Subject: Re: [RESEND PATCH] Allow passing tid or pid in SCM_CREDENTIALS without CAP_SYS_ADMIN

From: [email-masked] (Eric W. Biederman)
Date: Tue, 29 Aug 2017 19:10:50 -0500

"prakash.sangappa" <[email-masked]> writes:

> On 08/29/2017 04:02 PM, David Miller wrote:
>> From: Prakash Sangappa <[email-masked]>
>> Date: Mon, 28 Aug 2017 17:12:20 -0700
>>
>>> Currently passing tid(gettid(2)) of a thread in struct ucred in
>>> SCM_CREDENTIALS message requires CAP_SYS_ADMIN capability otherwise
>>> it fails with EPERM error. Some applications deal with thread id
>>> of a thread(tid) and so it would help to allow tid in SCM_CREDENTIALS
>>> message. Basically, either tgid(pid of the process) or the tid of
>>> the thread should be allowed without the need for CAP_SYS_ADMIN capability.
>>>
>>> SCM_CREDENTIALS will be used to determine the global id of a process or
>>> a thread running inside a pid namespace.
>>>
>>> This patch adds necessary check to accept tid in SCM_CREDENTIALS
>>> struct ucred.
>>>
>>> Signed-off-by: Prakash Sangappa <[email-masked]>
>> I'm pretty sure that by the descriptions in previous changes to this
>> function, what you are proposing is basically a minor form of PID
>> spoofing which we only want someone with CAP_SYS_ADMIN over the
>> PID namespace to be able to do.
>
> The fix is to allow passing tid of the calling thread itself not of any
> other thread or process. Curious why would this be considered
> as pid spoofing?
>
> This change would enable a thread in a multi threaded process, running
> inside a pid namespace to be identified by the recipient of the
> message easily.

I think a more practical problem is that change, changes what is being
passed in the SCM_CREDENTIALS from a pid of a process to a tid of a
thread.  That could be confusing and that confusion could be exploited.

It is definitely confusing because in some instances a value can be both
a tgid and a tid.

I definitely think this needs to be talked about in terms of changing
what is passed in that field and what the consequences could be.

I suspect you are ok.  As nothing allows passing a tid today.  But I
don't see any analysis on why passing a tid instead of a tgid will not
confuse the receiving application, and in such confusion introduce a
security hole.

Eric