lsm: separate security_task_getsecid() into subjective and objective variants

Of the three LSMs that implement the security_task_getsecid() LSM
hook, all three LSMs provide the task's objective security
credentials.  This turns out to be unfortunate as most of the hook's
callers seem to expect the task's subjective credentials, although
a small handful of callers do correctly expect the objective
credentials.

This patch is the first step towards fixing the problem: it splits
the existing security_task_getsecid() hook into two variants, one
for the subjective creds, one for the objective creds.

  void security_task_getsecid_subj(struct task_struct *p,
				   u32 *secid);
  void security_task_getsecid_obj(struct task_struct *p,
				  u32 *secid);

While this patch does fix all of the callers to use the correct
variant, in order to keep this patch focused on the callers and to
ease review, the LSMs continue to use the same implementation for
both hooks.  The net effect is that this patch should not change
the behavior of the kernel in any way, it will be up to the latter
LSM specific patches in this series to change the hook
implementations and return the correct credentials.

Acked-by: Mimi Zohar <zohar@linux.ibm.com> (IMA)
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index c119736ca56ac..61d235b6ccd82 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2700,7 +2700,16 @@ static void binder_transaction(struct binder_proc *proc,
 		u32 secid;
 		size_t added_size;
 
-		security_task_getsecid(proc->tsk, &secid);
+		/*
+		 * 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);
 		ret = security_secid_to_secctx(secid, &secctx, &secctx_sz);
 		if (ret) {
 			return_error = BR_FAILED_REPLY;