linux-kernel - Re: Documenting ptrace access mode checking

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Message-ID: <33209000-3fd4-0b26-f249-eb4e1a9e1651@gmail.com>
Date:	Thu, 23 Jun 2016 09:42:09 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Jann Horn <jann@...jh.net>
Cc:	mtk.manpages@...il.com, James Morris <jmorris@...ei.org>,
	linux-man <linux-man@...r.kernel.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	lkml <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: Documenting ptrace access mode checking

Hi Jann,

Thanks for your further review. Follow-up of one point below.

On 06/23/2016 12:44 AM, Jann Horn wrote:
> On Wed, Jun 22, 2016 at 09:21:29PM +0200, Michael Kerrisk (man-pages) wrote:
>> On 06/21/2016 10:55 PM, Jann Horn wrote:
>>> On Tue, Jun 21, 2016 at 11:41:16AM +0200, Michael Kerrisk (man-pages) wrote:

[...]

>>>>       The  algorithm  employed for ptrace access mode checking deter‐
>>>>       mines whether the calling process is  allowed  to  perform  the
>>>>       corresponding action on the target process, as follows:
>>>>
>>>>       1.  If the calling thread and the target thread are in the same
>>>>           thread group, access is always allowed.
>>>>
>>>>       2.  If the access mode specifies PTRACE_MODE_FSCREDS, then  for
>>>>           the  check in the next step, employ the caller's filesystem
>>>>           user ID and group ID (see credentials(7));  otherwise  (the
>>>>           access  mode  specifies  PTRACE_MODE_REALCREDS, so) use the
>>>>           caller's real user ID and group ID.
>>>
>>> Might want to add a "for historical reasons" or so here.
>>
>> Can you be a little more precise about "here", and maybe tell me why
>> you think it helps?
>
> I'm not sure, but it might be a good idea to add something like this at the
> end of 2.:
> "(Most other APIs that check one of the caller's UIDs use the effective one.
> This API uses the real UID instead for historical reasons.)"
>
> In my opinion, it is inconsistent to use the real UID/GID here, the
> effective one would be more appropriate. But since the existing code uses
> the real UID/GID and that's not a security issue for existing users of
> the ptrace API, this wasn't changed when I added the REALCREDS/FSCREDS
> distinction.
>
> I think that for a reader, it might help to point out that in most cases,
> when a process is the subject in an access check, its effective UID/GID
> are used, and this is (together with kill()) an exception to that rule.
> But you're the expert on writing documentation, if you think that that's
> too much detail / confusing here, it probably is.

Okay -- got it now, I think. I made this text:

        2.  If the access mode specifies PTRACE_MODE_FSCREDS, then, for
            the check in the next step, employ the caller's  filesystem
            UID  and  GID.  (As noted in credentials(7), the filesystem
            UID and GID almost always have the same values as the  cor‐
            responding effective IDs.)

            Otherwise, the access mode specifies PTRACE_MODE_REALCREDS,
            so use the caller's real UID and GID for the checks in  the
            next  step.  (Most APIs that check the caller's UID and GID
            use  the  effective  IDs.   For  historical  reasons,   the
            PTRACE_MODE_REALCREDS check uses the real IDs instead.)

[...]

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/