len == 2 // enqueue barrier // callback to CPU1s // rdp->cblist rcu_do_batch() // invoke CPU1s rdp->cblist // callback rcu_barrier_callback() rcu_barrier() mutex_lock(&rcu_state.barrier_mutex); // still see len == 2 // enqueue barrier callback // to CPU1s rdp->cblist rcu_segcblist_entrain() rcu_segcblist_add_len(1); // len == 3 // decrement len rcu_segcblist_add_len(-2); kthread_parkme()

// CPU1s rdp->cblist len == 1 // Warn because there is // still a pending barrier // trigger warning WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist)); cpus_read_unlock();

                                                            // wait CPU1 to comes online and
                                                            // invoke barrier callback on
                                                            // CPU1 rdps->cblist
                                                            wait_for_completion(&rcu_state.barrier_completion);

// deoffload CPU4 cpus_read_lock() rcu_barrier() mutex_lock(&rcu_state.barrier_mutex); // block on barrier_mutex // wait rcu_barrier() on // CPU3 to unlock barrier_mutex // but CPU3 unlock barrier_mutex // need to wait CPU1 comes online // when CPU1 going online will block on cpus_write_lock

The above scenario will not only trigger a WARN_ON_ONCE(), but also trigger a deadlock.

Thanks to nocb locking, a second racing rcu_barrier() on an offline CPU will either observe the decremented callback counter down to 0 and spare the callback enqueue, or rcuo will observe the new callback and keep rdp->nocb_cb_sleep to false.

Therefore check rdp->nocb_cb_sleep before parking to make sure no further rcu_barrier() is waiting on the rdp.

Affected Software

References

• https://git.kernel.org/stable/c/224b62028959858294789772d372dcb36cf5f820
• https://git.kernel.org/stable/c/2996980e20b7a54a1869df15b3445374b850b155