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

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Message-ID: <20240729.rayi3Chi9aef@digikod.net>
Date: Mon, 29 Jul 2024 17:02:41 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Jann Horn <jannh@...gle.com>
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 04:21:01PM +0200, Jann Horn wrote:
> On Mon, Jul 29, 2024 at 4:09 PM Mickaël Salaün <mic@...ikod.net> wrote:
> > On Mon, Jul 29, 2024 at 03:49:29PM +0200, Jann Horn wrote:
> > > 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.
> >
> > About Yama, the patched keyctl_session_to_parent() function already
> > check if the current's and the parent's credentials are the same before
> > this new ptrace_may_access() check.
> 
> prepare_exec_creds() in execve() always creates new credentials which
> are stored in bprm->cred and then later committed in begin_new_exec().
> Also, fork() always copies the credentials in copy_creds().
> So the "mycred == pcred" condition in keyctl_session_to_parent()
> basically never applies, I think.
> Also: When that condition is true, the whole operation is a no-op,
> since if the credentials are the same, then the session keyring that
> the credentials point to must also be the same.

Correct, it's not a content comparison.  We could compare the
credential's data for this specific KEYCTL_SESSION_TO_PARENT call, I
guess this should not be performance sensitive.