linux-kernel - Re: [PATCH v2] binder: use cred instead of task for selinux checks

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Date:   Fri, 1 Oct 2021 21:50:58 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Todd Kjos <tkjos@...gle.com>, gregkh@...uxfoundation.org,
        arve@...roid.com, tkjos@...roid.com, maco@...roid.com,
        christian@...uner.io, jmorris@...ei.org, serge@...lyn.com,
        paul@...l-moore.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org, keescook@...omium.org, jeffv@...gle.com,
        zohar@...ux.ibm.com, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org, joel@...lfernandes.org,
        kernel-team@...roid.com, stable@...r.kernel.org
Subject: Re: [PATCH v2] binder: use cred instead of task for selinux checks

On Fri, Oct 1, 2021 at 9:36 PM Jann Horn <jannh@...gle.com> wrote:
> On Fri, Oct 1, 2021 at 8:46 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
> > On 10/1/2021 10:55 AM, Todd Kjos wrote:
> > > Save the struct cred associated with a binder process
> > > at initial open to avoid potential race conditions
> > > when converting to a security ID.
> > >
> > > Since binder was integrated with selinux, it has passed
> > > 'struct task_struct' associated with the binder_proc
> > > to represent the source and target of transactions.
> > > The conversion of task to SID was then done in the hook
> > > implementations. It turns out that there are race conditions
> > > which can result in an incorrect security context being used.
> >
> > In the LSM stacking patch set I've been posting for a while
> > (on version 29 now) I use information from the task structure
> > to ensure that the security information passed via the binder
> > interface is agreeable to both sides. Passing the cred will
> > make it impossible to do this check. The task information
> > required is not appropriate to have in the cred.
>
> Why not? Why can't you put the security identity of the task into the creds?

Ah, I get it now, you're concerned about different processes wanting
to see security contexts formatted differently (e.g. printing the
SELinux label vs printing the AppArmor label), right?

But still, I don't think you can pull that information from the
receiving task. Maybe the easiest solution would be to also store that
in the creds? Or you'd have to manually grab that information when
/dev/binder is opened.