Revert "exec: load_script: don't blindly truncate shebang string"

commit cb5b020a upstream.

This reverts commit 8099b047.

It turns out that people do actually depend on the shebang string being
truncated, and on the fact that an interpreter (like perl) will often
just re-interpret it entirely to get the full argument list.
Reported-by: Samuel Dionne-Riel <samuel@dionne-riel.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/binfmt_script.c

@@ -42,14 +42,10 @@ static int load_script(struct linux_binprm *bprm)
 	fput(bprm->file);
 	bprm->file = NULL;
 
-	for (cp = bprm->buf+2;; cp++) {
-		if (cp >= bprm->buf + BINPRM_BUF_SIZE)
-			return -ENOEXEC;
-		if (!*cp || (*cp == '\n'))
-			break;
-	}
+	bprm->buf[BINPRM_BUF_SIZE - 1] = '\0';
+	if ((cp = strchr(bprm->buf, '\n')) == NULL)
+		cp = bprm->buf+BINPRM_BUF_SIZE-1;
 	*cp = '\0';
-
 	while (cp > bprm->buf) {
 		cp--;
 		if ((*cp == ' ') || (*cp == '\t'))