Re: [PATCH 1/5] net: Fix potential deadloop in skb_copy_ubufs()

[linmiaohe]

Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
>On Thu, Aug 6, 2020 at 1:48 PM linmiaohe <linmiaohe@xxxxxxxxxx> wrote:
>>
>> From: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>
>> 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.