Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754743Ab3JBQqn (ORCPT ); Wed, 2 Oct 2013 12:46:43 -0400 Received: from mail-la0-f47.google.com ([209.85.215.47]:40680 "EHLO mail-la0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754274Ab3JBQql (ORCPT ); Wed, 2 Oct 2013 12:46:41 -0400 MIME-Version: 1.0 In-Reply-To: <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> <20131002151430.GB2669@dztty> From: Andy Lutomirski Date: Wed, 2 Oct 2013 17:46:19 +0100 Message-ID: Subject: Re: [PATCH v2 6/9] procfs: add permission checks on the file's opener of /proc/*/stat To: Djalal Harouni Cc: "Eric W. Biederman" , Kees Cook , Al Viro , Andrew Morton , Linus Torvalds , Ingo Molnar , "Serge E. Hallyn" , Cyrill Gorcunov , David Rientjes , LKML , Linux FS Devel , kernel-hardening@lists.openwall.com, Djalal Harouni Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4118 Lines: 95 On Wed, Oct 2, 2013 at 4:14 PM, Djalal Harouni wrote: > 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 Never mind, then -- I read that wrong... > > 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" ? I mean to move the entire "check current->cred and f_cred" check into its own function rather than duplicating it at each call site. --Andy -- 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/