Syzkaller was able to trigger

    kernel BUG at net/core/gro.c:424 !
    RIP: 0010:gro_pull_from_frag0 net/core/gro.c:424 [inline]
    RIP: 0010:gro_try_pull_from_frag0 net/core/gro.c:446 [inline]
    RIP: 0010:dev_gro_receive+0x242f/0x24b0 net/core/gro.c:571

Due to using an incorrect NAPI_GRO_CB(skb)->network_offset.

The referenced commit sets this offset to 0 in skb_gro_reset_offset.
That matches the expected case in dev_gro_receive:

        pp = INDIRECT_CALL_INET(ptype->callbacks.gro_receive,
                                ipv6_gro_receive, inet_gro_receive,
                                &gro_list->list, skb);

But syzkaller injected an skb with protocol ETH_P_TEB into an ip6gre
device (by writing the IP6GRE encapsulated version to a TAP device).
The result was a first call to eth_gro_receive, and thus an extra
ETH_HLEN in network_offset that should not be there. First issue hit
is when computing offset from network header in ipv6_gro_pull_exthdrs.

Initialize both offsets in the network layer gro_receive.

This pairs with all reads in gro_receive, which use
skb_gro_receive_network_offset().

Fixes: 186b1ea7 ("net: gro: use cb instead of skb->network_header")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
CC: Richard Gobert <richardbgobert@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240523141434.1752483-1-willemdebruijn.kernel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>