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

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Message-Id: <70950159-11e4-40c8-a498-04971b7a4ada@app.fastmail.com>
Date:   Mon, 23 Jan 2023 18:14:03 -0500
From:   "Colin Walters" <walters@...bum.org>
To:     "Giuseppe Scrivano" <gscrivan@...hat.com>
Cc:     linux-kernel@...r.kernel.org, "Kees Cook" <keescook@...omium.org>,
        bristot@...hat.com, "Eric W. Biederman" <ebiederm@...ssion.com>,
        brauner@...nel.org, "Aleksa Sarai" <cyphar@...har.com>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "Alexander Larsson" <alexl@...hat.com>, peterz@...radead.org,
        bmasney@...hat.com
Subject: Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl

On Mon, Jan 23, 2023, at 5:54 PM, Giuseppe Scrivano wrote:
>
> I realize it seems like a one-off fix, but it is done only for backward
> compatibility.
>
> Other paths under /proc/self/map_files require CAP_SYS_ADMIN in the
> initial user namespace, or have CAP_CHECKPOINT_RESTORE in the user
> namespace.  Sure, it is not future-proof, but it would look weird if
> after CVE-2019-19814 there will be more ways to access files from the
> host without requiring some capabilities.

I think a way to rephrase what I'm saying is that it feels like this should be about making /proc/self/exe and /proc/self/map_files consistent.

> With the prctl a runtime would just need to do the following and live
> happily ever after:
>
> __attribute__ ((constructor)) static void hide_self_exe (void)
> {
> 	if (prctl(PR_SET_HIDE_SELF_EXE, 1, 0, 0, 0) == 0)
> 		return;
>
> 	/* ...reexec as we do today... */
> }
>
> and we won't have to worry about what mount options are supported or
> used by proc.

Yeah, OK - having the logical operation be on the process and not the view into it (procfs) definitely is more robust.

But how about calling this PR_SET_PROCFS_RESTRICTED or so, and then *also* changing the /proc/self/map_files lookup to deny if this is set?  Yes, it'd be mostly redundant, but it'd help clarify things for any future changes since it'd be clear that the logic *should* be consistent for /proc/self/exe and /proc/self/map_files.  And actually as a bonus, it would make the case of e.g. `podman run --cap-add=checkpoint_restore` secure right?  (Though honestly I don't know how common that is or whether one can practically use checkpoint_restore without other caps)