Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754098Ab3JBPOn (ORCPT ); Wed, 2 Oct 2013 11:14:43 -0400 Received: from numidia.opendz.org ([98.142.220.152]:47596 "EHLO numidia.opendz.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752949Ab3JBPOj (ORCPT ); Wed, 2 Oct 2013 11:14:39 -0400 Date: Wed, 2 Oct 2013 16:14:30 +0100 From: Djalal Harouni To: Andy Lutomirski Cc: "Eric W. Biederman" , Kees Cook , Al Viro , Andrew Morton , Linus Torvalds , Ingo Molnar , "Serge E. Hallyn" , Cyrill Gorcunov , David Rientjes , LKML , linux-fsdevel@vger.kernel.org, kernel-hardening@lists.openwall.com, tixxdz@gmail.com Subject: Re: [PATCH v2 6/9] procfs: add permission checks on the file's opener of /proc/*/stat Message-ID: <20131002151430.GB2669@dztty> References: <1380659178-28605-1-git-send-email-tixxdz@opendz.org> <1380659178-28605-7-git-send-email-tixxdz@opendz.org> <524B7934.9070606@amacapital.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <524B7934.9070606@amacapital.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4051 Lines: 98 On Tue, Oct 01, 2013 at 06:39:00PM -0700, Andy Lutomirski wrote: > On 10/01/2013 01:26 PM, Djalal Harouni wrote: > > Some fields of the /proc/*/stat are sensitive fields that need > > appropriate protection. > > > > However, /proc file descriptors can be passed to a more privileged > > process (e.g. a suid-exec) which will pass the classic > > ptrace_may_access() permission check during read(). > > > > To prevent it, use proc_same_open_cred() to detect if current's cred > > have changed between ->open() and ->read(), if so, call > > proc_allow_access() to check if the original file's opener had enough > > permissions to read these sensitive fields. This will prevent passing > > file descriptors to a more privileged process to leak data. > > > > The patch also adds a previously missing signal->cred_guard_mutex lock. > > > > This patch does not break userspace since it only hides the fields that > > were supposed to be protected. > > > > Cc: Kees Cook > > Cc: Eric W. Biederman > > Signed-off-by: Djalal Harouni > > --- > > fs/proc/array.c | 16 ++++++++++++++-- > > 1 file changed, 14 insertions(+), 2 deletions(-) > > > > diff --git a/fs/proc/array.c b/fs/proc/array.c > > index cbd0f1b..f034e05 100644 > > --- a/fs/proc/array.c > > +++ b/fs/proc/array.c > > @@ -394,7 +394,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns, > > char state; > > pid_t ppid = 0, pgid = -1, sid = -1; > > int num_threads = 0; > > - int permitted; > > + int permitted = 0; > > struct mm_struct *mm; > > unsigned long long start_time; > > unsigned long cmin_flt = 0, cmaj_flt = 0; > > @@ -404,10 +404,22 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns, > > unsigned long rsslim = 0; > > char tcomm[sizeof(task->comm)]; > > unsigned long flags; > > + struct file *file = m->private; > > + int same_cred = proc_same_open_cred(file->f_cred); > > + unsigned int ptrace_mode = PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT; > > > > state = *get_task_state(task); > > vsize = eip = esp = 0; > > - permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT); > > + > > + if (!mutex_lock_killable(&task->signal->cred_guard_mutex)) { > > + permitted = ptrace_may_access(task, ptrace_mode); > > + if (permitted && !same_cred) > > + permitted = proc_allow_access(file->f_cred, > > + task, ptrace_mode); > > + > > + mutex_unlock(&task->signal->cred_guard_mutex); > > + } > > + > > else permitted = false? permitted is initialized to 0 First the original ptrace_may_access() check did not hold cred_guard_mutex, so add it. If we can't grab mutex then let permitted to be zero. Yes this a change in behaviour and I think it's correct, IOW we were not able to perform the ptrace_may_access() check, otherwise permitted will depend on checks result. However, there is still a race here since we set the permitted value before gathering the appropriate info about task. At the read() data moment this target task may have been gone privileged... , acquiring an X lock on target task, will just break/slow things, as it has been shown before... Not to mention that the race window is small... > But surely this would be *much* more comprehensible if you had > proc_allow_access do the entire check. I don't understand what you mean by "do the entire check" ? the /proc/pid/stat file is used by "ps", "top" ... it's a vital file We don't want to break it, otherwise "ps" will hide processes. So just do the ptrace_may_access() check correctly with the help of proc_allow_access() and set the permitted variable only. The patch did not touch /proc/pid/stat fields. I've done tests on ps and top, and they work fine. -- Djalal Harouni http://opendz.org -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/