skbuff: in skb_copy_ubufs unclone before releasing zerocopy

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

Commit b90ddd568792 ("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: b90ddd568792 ("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>

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a3cb0be4c6f3b5b519b60ac8dde73c33e902763a..08f57408131523adec3cc36044952e14aa5a6b9a 100644 (file)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -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);