Re: [PATCH net v5 7/7] virtio_net: cleanup protocol calculation

[netdev-bot+sashiko@kernel.org]

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- [High] net: use-after-free read of iph->ihl after pskb_may_pull() in
  ip4_parse()

--

> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -638,7 +638,6 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
>  	 * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so __virtio_net_hdr_to_skb
>  	 * set CHECKSUM_PARTIAL.
>  	 */
> -	inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP);
>  	if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh,
>  						 inner_proto))
>  		return 0;

[Severity: High]
This isn't introduced by this patch, but since inner_proto is what
steers virtio_net_csum_partial_ok_at_offset() into the IPv4 parser,
it's worth raising here.

virtio_net_csum_partial_ok_at_offset() calls virtio_net_l4_start(),
which for ETH_P_IP calls ip4_parse(). In ip4_parse() the fixed header
is pulled, iph is set to skb->data + nhoff, and then a second
pskb_may_pull(skb, nhoff + iph->ihl * sizeof(u32)) pulls in the
options. That second pull can go through __pskb_pull_tail() and
pskb_expand_head() when the options sit in paged frags, the skb is
cloned, or CONFIG_FAIL_SKB_REALLOC fires, and that frees the old
skb->head. The next line then computes *thoff from iph->ihl through
the stale pointer, before the header is reloaded from skb->data.

That is a use-after-free read that a guest, or a tun/tap/af_packet
user, can trigger with an IPv4 header whose options straddle the
linear area. If the freed buffer has been reused, we can also end up
with a bogus *thoff that is then checked against csum_start. Since
ip4_parse() is shared with ip_rcv_core(), br_validate_ipv4() and
nf_reject_iphdr_validate(), plain IPv4 ingress hits it too.

Reading ihl into a local before the second pskb_may_pull(), or
reloading iph from skb->data right after it, should be enough.
Do you want to fold a fix for ip4_parse() into this series, or send
it separately to net with a Fixes: tag for the commit that added
ip4_parse()?

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/cover.v5.git.mst%40redhat.com