Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752906AbdFGXws (ORCPT <rfc822;w@1wt.eu>);
        Wed, 7 Jun 2017 19:52:48 -0400
Received: from wtarreau.pck.nerim.net ([62.212.114.60]:50462 "EHLO 1wt.eu"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1752317AbdFGXDy (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 7 Jun 2017 19:03:54 -0400
From: Willy Tarreau <w@1wt.eu>
To: linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        linux@roeck-us.net
Cc: Aleksa Sarai <asarai@suse.de>, dev@opencontainers.org,
        Al Viro <viro@zeniv.linux.org.uk>, Willy Tarreau <w@1wt.eu>
Subject: [PATCH 3.10 247/250] fs: exec: apply CLOEXEC before changing dumpable task flags
Date: Thu,  8 Jun 2017 01:00:33 +0200
Message-Id: <1496876436-32402-248-git-send-email-w@1wt.eu>
X-Mailer: git-send-email 2.8.0.rc2.1.gbe9624a
In-Reply-To: <1496876436-32402-1-git-send-email-w@1wt.eu>
References: <1496876436-32402-1-git-send-email-w@1wt.eu>
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2788
Lines: 77

From: Aleksa Sarai <asarai@suse.de>

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/<pid>/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 <crosbymichael@gmail.com>
Signed-off-by: Aleksa Sarai <asarai@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
 fs/exec.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index acbd7ac..c945a55 100644
--- 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 <linux/slab.h>
@@ -1091,6 +1091,13 @@ int flush_old_exec(struct linux_binprm * bprm)
 	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:
@@ -1141,7 +1148,6 @@ void setup_new_exec(struct linux_binprm * bprm)
 	current->self_exec_id++;
 			
 	flush_signal_handlers(current, 0);
-	do_close_on_exec(current->files);
 }
 EXPORT_SYMBOL(setup_new_exec);
 
-- 
2.8.0.rc2.1.gbe9624a