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Date: Thu, 3 Jun 2021 08:54:28 -0700
From: sdf@...gle.com
To: Tanner Love <tannerlove.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Eric Dumazet <edumazet@...gle.com>,
Willem de Bruijn <willemb@...gle.com>,
Petar Penkov <ppenkov@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Tanner Love <tannerlove@...gle.com>,
kernel test robot <lkp@...el.com>
Subject: Re: [PATCH net-next v3 2/3] virtio_net: add optional flow dissection
in virtio_net_hdr_to_skb
On 06/01, Tanner Love wrote:
> From: Tanner Love <tannerlove@...gle.com>
> Syzkaller bugs have resulted from loose specification of
> virtio_net_hdr[1]. Enable execution of a BPF flow dissector program
> in virtio_net_hdr_to_skb to validate the vnet header and drop bad
> input.
> The existing behavior of accepting these vnet headers is part of the
> ABI. But individual admins may want to enforce restrictions. For
> example, verifying that a GSO_TCPV4 gso_type matches packet contents:
> unencapsulated TCP/IPV4 packet with payload exceeding gso_size and
> hdr_len at payload offset.
> Introduce a new sysctl net.core.flow_dissect_vnet_hdr controlling a
> static key to decide whether to perform flow dissection. When the key
> is false, virtio_net_hdr_to_skb computes as before.
> [1]
> https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0
> [ um build error ]
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Tanner Love <tannerlove@...gle.com>
> Suggested-by: Willem de Bruijn <willemb@...gle.com>
> ---
> include/linux/virtio_net.h | 25 +++++++++++++++++++++----
> net/core/flow_dissector.c | 3 +++
> net/core/sysctl_net_core.c | 9 +++++++++
> 3 files changed, 33 insertions(+), 4 deletions(-)
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index b465f8f3e554..cdc6152b40c6 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -25,10 +25,13 @@ static inline int virtio_net_hdr_set_proto(struct
> sk_buff *skb,
> return 0;
> }
> +DECLARE_STATIC_KEY_FALSE(sysctl_flow_dissect_vnet_hdr_key);
> +
> static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> const struct virtio_net_hdr *hdr,
> bool little_endian)
> {
> + struct flow_keys_basic keys;
> unsigned int gso_type = 0;
> unsigned int thlen = 0;
> unsigned int p_off = 0;
> @@ -78,13 +81,24 @@ static inline int virtio_net_hdr_to_skb(struct
> sk_buff *skb,
> p_off = skb_transport_offset(skb) + thlen;
> if (!pskb_may_pull(skb, p_off))
> return -EINVAL;
> - } else {
> + }
> +
> + /* BPF flow dissection for optional strict validation.
> + *
> + * Admins can define permitted packets more strictly, such as dropping
> + * deprecated UDP_UFO packets and requiring skb->protocol to be non-zero
> + * and matching packet headers.
> + */
> + if (static_branch_unlikely(&sysctl_flow_dissect_vnet_hdr_key) &&
> + !__skb_flow_dissect_flow_keys_basic(NULL, skb, &keys, NULL, 0, 0, 0,
> + 0, hdr))
> + return -EINVAL;
> +
> + if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM)) {
> /* gso packets without NEEDS_CSUM do not set transport_offset.
> * probe and drop if does not match one of the above types.
> */
> if (gso_type && skb->network_header) {
> - struct flow_keys_basic keys;
> -
> if (!skb->protocol) {
> __be16 protocol = dev_parse_header_protocol(skb);
> @@ -92,8 +106,11 @@ static inline int virtio_net_hdr_to_skb(struct
> sk_buff *skb,
> if (protocol && protocol != skb->protocol)
> return -EINVAL;
> }
> +
> retry:
> - if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
> + /* only if flow dissection not already done */
> + if (!static_branch_unlikely(&sysctl_flow_dissect_vnet_hdr_key) &&
> + !skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
> NULL, 0, 0, 0,
> 0)) {
> /* UFO does not specify ipv4 or 6: try both */
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 4b171ebec084..ae2ce382f4f4 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -35,6 +35,9 @@
> #endif
> #include <linux/bpf-netns.h>
> +DEFINE_STATIC_KEY_FALSE(sysctl_flow_dissect_vnet_hdr_key);
> +EXPORT_SYMBOL(sysctl_flow_dissect_vnet_hdr_key);
> +
> static void dissector_set_key(struct flow_dissector *flow_dissector,
> enum flow_dissector_key_id key_id)
> {
> diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> index c8496c1142c9..c01b9366bb75 100644
> --- a/net/core/sysctl_net_core.c
> +++ b/net/core/sysctl_net_core.c
> @@ -36,6 +36,8 @@ static int net_msg_warn; /* Unused, but still a sysctl
> */
> int sysctl_fb_tunnels_only_for_init_net __read_mostly = 0;
> EXPORT_SYMBOL(sysctl_fb_tunnels_only_for_init_net);
> +DECLARE_STATIC_KEY_FALSE(sysctl_flow_dissect_vnet_hdr_key);
> +
> /* 0 - Keep current behavior:
> * IPv4: inherit all current settings from init_net
> * IPv6: reset all settings to default
> @@ -580,6 +582,13 @@ static struct ctl_table net_core_table[] = {
> .extra1 = SYSCTL_ONE,
> .extra2 = &int_3600,
> },
[..]
> + {
> + .procname = "flow_dissect_vnet_hdr",
This sounds generic, but iiuc, it makes sense only for bpf flow
dissection? Should it be bpf_flow_dissect_vnet_hdr instead?
Or should we drop this sysctl in favor of some per-program flag
(either attach_flags or some signal via btf)? That way,
individual bpf programs can signal their willingness to
parse vnet hdr.
> + .data = &sysctl_flow_dissect_vnet_hdr_key.key,
> + .maxlen = sizeof(sysctl_flow_dissect_vnet_hdr_key),
> + .mode = 0644,
> + .proc_handler = proc_do_static_key,
> + },
> { }
> };
> --
> 2.32.0.rc0.204.g9fa02ecfa5-goog
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