epoll: Keep a reference on files added to the check list
commit a9ed4a6560b8562b7e2e2bed9527e88001f7b682 upstream. 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> Signed-off-by: Marc Zyngier <maz@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@ -1890,9 +1890,11 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
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* not already there, and calling reverse_path_check()
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* during ep_insert().
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*/
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if (list_empty(&epi->ffd.file->f_tfile_llink))
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if (list_empty(&epi->ffd.file->f_tfile_llink)) {
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get_file(epi->ffd.file);
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list_add(&epi->ffd.file->f_tfile_llink,
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&tfile_check_list);
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}
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}
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}
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mutex_unlock(&ep->mtx);
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@ -1936,6 +1938,7 @@ static void clear_tfile_check_list(void)
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file = list_first_entry(&tfile_check_list, struct file,
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f_tfile_llink);
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list_del_init(&file->f_tfile_llink);
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fput(file);
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}
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INIT_LIST_HEAD(&tfile_check_list);
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}
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@ -2095,9 +2098,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
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clear_tfile_check_list();
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goto error_tgt_fput;
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}
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} else
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} else {
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get_file(tf.file);
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list_add(&tf.file->f_tfile_llink,
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&tfile_check_list);
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}
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mutex_lock_nested(&ep->mtx, 0);
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if (is_file_epoll(tf.file)) {
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tep = tf.file->private_data;
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