rcu: Prevent spurious-wakeup DoS attack on rcu_gp_kthread() (88d6df61) · Commits · 戴 / test

Spurious wakeups in the force-quiescent-state loop in rcu_gp_kthread() cause the timeout to be recalculated, which would prevent rcu_gp_fqs() from ever being called. This would in turn would prevent the grace period from ever ending for as long as there was at least one CPU in an extended quiescent state that had not yet passed through a quiescent state. This commit therefore avoids recalculating the timeout unless the previous pass's call to wait_event_interruptible_timeout() actually did time out, thus preventing the above scenario. Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

+8 −3

Original line number	Diff line number	Diff line
		@@ -1470,6 +1470,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
		static int __noreturn rcu_gp_kthread(void *arg)
		{
		int fqs_state;
		int gf;
		unsigned long j;
		int ret;
		struct rcu_state *rsp = arg;
		@@ -1495,10 +1496,13 @@ static int __noreturn rcu_gp_kthread(void *arg)
		j = HZ;
		jiffies_till_first_fqs = HZ;
		}
		ret = 0;
		for (;;) {
		if (!ret)
		rsp->jiffies_force_qs = jiffies + j;
		ret = wait_event_interruptible_timeout(rsp->gp_wq,
		(rsp->gp_flags & RCU_GP_FLAG_FQS) \|\|
		((gf = ACCESS_ONCE(rsp->gp_flags)) &
		RCU_GP_FLAG_FQS) \|\|
		(!ACCESS_ONCE(rnp->qsmask) &&
		!rcu_preempt_blocked_readers_cgp(rnp)),
		j);
		@@ -1507,7 +1511,8 @@ static int __noreturn rcu_gp_kthread(void *arg)
		!rcu_preempt_blocked_readers_cgp(rnp))
		break;
		/* If time for quiescent-state forcing, do it. */
		if (ret == 0 \|\| (rsp->gp_flags & RCU_GP_FLAG_FQS)) {
		if (ULONG_CMP_GE(jiffies, rsp->jiffies_force_qs) \|\|
		(gf & RCU_GP_FLAG_FQS)) {
		fqs_state = rcu_gp_fqs(rsp, fqs_state);
		cond_resched();
		} else {

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