Commit d4581a23 authored Dec 10, 2009 by Thomas Gleixner

sys: Fix missing rcu protection for __task_cred() access



commit c69e8d9c (CRED: Use RCU to access another task's creds and to
release a task's own creds) added non rcu_read_lock() protected access
to task creds of the target task in set_prio_one().

The comment above the function says:
 * - the caller must hold the RCU read lock

The calling code in sys_setpriority does read_lock(&tasklist_lock) but
not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
interrupt when they see no read side critical section.

There is another instance of __task_cred() in sys_setpriority() itself
which is equally unprotected.

Wrap the whole code section into a rcu read side critical section to
fix this quick and dirty.

Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
crusade.

Oleg noted further:

This also fixes another bug here. find_task_by_vpid() is not safe
without rcu_read_lock(). I do not mean it is not safe to use the
result, just find_pid_ns() by itself is not safe.

Usually tasklist gives enough protection, but if copy_process() fails
it calls free_pid() lockless and does call_rcu(delayed_put_pid().
This means, without rcu lock find_pid_ns() can't scan the hash table
safely.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <20091210004703.029784964@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

parent 7cf7db8d

kernel/sys.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
		if (niceval > 19)
		niceval = 19;

		rcu_read_lock();
		read_lock(&tasklist_lock);
		switch (which) {
		case PRIO_PROCESS:
		@@ -200,6 +201,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
		}
		out_unlock:
		read_unlock(&tasklist_lock);
		rcu_read_unlock();
		out:
		return error;
		}