exec: Convert security_bprm_set_creds into security_bprm_repopulate_creds

Rename bprm->cap_elevated to bprm->active_secureexec and initialize it
in prepare_binprm instead of in cap_bprm_set_creds.  Initializing
bprm->active_secureexec in prepare_binprm allows multiple
implementations of security_bprm_repopulate_creds to play nicely with
each other.

Rename security_bprm_set_creds to security_bprm_reopulate_creds to
emphasize that this path recomputes part of bprm->cred.  This
recomputation avoids the time of check vs time of use problems that
are inherent in unix #! interpreters.

In short two renames and a move in the location of initializing
bprm->active_secureexec.

Link: https://lkml.kernel.org/r/87o8qkzrxp.fsf_-_@x220.int.ebiederm.org
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/security/security.c b/security/security.c
index 4ee76a7..b890b7e 100644
--- a/security/security.c
+++ b/security/security.c
@@ -828,9 +828,9 @@ int security_bprm_creds_for_exec(struct linux_binprm *bprm)
 	return call_int_hook(bprm_creds_for_exec, 0, bprm);
 }
 
-int security_bprm_set_creds(struct linux_binprm *bprm)
+int security_bprm_repopulate_creds(struct linux_binprm *bprm)
 {
-	return call_int_hook(bprm_set_creds, 0, bprm);
+	return call_int_hook(bprm_repopulate_creds, 0, bprm);
 }
 
 int security_bprm_check(struct linux_binprm *bprm)