[PATCH net] skbuff: in skb_copy_ubufs unclone before releasing zerocopy

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

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>

---

I should have spotted this when preparing the above patch, of course.
Apologies for the added work.

Since I did not spot a path that triggers the issue, this can also
go to net-next, instead. It applies to both cleanly.
---
 net/core/skbuff.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a3cb0be4c6f3..08f574081315 100644
--- 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);
-- 
2.15.1.620.gb9897f4670-goog