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

commit cb5b020a8d38f77209d0472a0fea755299a8ec78 upstream.

This reverts commit 8099b047ecc431518b9bb6bdbba3549bbecdc343.

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: NSamuel Dionne-Riel <samuel@dionne-riel.com>
Acked-by: NKees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NGreg 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'))