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Date:   Sat, 22 Sep 2018 12:18:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Jiri Kosina <jikos@kernel.org>,
        "Schaufler, Casey" <casey.schaufler@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Woodhouse, David" <dwmw@amazon.co.uk>,
        Andi Kleen <ak@linux.intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>
Subject: Re: [PATCH v6 0/3] Harden spectrev2 userspace-userspace protection
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On Sat, Sep 22, 2018 at 11:53:14AM +0200, Thomas Gleixner wrote:
> @@ -86,6 +88,7 @@ extern void exit_ptrace(struct task_stru
>   * process_vm_writev or ptrace (and should use the real credentials).
>   */
>  extern bool ptrace_may_access(struct task_struct *task, unsigned int mode);
> +extern bool ptrace_may_access_sched(struct task_struct *task, unsigned int mode);

I like that..

>  static inline int ptrace_reparented(struct task_struct *child)
>  {
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c

> +bool ptrace_may_access_sched(struct task_struct *task, unsigned int mode)
> +{
> +	struct mm_struct *mm;
> +	int res;
> +
> +	res = __ptrace_may_access_basic(task, mode);
> +	if (res <= 0)
> +		return !res;
> +
> +	rcu_read_lock();
> +	res = __ptrace_may_access_cred(__task_cred(task), mode);
>  	rcu_read_unlock();
> +	if (res)
> +		return false;
> +
> +	mm = task->mm;
> +	if (mm && get_dumpable(mm) != SUID_DUMP_USER)
> +		return false;
> +	return true;
> +}
> +
> +/* Returns 0 on success, -errno on denial. */
> +static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> +{
> +	const struct cred *tcred;
> +	struct mm_struct *mm;
> +	int res;
> +
> +	res = __ptrace_may_access_basic(task, mode);
> +	if (res <= 0)
> +		return res;
> +
> +	rcu_read_lock();
> +	tcred = __task_cred(task);
> +	res = __ptrace_may_access_cred(tcred, mode);
> +	if (res > 0)
> +		res = ptrace_has_cap(tcred->user_ns, mode) ? 0 : -EPERM;
>  	rcu_read_unlock();
> +	if (res < 0)
> +		return res;
> +
>  	mm = task->mm;
> +	if (mm && (get_dumpable(mm) != SUID_DUMP_USER &&
> +		   !ptrace_has_cap(mm->user_ns, mode)))
> +		return -EPERM;
>  
>  	return security_ptrace_access_check(task, mode);
>  }

This has some unfortunate duplication.

Lets go with it for now, but I'll see if I can do something about that
later.