gro: ensure frag0 meets IP header alignment

After commit 0f6925b3 ("virtio_net: Do not pull payload in skb->head")
Guenter Roeck reported one failure in his tests using sh architecture.

After much debugging, we have been able to spot silent unaligned accesses
in inet_gro_receive()

The issue at hand is that upper networking stacks assume their header
is word-aligned. Low level drivers are supposed to reserve NET_IP_ALIGN
bytes before the Ethernet header to make that happen.

This patch hardens skb_gro_reset_offset() to not allow frag0 fast-path
if the fragment is not properly aligned.

Some arches like x86, arm64 and powerpc do not care and define NET_IP_ALIGN
as 0, this extra check will be a NOP for them.

Note that if frag0 is not used, GRO will call pskb_may_pull()
as many times as needed to pull network and transport headers.

Fixes: 0f6925b3 ("virtio_net: Do not pull payload in skb->head")
Fixes: 78a478d0 ("gro: Inline skb_gro_header and cache frag0 virtual address")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NGuenter Roeck <linux@roeck-us.net>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Tested-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5924,7 +5924,8 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
 	NAPI_GRO_CB(skb)->frag0_len = 0;
 
 	if (!skb_headlen(skb) && pinfo->nr_frags &&
-	    !PageHighMem(skb_frag_page(frag0))) {
+	    !PageHighMem(skb_frag_page(frag0)) &&
+	    (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
 		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
 		NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
 						    skb_frag_size(frag0),