linux-kernel - Re: [PATCH 1/5] net: Fix potential deadloop in skb_copy

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Date:   Fri, 14 Aug 2020 07:14:39 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC:     David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Florian Westphal <fw@...len.de>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "decui@...rosoft.com" <decui@...rosoft.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        "jeremy@...zel.net" <jeremy@...zel.net>,
        "mashirle@...ibm.com" <mashirle@...ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] net: Fix potential deadloop in skb_copy_ubufs()

Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
>On Thu, Aug 6, 2020 at 1:48 PM linmiaohe <linmiaohe@...wei.com> wrote:
>>
>> From: Miaohe Lin <linmiaohe@...wei.com>
>>
>> We could be trapped in deadloop when we try to copy userspace skb 
>> frags buffers to kernel with a cloned skb:
>> Reproduce code snippet:
>>         skb = alloc_skb(UBUF_DATA_LEN, GFP_ATOMIC);
>>         clone = skb_clone(skb, GFP_ATOMIC);
>>         skb_zcopy_set_nouarg(clone, NULL);
>>         pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
>>
>> Catch this unexpected case and return -EINVAL in skb_orphan_frags() 
>> before we call skb_copy_ubufs() to fix it.
>
>Is this a hypothetical codepath?
>
>skb zerocopy carefully tracks clone calls where necessary. See the call to skb_orphan_frags in skb_clone, and the implementation of that callee.
>
>The only caller of skb zerocopy with nouarg is tpacket_fill_skb, as of commit 5cd8d46ea156 ("packet: copy user buffers before orphan or clone").
>
>As the commit subject indicates, this sets skb_zcopy_set_nouarg exactly to be sure that any clone will trigger a copy of "zerocopy"
>user data to private kernel memory.
>
>No clone must happen between alloc_skb and skb_zcopy_set_nouarg, indeed. But AFAIK, none exists.

Since we always call skb_orphan_frags in skb_clone, is it unnecessary to call skb_orphan_frags in pskb_expand_head when skb is cloned ?

Thanks.