From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937303AbdADVIQ (ORCPT ); Wed, 4 Jan 2017 16:08:16 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:34928 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S937229AbdADVHm (ORCPT ); Wed, 4 Jan 2017 16:07:42 -0500 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, dev@opencontainers.org, Michael Crosby , Aleksa Sarai , Al Viro Subject: [PATCH 4.8 39/85] fs: exec: apply CLOEXEC before changing dumpable task flags Date: Wed, 4 Jan 2017 21:47:24 +0100 Message-Id: <20170104200705.126969858@linuxfoundation.org> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170104200703.349648590@linuxfoundation.org> References: <20170104200703.349648590@linuxfoundation.org> User-Agent: quilt/0.65 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 4.8-stable review patch. If anyone has any objections, please let me know. ------------------ From: Aleksa Sarai commit 613cc2b6f272c1a8ad33aefa21cad77af23139f7 upstream. If you have a process that has set itself to be non-dumpable, and it then undergoes exec(2), any CLOEXEC file descriptors it has open are "exposed" during a race window between the dumpable flags of the process being reset for exec(2) and CLOEXEC being applied to the file descriptors. This can be exploited by a process by attempting to access /proc//fd/... during this window, without requiring CAP_SYS_PTRACE. The race in question is after set_dumpable has been (for get_link, though the trace is basically the same for readlink): [vfs] -> proc_pid_link_inode_operations.get_link -> proc_pid_get_link -> proc_fd_access_allowed -> ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS); Which will return 0, during the race window and CLOEXEC file descriptors will still be open during this window because do_close_on_exec has not been called yet. As a result, the ordering of these calls should be reversed to avoid this race window. This is of particular concern to container runtimes, where joining a PID namespace with file descriptors referring to the host filesystem can result in security issues (since PRCTL_SET_DUMPABLE doesn't protect against access of CLOEXEC file descriptors -- file descriptors which may reference filesystem objects the container shouldn't have access to). Cc: dev@opencontainers.org Reported-by: Michael Crosby Signed-off-by: Aleksa Sarai Signed-off-by: Al Viro Signed-off-by: Greg Kroah-Hartman --- fs/exec.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --- a/fs/exec.c +++ b/fs/exec.c @@ -19,7 +19,7 @@ * current->executable is only used by the procfs. This allows a dispatch * table to check for several different types of binary formats. We keep * trying until we recognize the file or we run out of supported binary - * formats. + * formats. */ #include @@ -1261,6 +1261,13 @@ int flush_old_exec(struct linux_binprm * flush_thread(); current->personality &= ~bprm->per_clear; + /* + * We have to apply CLOEXEC before we change whether the process is + * dumpable (in setup_new_exec) to avoid a race with a process in userspace + * trying to access the should-be-closed file descriptors of a process + * undergoing exec(2). + */ + do_close_on_exec(current->files); return 0; out: @@ -1323,7 +1330,6 @@ void setup_new_exec(struct linux_binprm group */ current->self_exec_id++; flush_signal_handlers(current, 0); - do_close_on_exec(current->files); } EXPORT_SYMBOL(setup_new_exec);