linux-kernel - Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF

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Date:   Sun, 29 Jan 2023 13:12:45 -0500
From:   "Colin Walters" <walters@...bum.org>
To:     "Christian Brauner" <brauner@...nel.org>
Cc:     "Giuseppe Scrivano" <gscrivan@...hat.com>,
        "Aleksa Sarai" <cyphar@...har.com>, linux-kernel@...r.kernel.org,
        "Kees Cook" <keescook@...omium.org>, bristot@...hat.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "Alexander Larsson" <alexl@...hat.com>,
        "Peter Zijlstra" <peterz@...radead.org>, bmasney@...hat.com
Subject: Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl



On Sun, Jan 29, 2023, at 11:58 AM, Christian Brauner wrote:
> On Sun, Jan 29, 2023 at 08:59:32AM -0500, Colin Walters wrote:
>> 
>> 
>> On Wed, Jan 25, 2023, at 11:30 AM, Giuseppe Scrivano wrote:
>> > 
>> > After reading some comments on the LWN.net article, I wonder if
>> > PR_HIDE_SELF_EXE should apply to CAP_SYS_ADMIN in the initial user
>> > namespace or if in this case root should keep the privilege to inspect
>> > the binary of a process.  If a container runs with that many privileges
>> > then it has already other ways to damage the host anyway.
>> 
>> Right, that's what I was trying to express with the "make it work the same as map_files".  Hiding the entry entirely even for initial-namespace-root (real root) seems like it's going to potentially confuse profiling/tracing/debugging tools for no good reason.
>
> If this can be circumvented via CAP_SYS_ADMIN 

To be clear, I'm proposing CAP_SYS_ADMIN in the current user namespace at the time of the prctl().  (Or if keeping around a reference just for this is too problematic, perhaps hardcoding to the init ns)

A process with CAP_SYS_ADMIN in a child namespace would still not be able to read the binary.

> then this mitigation
> becomes immediately way less interesting because the userspace
> mitigation we came up with protects against CAP_SYS_ADMIN as well
> without any regression risk. 

The userspace mitigation here being "clone self to memfd"?  But that's a sufficiently ugly workaround that it's created new problems; see https://lwn.net/Articles/918106/