linux-kernel - Re: [PATCH v4 3/3] binder: use euid from cred instead of using task

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Date:   Mon, 11 Oct 2021 17:39:07 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Todd Kjos <tkjos@...gle.com>
Cc:     casey@...aufler-ca.com, gregkh@...uxfoundation.org,
        arve@...roid.com, tkjos@...roid.com, maco@...roid.com,
        christian@...uner.io, James Morris <jmorris@...ei.org>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>, keescook@...omium.org,
        jannh@...gle.com, Jeffrey Vander Stoep <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 v4 3/3] binder: use euid from cred instead of using task

On Fri, Oct 8, 2021 at 5:24 PM Todd Kjos <tkjos@...gle.com> wrote:
>
> On Fri, Oct 8, 2021 at 2:12 PM Paul Moore <paul@...l-moore.com> wrote:
> >
> > On Wed, Oct 6, 2021 at 8:46 PM Todd Kjos <tkjos@...gle.com> wrote:
> > >
> > > Set a transaction's sender_euid from the 'struct cred'
> > > saved at binder_open() instead of looking up the euid
> > > from the binder proc's 'struct task'. This ensures
> > > the euid is associated with the security context that
> > > of the task that opened binder.
> > >
> > > Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> > > Signed-off-by: Todd Kjos <tkjos@...gle.com>
> > > Suggested-by: Stephen Smalley <stephen.smalley.work@...il.com>
> > > Cc: stable@...r.kernel.org # 4.4+
> > > ---
> > > v3: added this patch to series
> > >
> > >  drivers/android/binder.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > This is an interesting ordering of the patches.  Unless I'm missing
> > something I would have expected patch 3/3 to come first, followed by
> > 2/3, with patch 1/3 at the end; basically the reverse of what was
> > posted here.
>
> 2/3 and 3/3 both depend on 1/3 (add "cred" member of struct
> binder_proc). I kept that in 1/3 to keep that patch the same as what
> had already been reviewed. I didn't think much about the ordering
> between 2/3 and 3/3 -- but I agree that it would have been sensible to
> reverse their order.
>
> >
> > My reading of the previous thread was that Casey has made his peace
> > with these changes so unless anyone has any objections I'll plan on
> > merging 2/3 and 3/3 into selinux/stable-5.15 and merging 1/3 into
> > selinux/next.
>
> Thanks Paul. I'm not familiar with the branch structure, but you need
> 1/3 in selinux/stable-5.15 to resolve the dependency on proc->cred.

Yep, thanks.  My eyes kinda skipped over that part when looking at the
patchset but that would have fallen out as soon as I merged them.

Unfortunately that pretty much defeats the purpose of splitting this
into three patches.  While I suppose one could backport patches 2/3
and 3/3 individually, both of them have a very small footprint
especially considering their patch 1/3 dependency.  At the very least
it looks like patch 2/3 needs to be respun to address the
!CONFIG_SECURITY case and seeing the split patches now I think the
smart thing is to just combine them into a single patch.  I apologize
for the bad recommendation earlier, I should have followed that thread
a bit closer after the discussion with Casey and Stephen.

-- 
paul moore
www.paul-moore.com