[PATCH 5.6 13/41] net: be more gentle about silly gso requests coming from user

Greg Kroah-Hartman
2020-06-09T17:56:12Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 7c6d2ecbda83150b2036a2b36b21381ad4667762 ]

Recent change in virtio_net_hdr_to_skb() broke some packetdrill tests.

When --mss=XXX option is set, packetdrill always provide gso_type & gso_size
for its inbound packets, regardless of packet size.

if (packet->tcp && packet->mss) {
if (packet->ipv4)
gso.gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
else
gso.gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
gso.gso_size = packet->mss;
}

Since many other programs could do the same, relax virtio_net_hdr_to_skb()
to no longer return an error, but instead ignore gso settings.

This keeps Willem intent to make sure no malicious packet could
reach gso stack.

Note that TCP stack has a special logic in tcp_set_skb_tso_segs()
to clear gso_size for small packets.

Fixes: 6dd912f82680 ("net: check untrusted gso_size at kernel entry")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/virtio_net.h | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)

--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -109,16 +109,17 @@ retry:

if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
u16 gso_size = __virtio16_to_cpu(little_endian, hdr->gso_size);
+ struct skb_shared_info *shinfo = skb_shinfo(skb);

- if (skb->len - p_off <= gso_size)
- return -EINVAL;
+ /* Too small packets are not really GSO ones. */
+ if (skb->len - p_off > gso_size) {
+ shinfo->gso_size = gso_size;
+ shinfo->gso_type = gso_type;

- skb_shinfo(skb)->gso_size = gso_size;
- skb_shinfo(skb)->gso_type = gso_type;
-
- /* Header must be checked, and gso_segs computed. */
- skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY;
- skb_shinfo(skb)->gso_segs = 0;
+ /* Header must be checked, and gso_segs computed. */
+ shinfo->gso_type |= SKB_GSO_DODGY;
+ shinfo->gso_segs = 0;
+ }
}

return 0;