From patchwork Mon Jul 31 23:51:20 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kees Cook X-Patchwork-Id: 9873403 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id E91266016C for ; Mon, 31 Jul 2017 23:55:32 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D390A28602 for ; Mon, 31 Jul 2017 23:55:32 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id C8303285F2; Mon, 31 Jul 2017 23:55:32 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=2.0 tests=BAYES_00,DKIM_SIGNED, RCVD_IN_DNSWL_HI,T_DKIM_INVALID autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 51739285CC for ; Mon, 31 Jul 2017 23:55:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751682AbdGaXvt (ORCPT ); Mon, 31 Jul 2017 19:51:49 -0400 Received: from mail-pg0-f53.google.com ([74.125.83.53]:34425 "EHLO mail-pg0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751535AbdGaXvq (ORCPT ); Mon, 31 Jul 2017 19:51:46 -0400 Received: by mail-pg0-f53.google.com with SMTP id u185so503676pgb.1 for ; Mon, 31 Jul 2017 16:51:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references; bh=SDl0qvuzjfWe7/ikUkqaqpO3HuChCIqt7ucNsWV8Z8w=; b=cld8TGkoCFgrYryDdneoyekFEpKX6l9Tjal/h9I81iQay69zVJ0+RmbDQGIwWQGzg6 23OcU7cadE7luH0AVGb/TAnT78WljjSlME2riUhw/E3KcEeHY75UCzKcra8Aa+MvUF+w R2ksq1lWOhXWOvcBEor8IWJO9YAjCGeBrOsms= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references; bh=SDl0qvuzjfWe7/ikUkqaqpO3HuChCIqt7ucNsWV8Z8w=; b=pWSEwZ09vwgb7geP8MInFudTKxnnhDjfB80lxSXhkv/bCbHV5iqAhBxeT//CZnp9qN AQI//H2ALaRGsskvtFCXcnWaglYMlLQKlS7bVU2JBhU7TQ6dMHk8oJBzryNy610h3FYZ Mvj9mvRd9pUt4eYdjTIIN4/xFykmmhr0zp+jAn/LKOYB8VS6p5rxHHRe4VwW2OtcgQhR nvwCri5q8XNPClBLfu5nq/Azi4kyd19/6Njd1DZIC4MtO29vPHN3AVqR9/a4CMOUxTss QGYUnrGfkYFEyQJlCLVNgXrUro7SE474gq7kQeDPkYxH2uRVAg72AJQVlTvSmjGjYhDJ kPow== X-Gm-Message-State: AIVw110dZhFlI+ZSBgWmWVJ8IlaaWes/UaV9upNEnu0lxwoN3/9vUdPe cfckD5eVMM5yEt3o X-Received: by 10.99.5.1 with SMTP id 1mr16946043pgf.222.1501545105715; Mon, 31 Jul 2017 16:51:45 -0700 (PDT) Received: from www.outflux.net (173-164-112-133-Oregon.hfc.comcastbusiness.net. [173.164.112.133]) by smtp.gmail.com with ESMTPSA id s87sm55780619pfa.86.2017.07.31.16.51.44 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 31 Jul 2017 16:51:44 -0700 (PDT) From: Kees Cook To: Andrew Morton Cc: Kees Cook , David Howells , "Eric W . Biederman" , John Johansen , "Serge E. Hallyn" , Paul Moore , Stephen Smalley , Casey Schaufler , Tetsuo Handa , James Morris , Andy Lutomirski , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 02/15] exec: Correct comments about "point of no return" Date: Mon, 31 Jul 2017 16:51:20 -0700 Message-Id: <1501545093-56634-3-git-send-email-keescook@chromium.org> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1501545093-56634-1-git-send-email-keescook@chromium.org> References: <1501545093-56634-1-git-send-email-keescook@chromium.org> Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: X-Virus-Scanned: ClamAV using ClamSMTP In commit 221af7f87b97 ("Split 'flush_old_exec' into two functions"), the comment about the point of no return should have stayed in flush_old_exec() since it refers to "bprm->mm = NULL;" line, but prior changes in commits c89681ed7d0e ("remove steal_locks()"), and fd8328be874f ("sanitize handling of shared descriptor tables in failing execve()") made it look like it meant the current->sas_ss_sp line instead. The comment was referring to the fact that once bprm->mm is NULL, all failures from a binfmt load_binary hook (e.g. load_elf_binary), will get SEGV raised against current. Move this comment and expand the explanation a bit, putting it above the assignment this time, and add details about the true nature of "point of no return" being the call to flush_old_exec() itself. This also removes an erroneous commet about when credentials are being installed. That has its own dedicated function, install_exec_creds(), which carries a similar (and correct) comment, so remove the bogus comment where installation is not actually happening. Cc: David Howells Cc: Eric W. Biederman Signed-off-by: Kees Cook Acked-by: "Eric W. Biederman" Acked-by: Serge Hallyn --- fs/exec.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index cedae1620d2f..90bd5b85814f 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1238,6 +1238,12 @@ void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec) perf_event_comm(tsk, exec); } +/* + * Calling this is the point of no return. None of the failures will be + * seen by userspace since either the process is already taking a fatal + * signal (via de_thread() or coredump), or will have SEGV raised + * (after exec_mmap()) by search_binary_handlers (see below). + */ int flush_old_exec(struct linux_binprm * bprm) { int retval; @@ -1265,7 +1271,13 @@ int flush_old_exec(struct linux_binprm * bprm) if (retval) goto out; - bprm->mm = NULL; /* We're using it now */ + /* + * After clearing bprm->mm (to mark that current is using the + * prepared mm now), we have nothing left of the original + * process. If anything from here on returns an error, the check + * in search_binary_handler() will SEGV current. + */ + bprm->mm = NULL; set_fs(USER_DS); current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD | @@ -1312,7 +1324,6 @@ void setup_new_exec(struct linux_binprm * bprm) { arch_pick_mmap_layout(current->mm); - /* This is the point of no return */ current->sas_ss_sp = current->sas_ss_size = 0; if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid())) @@ -1330,7 +1341,6 @@ void setup_new_exec(struct linux_binprm * bprm) */ current->mm->task_size = TASK_SIZE; - /* install the new credentials */ if (!uid_eq(bprm->cred->uid, current_euid()) || !gid_eq(bprm->cred->gid, current_egid())) { current->pdeath_signal = 0;