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

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Message-ID: <CAEjxPJ5YT36ZvrN6uSDOCNv3pYrWBzcutsnSjSzya-5e0v9Rpw@mail.gmail.com>
Date:   Tue, 12 Oct 2021 08:24:33 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Todd Kjos <tkjos@...gle.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Greg Kroah-Hartman <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>,
        Eric Paris <eparis@...isplace.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        SElinux list <selinux@...r.kernel.org>,
        devel@...verdev.osuosl.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v4 3/3] binder: use euid from cred instead of using task

On Mon, Oct 11, 2021 at 7:39 PM Todd Kjos <tkjos@...gle.com> wrote:
>
> On Mon, Oct 11, 2021 at 2:39 PM Paul Moore <paul@...l-moore.com> wrote:
> >
> > 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.
>
> I'm happy to submit a single patch for all of this. Another part of
> the rationale
> for splitting it into 3 patches was correctly identify the patch that introduced
> the patch that introduced the issue -- so each of the 3 had a different
> "Fixes:" tag. Should I cite the oldest (binder introduction) with the "Fixes"
> tag and perhaps mention the other two in the commit message?

Couldn't you just split patch 1 into the "add cred to binder proc"
part and "use cred in LSM/SELinux hooks" part, combine patch 3 with
the "add cred to binder proc" part to create new patch 1, then "use
cred in LSM/SELinux hooks" part is patch 2, and "switch task_getsecid
to cred_getsecid" to patch 3? Then patch 1 can be cherry-picked/ported
all the way back to the introduction of binder, patch 2 all the way
back to the introduction of binder LSM/SELinux hooks, and patch 3 just
back to where passing the secctx across binder was introduced.