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Subject: Re: [PATCH] proc: /proc/<pid>/timerslack_ns permissions fixes
From: Kees Cook <keescook@chromium.org>
To: John Stultz <john.stultz@linaro.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arjan van de Ven <arjan@linux.intel.com>,
        lkml <linux-kernel@vger.kernel.org>, Oren Laadan <orenl@cellrox.com>,
        Ruchi Kandoi <kandoiruchi@google.com>,
        Rom Lemarchand <romlem@android.com>,
        Android Kernel Team <kernel-team@android.com>
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On Wed, Feb 17, 2016 at 9:59 PM, John Stultz <john.stultz@linaro.org> wrote:
> This patch adjusts the timerslack_ns file permissions to be
> 0666 but requires PTRACE_MODE_ATTACH_FSCREDS to read or write
> the value.
>
> This allows tasks with sufficient privledges (CAP_SYS_PTRACE)
> to be able to modify a the timerslack for proccesses owned by
> a different user.
>
> This patch also fixes a return value from EINVAL to EPERM,
> and does task locking consistently, given we're handling u64s
> on 32bit systems. It also makes use of kstrtoull_from_user
> which simplifies some code.
>
> Cc: Arjan van de Ven <arjan@linux.intel.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Oren Laadan <orenl@cellrox.com>
> Cc: Ruchi Kandoi <kandoiruchi@google.com>
> Cc: Rom Lemarchand <romlem@android.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Android Kernel Team <kernel-team@android.com>
> Signed-off-by: John Stultz <john.stultz@linaro.org>

Acked-by: Kees Cook <keescook@chromium.org>

> ---
> This patch applies on top of the previous two patches
> which Andrew already added to -mm. It can be folded
> down or kept separate as desired.

Probably best to fold them together.

-Kees

>
> I've also wired up the Android userspace side to use
> this interface, and tested it there, and things seem
> to be working properly ( - with some selinux noise, I
> still need to figure out the selinux policy changes,
> but its working with permissive mode).
>
>  fs/proc/base.c | 28 +++++++++++++---------------
>  1 file changed, 13 insertions(+), 15 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index d7c51ca..35f583a 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2262,18 +2262,10 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
>  {
>         struct inode *inode = file_inode(file);
>         struct task_struct *p;
> -       char buffer[PROC_NUMBUF];
>         u64 slack_ns;
>         int err;
>
> -       memset(buffer, 0, sizeof(buffer));
> -       if (count > sizeof(buffer) - 1)
> -               count = sizeof(buffer) - 1;
> -
> -       if (copy_from_user(buffer, buf, count))
> -               return -EFAULT;
> -
> -       err = kstrtoull(strstrip(buffer), 10, &slack_ns);
> +       err = kstrtoull_from_user(buf, count, 10, &slack_ns);
>         if (err < 0)
>                 return err;
>
> @@ -2282,12 +2274,14 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
>                 return -ESRCH;
>
>         if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> +               task_lock(p);
>                 if (slack_ns == 0)
>                         p->timer_slack_ns = p->default_timer_slack_ns;
>                 else
>                         p->timer_slack_ns = slack_ns;
> +               task_unlock(p);
>         } else
> -               count = -EINVAL;
> +               count = -EPERM;
>
>         put_task_struct(p);
>
> @@ -2298,18 +2292,22 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>  {
>         struct inode *inode = m->private;
>         struct task_struct *p;
> +       int err =  0;
>
>         p = get_proc_task(inode);
>         if (!p)
>                 return -ESRCH;
>
> -       task_lock(p);
> -       seq_printf(m, "%llu\n", p->timer_slack_ns);
> -       task_unlock(p);
> +       if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> +               task_lock(p);
> +               seq_printf(m, "%llu\n", p->timer_slack_ns);
> +               task_unlock(p);
> +       } else
> +               err = -EPERM;
>
>         put_task_struct(p);
>
> -       return 0;
> +       return err;
>  }
>
>  static int timerslack_ns_open(struct inode *inode, struct file *filp)
> @@ -2899,7 +2897,7 @@ static const struct pid_entry tgid_base_stuff[] = {
>  #ifdef CONFIG_CHECKPOINT_RESTORE
>         REG("timers",     S_IRUGO, proc_timers_operations),
>  #endif
> -       REG("timerslack_ns", S_IRUGO|S_IWUSR, proc_pid_set_timerslack_ns_operations),
> +       REG("timerslack_ns", S_IRUGO|S_IWUGO, proc_pid_set_timerslack_ns_operations),
>  };
>
>  static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
> --
> 1.9.1
>



-- 
Kees Cook
Chrome OS & Brillo Security