linux-kernel - Re: [PATCH v1] keys: Restrict KEYCTL_SESSION_TO_PARENT according to ptrace_may

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Message-ID: <CAG48ez3DzxGMWN9GDhSqpHrDJnZDg2k=VEMD_DFiET5yDr07rw@mail.gmail.com>
Date: Mon, 29 Jul 2024 15:49:29 +0200
From: Jann Horn <jannh@...gle.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: David Howells <dhowells@...hat.com>, Jarkko Sakkinen <jarkko@...nel.org>, 
	Günther Noack <gnoack@...gle.com>, 
	James Morris <jmorris@...ei.org>, Kees Cook <kees@...nel.org>, Paul Moore <paul@...l-moore.com>, 
	keyrings@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH v1] keys: Restrict KEYCTL_SESSION_TO_PARENT according to ptrace_may_access()

On Mon, Jul 29, 2024 at 2:59 PM Mickaël Salaün <mic@...ikod.net> wrote:
> A process can modify its parent's credentials with
> KEYCTL_SESSION_TO_PARENT when their EUID and EGID are the same.  This
> doesn't take into account all possible access controls.
>
> Enforce the same access checks as for impersonating a process.
>
> The current credentials checks are untouch because they check against
> EUID and EGID, whereas ptrace_may_access() checks against UID and GID.

FWIW, my understanding is that the intended usecase of
KEYCTL_SESSION_TO_PARENT is that command-line tools (like "keyctl
new_session" and "e4crypt new_session") want to be able to change the
keyring of the parent process that spawned them (which I think is
usually a shell?); and Yama LSM, which I think is fairly widely used
at this point, by default prevents a child process from using
PTRACE_MODE_ATTACH on its parent.

I think KEYCTL_SESSION_TO_PARENT is not a great design, but I'm not
sure if we can improve it much without risking some breakage.