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skbuff: in skb_copy_ubufs unclone before releasing zerocopy

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skb_copy_ubufs must unclone before it is safe to modify its
skb_shared_info with skb_zcopy_clear.

Commit b90ddd5687 ("skbuff: skb_copy_ubufs must release uarg even
without user frags") ensures that all skbs release their zerocopy
state, even those without frags.

But I forgot an edge case where such an skb arrives that is cloned.

The stack does not build such packets. Vhost/tun skbs have their
frags orphaned before cloning. TCP skbs only attach zerocopy state
when a frag is added.

But if TCP packets can be trimmed or linearized, this might occur.
Tracing the code I found no instance so far (e.g., skb_linearize
ends up calling skb_zcopy_clear if !skb->data_len).

Still, it is non-obvious that no path exists. And it is fragile to
rely on this.

Fixes: b90ddd5687 ("skbuff: skb_copy_ubufs must release uarg even without user frags")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Willem de Bruijn 2017-12-28 12:38:13 -05:00 committed by David S. Miller
parent 517d7c79bd
commit f72c4ac695
1 changed files with 3 additions and 3 deletions
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6
net/core/skbuff.c
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@ -1177,12 +1177,12 @@ int skb_copy_ubufs(struct sk_buff *skb, gfp_t gfp_mask)
int i, new_frags;
u32 d_off;
if (!num_frags)
goto release;
if (skb_shared(skb) || skb_unclone(skb, gfp_mask))
return -EINVAL;
if (!num_frags)
goto release;
new_frags = (__skb_pagelen(skb) + PAGE_SIZE - 1) >> PAGE_SHIFT;
for (i = 0; i < new_frags; i++) {
page = alloc_page(gfp_mask);