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Date:   Wed, 16 Dec 2020 00:23:16 +0100
From:   "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To:     Jann Horn <jann@...jh.net>
Cc:     Pavel Emelyanov <xemul@...nvz.org>,
        Oleg Nesterov <oleg@...sign.ru>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Ted Estes <ted@...twarecrafters.com>,
        linux-man <linux-man@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [Bug 210655] ptrace.2: documentation is incorrect about access
 checking threads in same thread group

Hi Jann,

On 12/16/20 12:07 AM, Jann Horn wrote:
> Am Tue, Dec 15, 2020 at 06:01:25PM +0100 schrieb Alejandro Colomar (man-pages):
>> Hi,
>>
>> There's a bug report: https://bugzilla.kernel.org/show_bug.cgi?id=210655
>>
>> [[
>> Under "Ptrace access mode checking", the documentation states:
>>   "1. If the calling thread and the target thread are in the same thread
>> group, access is always allowed."
>>
>> This is incorrect. A thread may never attach to another in the same group.
> 
> No, that is correct. ptrace-mode access checks do always short-circuit for
> tasks in the same thread group:
> 
> /* Returns 0 on success, -errno on denial. */
> static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> {
> [...]
>         /* May we inspect the given task?
>          * This check is used both for attaching with ptrace
>          * and for allowing access to sensitive information in /proc.
>          *
>          * ptrace_attach denies several cases that /proc allows
>          * because setting up the necessary parent/child relationship
>          * or halting the specified task is impossible.
>          */
> 
>         /* Don't let security modules deny introspection */
>         if (same_thread_group(task, current))
>                 return 0;
> [...]
> }

AFAICS, that code always returns non-zero,
at least when called from ptrace_attach().

As you can see below,
__ptrace_may_access() is called some lines after
the code pointed to by the bug report.


static int ptrace_attach(struct task_struct *task, long request,
			 unsigned long addr,
			 unsigned long flags)
{
[...]
	if (same_thread_group(task, current))
		goto out;

	/*
	 * Protect exec's credential calculations against our interference;
	 * SUID, SGID and LSM creds get determined differently
	 * under ptrace.
	 */
	retval = -ERESTARTNOINTR;
	if (mutex_lock_interruptible(&task->signal->cred_guard_mutex))
		goto out;

	task_lock(task);
	retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
[...]
}


Thanks,

Alex

> 
> As the comment explains, you can't actually *attach*
> to another task in the same thread group; but that's
> not because of the ptrace-style access check rules,
> but because specifically *attaching* to another task
> in the same thread group doesn't work.
> 

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