From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5F34DC4167D for ; Mon, 28 Nov 2022 17:38:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233155AbiK1Ri4 (ORCPT ); Mon, 28 Nov 2022 12:38:56 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51492 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233113AbiK1Rif (ORCPT ); Mon, 28 Nov 2022 12:38:35 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 87D6EA450; Mon, 28 Nov 2022 09:38:25 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id F1B88B80E9C; Mon, 28 Nov 2022 17:38:23 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 20452C433B5; Mon, 28 Nov 2022 17:38:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1669657102; bh=OvIH+/rzbE4iIJMXArCK3MshVpQL1tOZGfQMHYX+yj0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=eoDM5jRpjsRGljFg1j/wCGiD9sFLtLYvIJP93yp/3FFe7nSl9UPBKQZMCf8mQ+H5w gfG11N2r7PoZ6exOGtBCy6Cv2Ve0DPlTr4PrnQxotYf1fSHQnKACvU3MNKhEp19c/x CXfvDspgV2yGBITOzP3ydHOLknDJVrr7LpNRwnWgidkAqxXdLSFF9TSDY6IiQIUxHQ kgePVySZhpVcaagdi5dYNprq1tmKYEipib0VC/+YetGFITiIv/xMsjtNx1cB8X1EJQ lr77ax3e2RPnQGMs1ZkuOfFnOZrFFVgaRaNnW/kSi7FvEsANyPj2nI+GbU4XaCPFO6 LLtAU6oOoMk5g== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Jann Horn , Al Viro , Sasha Levin , linux-fsdevel@vger.kernel.org Subject: [PATCH AUTOSEL 6.0 13/39] fs: use acquire ordering in __fget_light() Date: Mon, 28 Nov 2022 12:35:53 -0500 Message-Id: <20221128173642.1441232-13-sashal@kernel.org> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20221128173642.1441232-1-sashal@kernel.org> References: <20221128173642.1441232-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org From: Jann Horn [ Upstream commit 7ee47dcfff1835ff75a794d1075b6b5f5462cfed ] We must prevent the CPU from reordering the files->count read with the FD table access like this, on architectures where read-read reordering is possible: files_lookup_fd_raw() close_fd() put_files_struct() atomic_read(&files->count) I would like to mark this for stable, but the stable rules explicitly say "no theoretical races", and given that the FD table pointer and files->count are explicitly stored in the same cacheline, this sort of reordering seems quite unlikely in practice... Signed-off-by: Jann Horn Signed-off-by: Al Viro Signed-off-by: Sasha Levin --- fs/file.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/fs/file.c b/fs/file.c index 3bcc1ecc314a..57af5f8375fd 100644 --- a/fs/file.c +++ b/fs/file.c @@ -1002,7 +1002,16 @@ static unsigned long __fget_light(unsigned int fd, fmode_t mask) struct files_struct *files = current->files; struct file *file; - if (atomic_read(&files->count) == 1) { + /* + * If another thread is concurrently calling close_fd() followed + * by put_files_struct(), we must not observe the old table + * entry combined with the new refcount - otherwise we could + * return a file that is concurrently being freed. + * + * atomic_read_acquire() pairs with atomic_dec_and_test() in + * put_files_struct(). + */ + if (atomic_read_acquire(&files->count) == 1) { file = files_lookup_fd_raw(files, fd); if (!file || unlikely(file->f_mode & mask)) return 0; -- 2.35.1