[PATCH net-next] bpf: restore skb->sk before pskb_trim() call

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

While testing a fix [1] in ___pskb_trim(), addressing the WARN_ON_ONCE()
in skb_try_coalesce() reported by Andrey, I found that we had an skb
with skb->sk set but no skb->destructor.

This invalidated heuristic found in commit 158f323b9868 ("net: adjust
skb->truesize in pskb_expand_head()") and in cited patch.

Considering the BUG_ON(skb->sk) we have in skb_orphan(), we should
restrain the temporary setting to a minimal section.

[1] https://patchwork.ozlabs.org/patch/755570/ 
    net: adjust skb->truesize in ___pskb_trim()

Fixes: 8f917bba0042 ("bpf: pass sk to helper functions")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Willem de Bruijn <will...@google.com>
Cc: Andrey Konovalov <andreyk...@google.com>
---
 net/core/filter.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 
9a37860a80fc78378705b681ec3b0468824cbcf4..a253a6197e6b37a7ae2fe451c646b01c861a3e22
 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -98,8 +98,8 @@ int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb, 
unsigned int cap)
 
                skb->sk = sk;
                pkt_len = bpf_prog_run_save_cb(filter->prog, skb);
-               err = pkt_len ? pskb_trim(skb, max(cap, pkt_len)) : -EPERM;
                skb->sk = save_sk;
+               err = pkt_len ? pskb_trim(skb, max(cap, pkt_len)) : -EPERM;
        }
        rcu_read_unlock();