[PATCH 4.19 1/2] binder: use euid from cred instead of using task

[Todd Kjos]

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 at vger.kernel.org # 4.4+
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Signed-off-by: Todd Kjos <tkjos at google.com>
Suggested-by: Stephen Smalley <stephen.smalley.work at gmail.com>
Suggested-by: Jann Horn <jannh at google.com>
Acked-by: Casey Schaufler <casey at schaufler-ca.com>
Signed-off-by: Paul Moore <paul at paul-moore.com>
Change-Id: I91922e7f359df5901749f1b09094c3c68d45aed4
---
 drivers/android/binder.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index cda4f7eb58ea..58e497db26b4 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -483,6 +483,9 @@ enum binder_deferred_state {
  * @files                 files_struct for process
  *                        (protected by @files_lock)
  * @files_lock            mutex to protect @files
+ * @cred                  struct cred associated with the `struct file`
+ *                        in binder_open()
+ *                        (invariant after initialized)
  * @deferred_work_node:   element for binder_deferred_list
  *                        (protected by binder_deferred_lock)
  * @deferred_work:        bitmap of deferred work to perform
@@ -529,6 +532,7 @@ struct binder_proc {
 	struct task_struct *tsk;
 	struct files_struct *files;
 	struct mutex files_lock;
+	const struct cred *cred;
 	struct hlist_node deferred_work_node;
 	int deferred_work;
 	bool is_dead;
@@ -2962,7 +2966,7 @@ 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;
@@ -4341,6 +4345,7 @@ static void binder_free_proc(struct binder_proc *proc)
 	BUG_ON(!list_empty(&proc->delivered_death));
 	binder_alloc_deferred_release(&proc->alloc);
 	put_task_struct(proc->tsk);
+	put_cred(proc->cred);
 	binder_stats_deleted(BINDER_STAT_PROC);
 	kfree(proc);
 }
@@ -4799,6 +4804,7 @@ static int binder_open(struct inode *nodp, struct file *filp)
 	get_task_struct(current->group_leader);
 	proc->tsk = current->group_leader;
 	mutex_init(&proc->files_lock);
+	proc->cred = get_cred(filp->f_cred);
 	INIT_LIST_HEAD(&proc->todo);
 	proc->default_priority = task_nice(current);
 	binder_dev = container_of(filp->private_data, struct binder_device,
-- 
2.34.0.rc0.344.g81b53c2807-goog