bpf: pull before calling skb_postpull_rcsum()

Anand hit a BUG() when pulling off headers on egress to a SW tunnel.
We get to skb_checksum_help() with an invalid checksum offset
(commit d7ea0d9df2a6 ("net: remove two BUG() from skb_checksum_help()")
converted those BUGs to WARN_ONs()).
He points out oddness in how skb_postpull_rcsum() gets used.
Indeed looks like we should pull before "postpull", otherwise
the CHECKSUM_PARTIAL fixup from skb_postpull_rcsum() will not
be able to do its job:

	if (skb->ip_summed == CHECKSUM_PARTIAL &&
	    skb_checksum_start_offset(skb) < 0)
		skb->ip_summed = CHECKSUM_NONE;

Reported-by: Anand Parthasarathy <anpartha@meta.com>
Fixes: 6578171a7ff0 ("bpf: add bpf_skb_change_proto helper")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20221220004701.402165-1-kuba@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

diff --git a/net/core/filter.c b/net/core/filter.c
index 9293586..43cc1fe 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3180,15 +3180,18 @@ static int bpf_skb_generic_push(struct sk_buff *skb, u32 off, u32 len)
 
 static int bpf_skb_generic_pop(struct sk_buff *skb, u32 off, u32 len)
 {
+       void *old_data;
+
        /* skb_ensure_writable() is not needed here, as we're
         * already working on an uncloned skb.
         */
        if (unlikely(!pskb_may_pull(skb, off + len)))
                return -ENOMEM;
 
-       skb_postpull_rcsum(skb, skb->data + off, len);
-       memmove(skb->data + len, skb->data, off);
+       old_data = skb->data;
        __skb_pull(skb, len);
+       skb_postpull_rcsum(skb, old_data + off, len);
+       memmove(skb->data, old_data, off);
 
        return 0;
 }