[PATCH 4.4 1/2] binder: use euid from cred instead of using task

[Todd Kjos <tkjos@google.com>]

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.

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>
Change-Id: I91922e7f359df5901749f1b09094c3c68d45aed4
---
 drivers/android/binder.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 3bc5a7caccbf..0b27238b42c7 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -303,6 +303,7 @@ struct binder_proc {
 	struct mm_struct *vma_vm_mm;
 	struct task_struct *tsk;
 	struct files_struct *files;
+	const struct cred *cred;
 	struct hlist_node deferred_work_node;
 	int deferred_work;
 	void *buffer;
@@ -1493,7 +1494,7 @@ static void binder_transaction(struct binder_proc *proc,
 		t->from = thread;
 	else
 		t->from = NULL;
-	t->sender_euid = task_euid(proc->tsk);
+	t->sender_euid = proc->cred->euid;
 	t->to_proc = target_proc;
 	t->to_thread = target_thread;
 	t->code = tr->code;
@@ -3015,6 +3016,7 @@ static int binder_open(struct inode *nodp, struct file *filp)
 		return -ENOMEM;
 	get_task_struct(current->group_leader);
 	proc->tsk = current->group_leader;
+	proc->cred = get_cred(filp->f_cred);
 	INIT_LIST_HEAD(&proc->todo);
 	init_waitqueue_head(&proc->wait);
 	proc->default_priority = task_nice(current);
@@ -3220,6 +3222,7 @@ static void binder_deferred_release(struct binder_proc *proc)
 	}
 
 	put_task_struct(proc->tsk);
+	put_cred(proc->cred);
 
 	binder_debug(BINDER_DEBUG_OPEN_CLOSE,
 		     "%s: %d threads %d, nodes %d (ref %d), refs %d, active transactions %d, buffers %d, pages %d\n",
-- 
2.34.0.rc0.344.g81b53c2807-goog