binder: use cred instead of task for getsecid

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.

Cc: stable@vger.kernel.org # 5.4+
Fixes: ec74136 ("binder: create node flag to request sender's security context")
Signed-off-by: Todd Kjos <tkjos@google.com>
Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Change-Id: I8e10e5e182b17b70ccd21288d4c2590469586989

drivers/android/binder.c

@@ -3137,7 +3137,7 @@ static void binder_transaction(struct binder_proc *proc,
 		size_t added_size;
 		int max_retries = 100;
 
-		security_task_getsecid(proc->tsk, &secid);
+		security_cred_getsecid(proc->cred, &secid);
  retry_alloc:
 		ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
 		if (ret == -ENOMEM && max_retries-- > 0) {

include/linux/security.h

@@ -850,6 +850,11 @@ static inline void security_transfer_creds(struct cred *new,
 {
 }
 
+static inline void security_cred_getsecid(const struct cred *c, u32 *secid)
+{
+	*secid = 0;
+}
+
 static inline int security_kernel_act_as(struct cred *cred, u32 secid)
 {
 	return 0;