[Patch net] gre: refetch erspan header from skb->data after pskb_may_pull()

[Cong Wang <xiyou.wangcong@gmail.com>]

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: Cong Wang <xiyou.wangcong@gmail.com>
---
 net/ipv4/gre_demux.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/gre_demux.c b/net/ipv4/gre_demux.c
index 44bfeecac33e..5fd6e8ed02b5 100644
--- a/net/ipv4/gre_demux.c
+++ b/net/ipv4/gre_demux.c
@@ -127,7 +127,7 @@ 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));
 	}
 
-- 
2.21.0