From: "Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>
To: Jann Horn <jann@thejh.net>
Cc: Pavel Emelyanov <xemul@openvz.org>,
Oleg Nesterov <oleg@tv-sign.ru>,
Andrew Morton <akpm@linux-foundation.org>,
Michael Kerrisk <mtk.manpages@gmail.com>,
Kees Cook <keescook@chromium.org>,
Ted Estes <ted@softwarecrafters.com>,
linux-man <linux-man@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Jann Horn <jannh@google.com>
Subject: Re: [Bug 210655] ptrace.2: documentation is incorrect about access checking threads in same thread group
Date: Wed, 16 Dec 2020 00:25:13 +0100 [thread overview]
Message-ID: <d38c5e8b-1653-d89a-a0c8-b95cb1844fba@gmail.com> (raw)
In-Reply-To: <0e5189c0-9e9b-ac34-825c-619a9a6ef682@gmail.com>
On 12/16/20 12:23 AM, Alejandro Colomar (man-pages) wrote:
> 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,
Sorry, I should have said "that code never returns 0".
> 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.
>>
next prev parent reply other threads:[~2020-12-16 0:08 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-15 17:01 [Bug 210655] ptrace.2: documentation is incorrect about access checking threads in same thread group Alejandro Colomar (man-pages)
2020-12-15 18:31 ` Ted Estes
2020-12-15 18:34 ` Alejandro Colomar (man-pages)
2020-12-15 22:48 ` Alejandro Colomar (man-pages)
2020-12-16 0:55 ` Linus Torvalds
2020-12-16 7:34 ` Oleg Nesterov
2020-12-15 23:07 ` Jann Horn
2020-12-15 23:23 ` Alejandro Colomar (man-pages)
2020-12-15 23:25 ` Alejandro Colomar (man-pages) [this message]
2020-12-16 0:01 ` Jann Horn
2020-12-16 2:21 ` Ted Estes
2020-12-16 2:33 ` Jann Horn
2020-12-16 9:22 ` Alejandro Colomar (man-pages)
2020-12-17 18:45 ` Eric W. Biederman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=d38c5e8b-1653-d89a-a0c8-b95cb1844fba@gmail.com \
--to=alx.manpages@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=jann@thejh.net \
--cc=jannh@google.com \
--cc=keescook@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-man@vger.kernel.org \
--cc=mtk.manpages@gmail.com \
--cc=oleg@tv-sign.ru \
--cc=ted@softwarecrafters.com \
--cc=xemul@openvz.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).