[PATCH 6/6] io_uring/epoll: disallow adding an epoll file to an epoll context

[Jens Axboe <axboe@kernel.dk>]

One of the nastier things about epoll is how it allows adding epoll
files to epoll contexts. This leads to all sorts of loop detection
code, and has been a source of issues in the past.

Arguably adding IORING_EPOLL_CTL is a historical mistake on the
io_uring side, but we're kind of stuck with it now as it does seem
to be in use according to code searches. But we can at least minimize
the damage a bit and just disallow this part of epoll, where nesting
issues can arise.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/epoll.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/io_uring/epoll.c b/io_uring/epoll.c
index b9db8bde27ec..eecd748cad01 100644
--- a/io_uring/epoll.c
+++ b/io_uring/epoll.c
@@ -62,6 +62,9 @@ int io_epoll_ctl(struct io_kiocb *req, unsigned int issue_flags)
 	CLASS(fd, tf)(ie->fd);
 	if (fd_empty(tf))
 		return -EBADF;
+	/* disallow adding an epoll context to another epoll context */
+	if (ie->op == EPOLL_CTL_ADD && is_file_epoll(fd_file(tf)))
+		return -EINVAL;
 
 	key.file = fd_file(tf);
 	key.fd = ie->fd;
-- 
2.53.0