Re: [ISSUE] mm: Add a user_ns owner to mm_struct and fix ptrace_may_access

[Andrey Vagin <avagin@virtuozzo.com>]

On Mon, Oct 24, 2016 at 01:59:59PM +0300, Cyrill Gorcunov wrote:
> Hi Eric! A few days ago we've noticed that our zombie00 test case started
> failing: https://ci.openvz.org/job/CRIU/view/All/job/CRIU-linux-next/406/console
> ---
> ======================== Run zdtm/static/zombie00 in h =========================
> Start test
> ./zombie00 --pidfile=zombie00.pid --outfile=zombie00.out
> Run criu dump
> Run criu restore
> Send the 15 signal to  30
> Wait for zdtm/static/zombie00(30) to die for 0.100000
> ################ Test zdtm/static/zombie00 FAIL at result check ################
> 
> I've narrowed problem down to commit
> 
>  | From ce99dd5fd5f600f9f4f0d37bb8847c1cb7c6e4fc Mon Sep 17 00:00:00 2001
>  | From: "Eric W. Biederman" <ebiederm@xmission.com>
>  | Date: Thu, 13 Oct 2016 21:23:16 -0500
>  | Subject: [PATCH] mm: Add a user_ns owner to mm_struct and fix
>  |  ptrace_may_access
>  |
>  | During exec dumpable is cleared if the file that is being executed is
>  | not readable by the user executing the file.  A bug in
>  | ptrace_may_access allows reading the file if the executable happens to
>  | enter into a subordinate user namespace (aka clone(CLONE_NEWUSER),
>  | unshare(CLONE_NEWUSER), or setns(fd, CLONE_NEWUSER).
> 
> and the reason is that the zombie tasks do not have task::mm and in resut
> we're obtaining -EPERM when trying to read task->exit_code from /proc/pid/stat.

To be precise,
we are obtaining 0 instead of task->exit_ode, because permitted is
false:

static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,

...

        permitted = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS |
PTRACE_MODE_NOAUDIT);

...

        if (permitted)
                seq_put_decimal_ll(m, " ", task->exit_code);
        else
                seq_puts(m, " 0");

> 
> Looking into commit I suspect when mm = NULL we've to move back the test
> ptrace_has_cap(__task_cred(task)->user_ns, mode)?