Re: Linus GIT - INFO: possible circular locking dependency detected

[Vasiliy Kulikov <segoon@openwall.com>]

(cc'ed Andrew and Alexey)

On Thu, Nov 03, 2011 at 20:49 +0000, Luis Henriques wrote:
> On Thu, Nov 03, 2011 at 11:57:20AM -0400, Miles Lane wrote:
> ...
> 
> I'm hitting the exact same problem, using a minimal .config file (can send
> it if required), by just running "find /".
> 
> I have bisected the problem and found that commit
> aa6afca5bcaba8101f3ea09d5c3e4100b2b9f0e5 seems to be the cause of it.

procfs holds sig->cred_guard_mutex to ensure the target's credentials are
not changed.  It is held for a little timeslice.  From the stack trace I
don't understand how sys_execve() can happen with ->cred_guard_mutex
held:

static struct dentry *proc_lookupfd_common(struct inode *dir,
					   struct dentry *dentry,
					   instantiate_t instantiate)
{
    ...
	if (lock_trace(task))
		goto out;

	result = instantiate(dir, dentry, task, &fd);
	unlock_trace(task);
    ...
}


static int lock_trace(struct task_struct *task)
{
	int err = mutex_lock_killable(&task->signal->cred_guard_mutex);
	if (err)
		return err;
	if (!ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
		mutex_unlock(&task->signal->cred_guard_mutex);
		return -EPERM;
	}
	return 0;
}

proc_lookupfd_common() always exits without ->cred_guard_mutex held.


Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments