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epoll: Keep a reference on files added to the check list 

When adding a new fd to an epoll, and that this new fd is an
epoll fd itself, we recursively scan the fds attached to it
to detect cycles, and add non-epool files to a "check list"
that gets subsequently parsed.

However, this check list isn't completely safe when deletions
can happen concurrently. To sidestep the issue, make sure that
a struct file placed on the check list sees its f_count increased,
ensuring that a concurrent deletion won't result in the file
disapearing from under our feet.

Cc: stable@vger.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Marc Zyngier 2020-08-19 17:12:17 +01:00 committed by Al Viro
parent 9123e3a74e
commit a9ed4a6560
1 changed files with 9 additions and 2 deletions
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11
fs/eventpoll.c
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@ -1994,9 +1994,11 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
* not already there, and calling reverse_path_check() * not already there, and calling reverse_path_check()
* during ep_insert(). * during ep_insert().
*/ */
if (list_empty(&epi->ffd.file->f_tfile_llink)) if (list_empty(&epi->ffd.file->f_tfile_llink)) {
get_file(epi->ffd.file);
list_add(&epi->ffd.file->f_tfile_llink, list_add(&epi->ffd.file->f_tfile_llink,
&tfile_check_list); &tfile_check_list);
}
} }
} }
mutex_unlock(&ep->mtx); mutex_unlock(&ep->mtx);
@ -2040,6 +2042,7 @@ static void clear_tfile_check_list(void)
file = list_first_entry(&tfile_check_list, struct file, file = list_first_entry(&tfile_check_list, struct file,
f_tfile_llink); f_tfile_llink);
list_del_init(&file->f_tfile_llink); list_del_init(&file->f_tfile_llink);
fput(file);
} }
INIT_LIST_HEAD(&tfile_check_list); INIT_LIST_HEAD(&tfile_check_list);
} }
@ -2204,13 +2207,17 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
clear_tfile_check_list(); clear_tfile_check_list();
goto error_tgt_fput; goto error_tgt_fput;
} }
} else } else {
get_file(tf.file);
list_add(&tf.file->f_tfile_llink, list_add(&tf.file->f_tfile_llink,
&tfile_check_list); &tfile_check_list);
}
error = epoll_mutex_lock(&ep->mtx, 0, nonblock); error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
if (error) { if (error) {
out_del: out_del:
list_del(&tf.file->f_tfile_llink); list_del(&tf.file->f_tfile_llink);
if (!is_file_epoll(tf.file))
fput(tf.file);
goto error_tgt_fput; goto error_tgt_fput;
} }
if (is_file_epoll(tf.file)) { if (is_file_epoll(tf.file)) {