linux-kernel - Re: [PATCH v4 2/3] binder: use cred instead of task for getsecid

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Date:   Tue, 12 Oct 2021 10:13:06 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        Todd Kjos <tkjos@...gle.com>, zohar@...ux.ibm.com,
        arve@...roid.com, joel@...lfernandes.org,
        devel@...verdev.osuosl.org,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        James Morris <jmorris@...ei.org>, kernel-team@...roid.com,
        tkjos@...roid.com, keescook@...omium.org, jannh@...gle.com,
        selinux@...r.kernel.org, Eric Paris <eparis@...isplace.org>,
        maco@...roid.com, christian@...uner.io, gregkh@...uxfoundation.org,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v4 2/3] binder: use cred instead of task for getsecid

On Tue, Oct 12, 2021 at 5:41 AM Dan Carpenter <dan.carpenter@...cle.com> wrote:
>
> On Mon, Oct 11, 2021 at 02:59:13PM -0700, Casey Schaufler wrote:
> > On 10/11/2021 2:33 PM, Paul Moore wrote:
> > > On Wed, Oct 6, 2021 at 8:46 PM Todd Kjos <tkjos@...gle.com> wrote:
> > >> Use the 'struct cred' saved at binder_open() to lookup
> > >> the security ID via security_cred_getsecid(). This
> > >> ensures that the security context that opened binder
> > >> is the one used to generate the secctx.
> > >>
> > >> Fixes: ec74136ded79 ("binder: create node flag to request sender's
> > >> security context")
> > >> Signed-off-by: Todd Kjos <tkjos@...gle.com>
> > >> Suggested-by: Stephen Smalley <stephen.smalley.work@...il.com>
> > >> Reported-by: kernel test robot <lkp@...el.com>
> > >> Cc: stable@...r.kernel.org # 5.4+
> > >> ---
> > >> v3: added this patch to series
> > >> v4: fix build-break for !CONFIG_SECURITY
> > >>
> > >>  drivers/android/binder.c | 11 +----------
> > >>  include/linux/security.h |  4 ++++
> > >>  2 files changed, 5 insertions(+), 10 deletions(-)
> > >>
> > >> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > >> index ca599ebdea4a..989afd0804ca 100644
> > >> --- a/drivers/android/binder.c
> > >> +++ b/drivers/android/binder.c
> > >> @@ -2722,16 +2722,7 @@ static void binder_transaction(struct binder_proc *proc,
> > >>                 u32 secid;
> > >>                 size_t added_size;
> > >>
> > >> -               /*
> > >> -                * Arguably this should be the task's subjective LSM secid but
> > >> -                * we can't reliably access the subjective creds of a task
> > >> -                * other than our own so we must use the objective creds, which
> > >> -                * are safe to access.  The downside is that if a task is
> > >> -                * temporarily overriding it's creds it will not be reflected
> > >> -                * here; however, it isn't clear that binder would handle that
> > >> -                * case well anyway.
> > >> -                */
> > >> -               security_task_getsecid_obj(proc->tsk, &secid);
> > >> +               security_cred_getsecid(proc->cred, &secid);
> > >>                 ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
> > >>                 if (ret) {
> > >>                         return_error = BR_FAILED_REPLY;
> > >> diff --git a/include/linux/security.h b/include/linux/security.h
> > >> index 6344d3362df7..f02cc0211b10 100644
> > >> --- a/include/linux/security.h
> > >> +++ b/include/linux/security.h
> > >> @@ -1041,6 +1041,10 @@ static inline void security_transfer_creds(struct cred *new,
> > >>  {
> > >>  }
> > >>
> > >> +static inline void security_cred_getsecid(const struct cred *c, u32 *secid)
> > >> +{
> > >> +}
> > > Since security_cred_getsecid() doesn't return an error code we should
> > > probably set the secid to 0 in this case, for example:
> > >
> > >   static inline void security_cred_getsecid(...)
> > >   {
> > >     *secid = 0;
> > >   }
> >
> > If CONFIG_SECURITY is unset there shouldn't be any case where
> > the secid value is ever used for anything. Are you suggesting that
> > it be set out of an abundance of caution?
>
> The security_secid_to_secctx() function is probably inlined so probably
> KMSan will not warn about this.  But Smatch will warn about passing
> unitialized variables.  You probably wouldn't recieve and email about
> it, and I would just add an exception that security_cred_getsecid()
> should be ignored.

I'd much rather just see the secid set to zero in the !CONFIG_SECURITY case.

-- 
paul moore
www.paul-moore.com