exec: load_script: don't blindly truncate shebang string

load_script() simply truncates bprm->buf and this is very wrong if the
length of shebang string exceeds BINPRM_BUF_SIZE-2.  This can silently
truncate i_arg or (worse) we can execute the wrong binary if buf[2:126]
happens to be the valid executable path.

Change load_script() to return ENOEXEC if it can't find '\n' or zero in
bprm->buf.  Note that '\0' can come from either
prepare_binprm()->memset() or from kernel_read(), we do not care.

Link: http://lkml.kernel.org/r/20181112160931.GA28463@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Ben Woodard <woodard@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_script.c

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