Re: [PATCH v2 6/9] procfs: add permission checks on the file's opener of /proc/*/stat

Djalal Harouni
2013-10-02T19:00:42Z

On Wed, Oct 02, 2013 at 05:46:19PM +0100, Andy Lutomirski wrote:
> On Wed, Oct 2, 2013 at 4:14 PM, Djalal Harouni <tixxdz@opendz.org> 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 <keescook@chromium.org>
> >> > Cc: Eric W. Biederman <ebiederm@xmission.com>
> >> > Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
> >> > ---
> >> > 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.
We can perhaps do this, yes.

In other places the check is done and protected by
lock_trace()/unlock_trace(). Will need to see if we can do it.

> --Andy

--
Djalal Harouni