binder: use euid from cred instead of using task

binder: use euid from cred instead of using task

commit 29bc22ac5e5bc63275e850f0c8fc549e3d0e306b upstream.

Save the 'struct cred' associated with a binder process
at initial open to avoid potential race conditions
when converting to an euid.

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.

Mot-CRs-fixed: (CR)
CVE-Fixed: CVE-2021-39686
Bug: 200688826

Change-Id: Ia308ffe189b5e6f740c310d4d9d5e39451fcaed4
Cc: stable@vger.kernel.org # 4.4+
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Signed-off-by: Todd Kjos <tkjos@google.com>
Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Suggested-by: Jann Horn <jannh@google.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Gajjala Chakradhar <gajjalac@motorola.com>
Reviewed-on: https://gerrit.mot.com/2197695
SME-Granted: SME Approvals Granted
SLTApproved: Slta Waiver
Tested-by: Jira Key
Reviewed-by: Xiangpo Zhao <zhaoxp3@motorola.com>
Submit-Approved: Jira Key