On Fri, Aug 28, 2020 at 06:54:13PM +0100, Al Viro wrote: > On Fri, Aug 28, 2020 at 04:38:25PM +0100, Al Viro wrote: > > On Fri, Aug 28, 2020 at 06:18:17AM -0700, syzbot wrote: > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: d012a719 Linux 5.9-rc2 > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=15aa650e900000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=891ca5711a9f1650 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=c92c93d1f1aaaacdb9db > > > compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81) > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12ecb939900000 > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=140a19a9900000 > > > > Trying to reproduce... > > OK, I think I see what's going on. ep_loop_check_proc() runs into an already > doomed file that has already committed to getting killed (->f_count is already > at 0), but still hadn't gotten through its epitems removal (e.g. has its > eventpoll_release_file() sitting there trying to get epmutex). > > Blindly bumping refcount here is worse than useless. Try this, to verify that > this is what's going on; it's _not_ a proper fix, but it should at least tell > if we have something else going on. ... and what I think is the right way to fix the original race is (on top of mainline) this: [PATCH] Use list_empty_careful() in eventpoll_release() ... to avoid races with list_del_init() in clear_tfile_check_list(). Get rid of pinning files on check list in eventpoll.c - it's not needed there. Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> --- diff --git a/fs/eventpoll.c b/fs/eventpoll.c index e0decff22ae2..39eae45bff18 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1995,7 +1995,6 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests) * during ep_insert(). */ if (list_empty(&epi->ffd.file->f_tfile_llink)) { - get_file(epi->ffd.file); list_add(&epi->ffd.file->f_tfile_llink, &tfile_check_list); } @@ -2042,7 +2041,6 @@ static void clear_tfile_check_list(void) file = list_first_entry(&tfile_check_list, struct file, f_tfile_llink); list_del_init(&file->f_tfile_llink); - fput(file); } INIT_LIST_HEAD(&tfile_check_list); } @@ -2206,7 +2204,6 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds, if (ep_loop_check(ep, tf.file) != 0) goto error_tgt_fput; } else { - get_file(tf.file); list_add(&tf.file->f_tfile_llink, &tfile_check_list); } diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h index 8f000fada5a4..e2bdefd90cf8 100644 --- a/include/linux/eventpoll.h +++ b/include/linux/eventpoll.h @@ -46,11 +46,9 @@ static inline void eventpoll_release(struct file *file) * Fast check to avoid the get/release of the semaphore. Since * we're doing this outside the semaphore lock, it might return * false negatives, but we don't care. It'll help in 99.99% of cases - * to avoid the semaphore lock. False positives simply cannot happen - * because the file in on the way to be removed and nobody ( but - * eventpoll ) has still a reference to this file. + * to avoid the semaphore lock. */ - if (likely(list_empty(&file->f_ep_links))) + if (likely(list_empty_careful(&file->f_ep_links))) return; /*
