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Date:   Tue, 15 Dec 2020 20:21:26 -0600
From:   Ted Estes <ted@...twarecrafters.com>
To:     Jann Horn <jannh@...gle.com>,
        "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
Cc:     Jann Horn <jann@...jh.net>, 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>,
        linux-man <linux-man@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Bug 210655] ptrace.2: documentation is incorrect about access
 checking threads in same thread group

On 12/15/2020 6:01 PM, Jann Horn wrote:
> On Wed, Dec 16, 2020 at 12:25 AM Alejandro Colomar (man-pages)
> <alx.manpages@...il.com> wrote:
>> On 12/16/20 12:23 AM, Alejandro Colomar (man-pages) wrote:
>>> 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):
>>>>> 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,
>> Sorry, I should have said "that code never returns 0".
>>
>>> at least when called from ptrace_attach().
> Yes.
>
>>> 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);
>>> [...]
>>> }
> I said exactly that in my last mail:
>
>>>> 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.
> As I said, attaching indeed doesn't work. But that's not what "Ptrace
> access mode checking" means. As the first sentence of that section
> says:
>
> | Various parts of the kernel-user-space API (not just ptrace()
> | operations), require so-called "ptrace access mode" checks,
> | whose outcome determines whether an operation is
> | permitted (or, in a  few cases,  causes  a "read" operation
> | to return sanitized data).
>
> You can find these places by grepping for \bptrace_may_access\b -
> operations like e.g. the get_robust_list() syscall will always succeed
> when inspecting other tasks in the caller's thread group thanks to
> this rule.

Ah, yes.  I missed that back reference while trying to digest that 
rather meaty man page.  A grep on the man page source tree does show a 
number of references to "ptrace access mode".

That said, the ptrace(2) man page also directly references the ptrace 
access mode check under both PTRACE_ATTACH and PTACE_SEIZE:

| Permission to perform a PTRACE_ATTACH is governed by a ptrace | access 
mode PTRACE_MODE_ATTACH_REALCREDS check; see below. As confirmed, the 
"same thread group" rule does not apply to either of those operations. A 
re-wording of rule 1 similar to this might help avoid confusion: 1. If 
the calling thread and the target thread are in the same thread group: 
a. For ptrace() called with PTRACE_ATTACH or PTRACE_SEIZE, access is 
NEVER allowed. b. For all other so-called "ptrace access mode checks", 
access is ALWAYS allowed. --Ted

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