netdev - Re: [PATCH v2] net/packet: initialize val in packet

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Date:   Tue, 25 Apr 2017 11:44:33 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     glider@...gle.com
Cc:     dvyukov@...gle.com, kcc@...gle.com, edumazet@...gle.com,
        kuznet@....inr.ac.ru, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] net/packet: initialize val in packet_getsockopt()

From: Alexander Potapenko <glider@...gle.com>
Date: Mon, 24 Apr 2017 14:59:14 +0200

> In the case getsockopt() is called with PACKET_HDRLEN and optlen < 4
> |val| remains uninitialized and the syscall may behave differently
> depending on its value. This doesn't have security consequences (as the
> uninit bytes aren't copied back), but it's still cleaner to initialize
> |val| and ensure optlen is not less than sizeof(int).
> 
> This bug has been detected with KMSAN.
> 
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
> v2: - if len < sizeof(int), make it 0

No, you should signal an error if the len is too small.

Returning zero bytes to userspace silently makes the user think that
he got the data he asked for.