Commit f358166a authored May 11, 2006 by Linus Torvalds

ptrace_attach: fix possible deadlock schenario with irqs



Eric Biederman points out that we can't take the task_lock while holding
tasklist_lock for writing, because another CPU that holds the task lock
might take an interrupt that then tries to take tasklist_lock for writing.

Which would be a nasty deadlock, with one CPU spinning forever in an
interrupt handler (although admittedly you need to really work at
triggering it ;)

Since the ptrace_attach() code is special and very unusual, just make it
be extra careful, and use trylock+repeat to avoid the possible deadlock.

Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 0e44dc38

kernel/ptrace.c

+19 −1

Original line number	Diff line number	Diff line
		@@ -155,8 +155,26 @@ int ptrace_attach(struct task_struct *task)
		if (task->tgid == current->tgid)
		goto out;

		write_lock_irq(&tasklist_lock);
		repeat:
		/*
		* Nasty, nasty.
		*
		* We want to hold both the task-lock and the
		* tasklist_lock for writing at the same time.
		* But that's against the rules (tasklist_lock
		* is taken for reading by interrupts on other
		* cpu's that may have task_lock).
		*/
		task_lock(task);
		local_irq_disable();
		if (!write_trylock(&tasklist_lock)) {
		local_irq_enable();
		task_unlock(task);
		do {
		cpu_relax();
		} while (!write_can_lock(&tasklist_lock));
		goto repeat;
		}

		/* the same process cannot be attached many times */
		if (task->ptrace & PT_PTRACED)