Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756255AbcLUF3T (ORCPT ); Wed, 21 Dec 2016 00:29:19 -0500 Received: from mx2.suse.de ([195.135.220.15]:58392 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753821AbcLUF3R (ORCPT ); Wed, 21 Dec 2016 00:29:17 -0500 From: Aleksa Sarai To: oleg@redhat.com, viro@zeniv.linux.org.uk, ebiederm@xmission.com Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, cyphar@cyphar.com, Aleksa Sarai , dev@opencontainers.org, "# v3 . 2+" Subject: [PATCH v2] fs: exec: apply CLOEXEC before changing dumpable task flags Date: Wed, 21 Dec 2016 16:26:24 +1100 Message-Id: <20161221052624.16270-1-asarai@suse.de> X-Mailer: git-send-email 2.11.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2645 Lines: 72 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 Cc: # v3.2+ Reported-by: Michael Crosby Signed-off-by: Aleksa Sarai --- fs/exec.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index 4e497b9ee71e..b0a98ef03253 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 @@ -1266,6 +1266,13 @@ 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: @@ -1315,7 +1322,6 @@ void setup_new_exec(struct linux_binprm * bprm) group */ current->self_exec_id++; flush_signal_handlers(current, 0); - do_close_on_exec(current->files); } EXPORT_SYMBOL(setup_new_exec); -- 2.11.0