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

[Eric Dumazet]

On Fri, Aug 14, 2020 at 12:14 AM linmiaohe <linmiaohe@xxxxxxxxxx> wrote:
>
> 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 ?

Please give us a real case.

I fear that your patches are coming directly from some kind of
automated tool, that really misses how the code is really used
from _current_ code base, not _hypothetical_ one.

This is very time consuming. Please provide evidence first.

Thank you.