Commit 4efaa5ac authored by Linus Torvalds's avatar Linus Torvalds
Browse files

epoll: be better about file lifetimes 

epoll can call out to vfs_poll() with a file pointer that may race with
the last 'fput()'. That would make f_count go down to zero, and while
the ep->mtx locking means that the resulting file pointer tear-down will
be blocked until the poll returns, it means that f_count is already
dead, and any use of it won't actually get a reference to the file any
more: it's dead regardless.

Make sure we have a valid ref on the file pointer before we call down to
vfs_poll() from the epoll routines.

Link: https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/


Reported-by: default avatar <syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com>
Reviewed-by: default avatarJens Axboe <axboe@kernel.dk>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f462ae0e

fs/eventpoll.c

+37 −1
Original line number Diff line number Diff line
@@ -979,6 +979,34 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep 
	return res;

}



/*

 * The ffd.file pointer may be in the process of being torn down due to

 * being closed, but we may not have finished eventpoll_release() yet.

 *

 * Normally, even with the atomic_long_inc_not_zero, the file may have

 * been free'd and then gotten re-allocated to something else (since

 * files are not RCU-delayed, they are SLAB_TYPESAFE_BY_RCU).

 *

 * But for epoll, users hold the ep->mtx mutex, and as such any file in

 * the process of being free'd will block in eventpoll_release_file()

 * and thus the underlying file allocation will not be free'd, and the

 * file re-use cannot happen.

 *

 * For the same reason we can avoid a rcu_read_lock() around the

 * operation - 'ffd.file' cannot go away even if the refcount has

 * reached zero (but we must still not call out to ->poll() functions

 * etc).

 */

static struct file *epi_fget(const struct epitem *epi)

{

	struct file *file;



	file = epi->ffd.file;

	if (!atomic_long_inc_not_zero(&file->f_count))

		file = NULL;

	return file;

}



/*

 * Differs from ep_eventpoll_poll() in that internal callers already have

 * the ep->mtx so we need to start from depth=1, such that mutex_lock_nested()
@@ -987,14 +1015,22 @@ static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int dep 
static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,

				 int depth)

{

	struct file *file = epi->ffd.file;

	struct file *file = epi_fget(epi);

	__poll_t res;



	/*

	 * We could return EPOLLERR | EPOLLHUP or something, but let's

	 * treat this more as "file doesn't exist, poll didn't happen".

	 */

	if (!file)

		return 0;



	pt->_key = epi->event.events;

	if (!is_file_epoll(file))

		res = vfs_poll(file, pt);

	else

		res = __ep_eventpoll_poll(file, pt, depth);

	fput(file);

	return res & epi->event.events;

}