gre: refetch erspan header from skb->data after pskb_may_pull()

[ Upstream commit 0e4940928c26527ce8f97237fef4c8a91cd34207 ]

After pskb_may_pull() we should always refetch the header
pointers from the skb->data in case it got reallocated.

In gre_parse_header(), the erspan header is still fetched
from the 'options' pointer which is fetched before
pskb_may_pull().

Found this during code review of a KMSAN bug report.

Fixes: cb73ee40b1b3 ("net: ip_gre: use erspan key field for tunnel lookup")
Cc: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Acked-by: NLorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: NWilliam Tu <u9012063@gmail.com>
Reviewed-by: NSimon Horman <simon.horman@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>

net/ipv4/gre_demux.c

@@ -132,7 +132,7 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 		if (!pskb_may_pull(skb, nhs + hdr_len + sizeof(*ershdr)))
 			return -EINVAL;
 
-		ershdr = (struct erspan_base_hdr *)options;
+		ershdr = (struct erspan_base_hdr *)(skb->data + nhs + hdr_len);
 		tpi->key = cpu_to_be32(get_session_id(ershdr));
 	}