Commit message: epoll: clear the tfile_check_list on -ELOOP

An epoll_ctl(,EPOLL_CTL_ADD,,) operation can return '-ELOOP' to prevent
circular epoll dependencies from being created.  However, in that case we
do not properly clear the 'tfile_check_list'.  Thus, add a call to
clear_tfile_check_list() for the -ELOOP case.

Signed-off-by: Jason Baron <jbaron@redhat.com>
Reported-by: Yurij M. Plotnikov <Yurij.Plotnikov@oktetlabs.ru>
Cc: Nelson Elhage <nelhage@nelhage.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Tested-by: Alexandra N. Kossovsky <Alexandra.Kossovsky@oktetlabs.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2012-3375
Vulnerability description: The epoll_ctl system call in fs/eventpoll.c in the Linux kernel before 3.2.24 does not properly handle ELOOP errors in EPOLL_CTL_ADD operations, which allows local users to cause a denial of service (file-descriptor consumption and system crash) via a crafted application that attempts to create a circular epoll dependency.  NOTE: this vulnerability exists because of an incorrect fix for CVE-2011-1083.

Project: linux
File: eventpoll.c

SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
		struct epoll_event __user *, event)
{
	int error;
	int did_lock_epmutex = 0;
	struct file *file, *tfile;
	struct eventpoll *ep;
	struct epitem *epi;
	struct epoll_event epds;

	error = -EFAULT;
	if (ep_op_has_event(op) &&
	    copy_from_user(&epds, event, sizeof(struct epoll_event)))
		goto error_return;

	/* Get the "struct file *" for the eventpoll file */
	error = -EBADF;
	file = fget(epfd);
	if (!file)
		goto error_return;

	/* Get the "struct file *" for the target file */
	tfile = fget(fd);
	if (!tfile)
		goto error_fput;

	/* The target file descriptor must support poll */
	error = -EPERM;
	if (!tfile->f_op || !tfile->f_op->poll)
		goto error_tgt_fput;

	/*
	 * We have to check that the file structure underneath the file descriptor
	 * the user passed to us _is_ an eventpoll file. And also we do not permit
	 * adding an epoll file descriptor inside itself.
	 */
	error = -EINVAL;
	if (file == tfile || !is_file_epoll(file))
		goto error_tgt_fput;

	/*
	 * At this point it is safe to assume that the "private_data" contains
	 * our own data structure.
	 */
	ep = file->private_data;

	/*
	 * When we insert an epoll file descriptor, inside another epoll file
	 * descriptor, there is the change of creating closed loops, which are
	 * better be handled here, than in more critical paths. While we are
	 * checking for loops we also determine the list of files reachable
	 * and hang them on the tfile_check_list, so we can check that we
	 * haven't created too many possible wakeup paths.
	 *
	 * We need to hold the epmutex across both ep_insert and ep_remove
	 * b/c we want to make sure we are looking at a coherent view of
	 * epoll network.
	 */
	if (op == EPOLL_CTL_ADD || op == EPOLL_CTL_DEL) {
		mutex_lock(&epmutex);
		did_lock_epmutex = 1;
	}
 	if (op == EPOLL_CTL_ADD) {
 		if (is_file_epoll(tfile)) {
 			error = -ELOOP;
			if (ep_loop_check(ep, tfile) != 0) {
				clear_tfile_check_list();
 				goto error_tgt_fput;
			}
 		} else
 			list_add(&tfile->f_tfile_llink, &tfile_check_list);
 	}

	mutex_lock_nested(&ep->mtx, 0);

	/*
	 * Try to lookup the file inside our RB tree, Since we grabbed "mtx"
	 * above, we can be sure to be able to use the item looked up by
	 * ep_find() till we release the mutex.
	 */
	epi = ep_find(ep, tfile, fd);

	error = -EINVAL;
	switch (op) {
	case EPOLL_CTL_ADD:
		if (!epi) {
			epds.events |= POLLERR | POLLHUP;
			error = ep_insert(ep, &epds, tfile, fd);
		} else
			error = -EEXIST;
		clear_tfile_check_list();
		break;
	case EPOLL_CTL_DEL:
		if (epi)
			error = ep_remove(ep, epi);
		else
			error = -ENOENT;
		break;
	case EPOLL_CTL_MOD:
		if (epi) {
			epds.events |= POLLERR | POLLHUP;
			error = ep_modify(ep, epi, &epds);
		} else
			error = -ENOENT;
		break;
	}
	mutex_unlock(&ep->mtx);

error_tgt_fput:
	if (did_lock_epmutex)
		mutex_unlock(&epmutex);

	fput(tfile);
error_fput:
	fput(file);
error_return:

	return error;
}