[PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Daniel Xu]

=== Context ===

In the context of a middlebox, fragmented packets are tricky to handle.
The full 5-tuple of a packet is often only available in the first
fragment which makes enforcing consistent policy difficult. There are
really only two stateless options, neither of which are very nice:

1. Enforce policy on first fragment and accept all subsequent fragments.
   This works but may let in certain attacks or allow data exfiltration.

2. Enforce policy on first fragment and drop all subsequent fragments.
   This does not really work b/c some protocols may rely on
   fragmentation. For example, DNS may rely on oversized UDP packets for
   large responses.

So stateful tracking is the only sane option. RFC 8900 [0] calls this
out as well in section 6.3:

    Middleboxes [...] should process IP fragments in a manner that is
    consistent with [RFC0791] and [RFC8200]. In many cases, middleboxes
    must maintain state in order to achieve this goal.

=== BPF related bits ===

However, when policy is enforced through BPF, the prog is run before the
kernel reassembles fragmented packets. This leaves BPF developers in a
awkward place: implement reassembly (possibly poorly) or use a stateless
method as described above.

Fortunately, the kernel has robust support for fragmented IP packets.
This patchset wraps the existing defragmentation facilities in kfuncs so
that BPF progs running on middleboxes can reassemble fragmented packets
before applying policy.

=== Patchset details ===

This patchset is (hopefully) relatively straightforward from BPF perspective.
One thing I'd like to call out is the skb_copy()ing of the prog skb. I
did this to maintain the invariant that the ctx remains valid after prog
has run. This is relevant b/c ip_defrag() and ip_check_defrag() may
consume the skb if the skb is a fragment.

Originally I did play around with teaching the verifier about kfuncs
that may consume the ctx and disallowing ctx accesses in ret != 0
branches. It worked ok, but it seemed too complex to modify the
surrounding assumptions about ctx validity.

[0]: https://datatracker.ietf.org/doc/html/rfc8900

===

Changes from v1:
* Add support for ipv6 defragmentation


Daniel Xu (8):
  ip: frags: Return actual error codes from ip_check_defrag()
  bpf: verifier: Support KF_CHANGES_PKT flag
  bpf, net, frags: Add bpf_ip_check_defrag() kfunc
  net: ipv6: Factor ipv6_frag_rcv() to take netns and user
  bpf: net: ipv6: Add bpf_ipv6_frag_rcv() kfunc
  bpf: selftests: Support not connecting client socket
  bpf: selftests: Support custom type and proto for client sockets
  bpf: selftests: Add defrag selftests

 Documentation/bpf/kfuncs.rst                  |   7 +
 drivers/net/macvlan.c                         |   2 +-
 include/linux/btf.h                           |   1 +
 include/net/ip.h                              |  11 +
 include/net/ipv6.h                            |   1 +
 include/net/ipv6_frag.h                       |   1 +
 include/net/transp_v6.h                       |   1 +
 kernel/bpf/verifier.c                         |   8 +
 net/ipv4/Makefile                             |   1 +
 net/ipv4/ip_fragment.c                        |  15 +-
 net/ipv4/ip_fragment_bpf.c                    |  98 ++++++
 net/ipv6/Makefile                             |   1 +
 net/ipv6/af_inet6.c                           |   4 +
 net/ipv6/reassembly.c                         |  16 +-
 net/ipv6/reassembly_bpf.c                     | 143 ++++++++
 net/packet/af_packet.c                        |   2 +-
 tools/testing/selftests/bpf/Makefile          |   3 +-
 .../selftests/bpf/generate_udp_fragments.py   |  90 +++++
 .../selftests/bpf/ip_check_defrag_frags.h     |  57 +++
 tools/testing/selftests/bpf/network_helpers.c |  26 +-
 tools/testing/selftests/bpf/network_helpers.h |   3 +
 .../bpf/prog_tests/ip_check_defrag.c          | 327 ++++++++++++++++++
 .../selftests/bpf/progs/bpf_tracing_net.h     |   1 +
 .../selftests/bpf/progs/ip_check_defrag.c     | 133 +++++++
 24 files changed, 931 insertions(+), 21 deletions(-)
 create mode 100644 net/ipv4/ip_fragment_bpf.c
 create mode 100644 net/ipv6/reassembly_bpf.c
 create mode 100755 tools/testing/selftests/bpf/generate_udp_fragments.py
 create mode 100644 tools/testing/selftests/bpf/ip_check_defrag_frags.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/ip_check_defrag.c
 create mode 100644 tools/testing/selftests/bpf/progs/ip_check_defrag.c

-- 
2.39.1

[PATCH bpf-next v2 1/8] ip: frags: Return actual error codes from ip_check_defrag()

[Daniel Xu]

Once we wrap ip_check_defrag() in a kfunc, it may be useful for progs to
know the exact error condition ip_check_defrag() encountered.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 drivers/net/macvlan.c  |  2 +-
 net/ipv4/ip_fragment.c | 13 ++++++++-----
 net/packet/af_packet.c |  2 +-
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index 99a971929c8e..b8310e13d7e1 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -456,7 +456,7 @@ static rx_handler_result_t macvlan_handle_frame(struct sk_buff **pskb)
 		unsigned int hash;
 
 		skb = ip_check_defrag(dev_net(skb->dev), skb, IP_DEFRAG_MACVLAN);
-		if (!skb)
+		if (IS_ERR(skb))
 			return RX_HANDLER_CONSUMED;
 		*pskb = skb;
 		eth = eth_hdr(skb);
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 69c00ffdcf3e..959d2c4260ea 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -514,6 +514,7 @@ struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb, u32 user)
 	struct iphdr iph;
 	int netoff;
 	u32 len;
+	int err;
 
 	if (skb->protocol != htons(ETH_P_IP))
 		return skb;
@@ -535,15 +536,17 @@ struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb, u32 user)
 		if (skb) {
 			if (!pskb_may_pull(skb, netoff + iph.ihl * 4)) {
 				kfree_skb(skb);
-				return NULL;
+				return ERR_PTR(-ENOMEM);
 			}
-			if (pskb_trim_rcsum(skb, netoff + len)) {
+			err = pskb_trim_rcsum(skb, netoff + len);
+			if (err) {
 				kfree_skb(skb);
-				return NULL;
+				return ERR_PTR(err);
 			}
 			memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
-			if (ip_defrag(net, skb, user))
-				return NULL;
+			err = ip_defrag(net, skb, user);
+			if (err)
+				return ERR_PTR(err);
 			skb_clear_hash(skb);
 		}
 	}
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index d4e76e2ae153..1ef94828c8da 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1470,7 +1470,7 @@ static int packet_rcv_fanout(struct sk_buff *skb, struct net_device *dev,
 
 	if (fanout_has_flag(f, PACKET_FANOUT_FLAG_DEFRAG)) {
 		skb = ip_check_defrag(net, skb, IP_DEFRAG_AF_PACKET);
-		if (!skb)
+		if (IS_ERR(skb))
 			return 0;
 	}
 	switch (f->type) {
-- 
2.39.1

[PATCH bpf-next v2 3/8] bpf, net, frags: Add bpf_ip_check_defrag() kfunc

[Daniel Xu]

This kfunc is used to defragment IPv4 packets. The idea is that if you
see a fragmented packet, you call this kfunc. If the kfunc returns 0,
then the skb has been updated to contain the entire reassembled packet.

If the kfunc returns an error (most likely -EINPROGRESS), then it means
the skb is part of a yet-incomplete original packet. A reasonable
response to -EINPROGRESS is to drop the packet, as the ip defrag
infrastructure is already hanging onto the frag for future reassembly.

Care has been taken to ensure the prog skb remains valid no matter what
the underlying ip_check_defrag() call does. This is in contrast to
ip_defrag(), which may consume the skb if the skb is part of a
yet-incomplete original packet.

So far this kfunc is only callable from TC clsact progs.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 include/net/ip.h           | 11 +++++
 net/ipv4/Makefile          |  1 +
 net/ipv4/ip_fragment.c     |  2 +
 net/ipv4/ip_fragment_bpf.c | 98 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 112 insertions(+)
 create mode 100644 net/ipv4/ip_fragment_bpf.c

diff --git a/include/net/ip.h b/include/net/ip.h
index c3fffaa92d6e..f3796b1b5cac 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -680,6 +680,7 @@ enum ip_defrag_users {
 	IP_DEFRAG_VS_FWD,
 	IP_DEFRAG_AF_PACKET,
 	IP_DEFRAG_MACVLAN,
+	IP_DEFRAG_BPF,
 };
 
 /* Return true if the value of 'user' is between 'lower_bond'
@@ -693,6 +694,16 @@ static inline bool ip_defrag_user_in_between(u32 user,
 }
 
 int ip_defrag(struct net *net, struct sk_buff *skb, u32 user);
+
+#ifdef CONFIG_DEBUG_INFO_BTF
+int register_ip_frag_bpf(void);
+#else
+static inline int register_ip_frag_bpf(void)
+{
+	return 0;
+}
+#endif
+
 #ifdef CONFIG_INET
 struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb, u32 user);
 #else
diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
index 880277c9fd07..950efb166d37 100644
--- a/net/ipv4/Makefile
+++ b/net/ipv4/Makefile
@@ -65,6 +65,7 @@ obj-$(CONFIG_TCP_CONG_ILLINOIS) += tcp_illinois.o
 obj-$(CONFIG_NET_SOCK_MSG) += tcp_bpf.o
 obj-$(CONFIG_BPF_SYSCALL) += udp_bpf.o
 obj-$(CONFIG_NETLABEL) += cipso_ipv4.o
+obj-$(CONFIG_DEBUG_INFO_BTF) += ip_fragment_bpf.o
 
 obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
 		      xfrm4_output.o xfrm4_protocol.o
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 959d2c4260ea..e3fda5203f09 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -759,5 +759,7 @@ void __init ipfrag_init(void)
 	if (inet_frags_init(&ip4_frags))
 		panic("IP: failed to allocate ip4_frags cache\n");
 	ip4_frags_ctl_register();
+	if (register_ip_frag_bpf())
+		panic("IP: bpf: failed to register ip_frag_bpf\n");
 	register_pernet_subsys(&ip4_frags_ops);
 }
diff --git a/net/ipv4/ip_fragment_bpf.c b/net/ipv4/ip_fragment_bpf.c
new file mode 100644
index 000000000000..a9e5908ed216
--- /dev/null
+++ b/net/ipv4/ip_fragment_bpf.c
@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Unstable ipv4 fragmentation helpers for TC-BPF hook
+ *
+ * These are called from SCHED_CLS BPF programs. Note that it is allowed to
+ * break compatibility for these functions since the interface they are exposed
+ * through to BPF programs is explicitly unstable.
+ */
+
+#include <linux/bpf.h>
+#include <linux/btf_ids.h>
+#include <linux/ip.h>
+#include <linux/filter.h>
+#include <linux/netdevice.h>
+#include <net/ip.h>
+#include <net/sock.h>
+
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "Global functions as their definitions will be in ip_fragment BTF");
+
+/* bpf_ip_check_defrag - Defragment an ipv4 packet
+ *
+ * This helper takes an skb as input. If this skb successfully reassembles
+ * the original packet, the skb is updated to contain the original, reassembled
+ * packet.
+ *
+ * Otherwise (on error or incomplete reassembly), the input skb remains
+ * unmodified.
+ *
+ * Parameters:
+ * @ctx		- Pointer to program context (skb)
+ * @netns	- Child network namespace id. If value is a negative signed
+ *		  32-bit integer, the netns of the device in the skb is used.
+ *
+ * Return:
+ * 0 on successfully reassembly or non-fragmented packet. Negative value on
+ * error or incomplete reassembly.
+ */
+int bpf_ip_check_defrag(struct __sk_buff *ctx, u64 netns)
+{
+	struct sk_buff *skb = (struct sk_buff *)ctx;
+	struct sk_buff *skb_cpy, *skb_out;
+	struct net *caller_net;
+	struct net *net;
+	int mac_len;
+	void *mac;
+
+	if (unlikely(!((s32)netns < 0 || netns <= S32_MAX)))
+		return -EINVAL;
+
+	caller_net = skb->dev ? dev_net(skb->dev) : sock_net(skb->sk);
+	if ((s32)netns < 0) {
+		net = caller_net;
+	} else {
+		net = get_net_ns_by_id(caller_net, netns);
+		if (unlikely(!net))
+			return -EINVAL;
+	}
+
+	mac_len = skb->mac_len;
+	skb_cpy = skb_copy(skb, GFP_ATOMIC);
+	if (!skb_cpy)
+		return -ENOMEM;
+
+	skb_out = ip_check_defrag(net, skb_cpy, IP_DEFRAG_BPF);
+	if (IS_ERR(skb_out))
+		return PTR_ERR(skb_out);
+
+	skb_morph(skb, skb_out);
+	kfree_skb(skb_out);
+
+	/* ip_check_defrag() does not maintain mac header, so push empty header
+	 * in so prog sees the correct layout. The empty mac header will be
+	 * later pulled from cls_bpf.
+	 */
+	mac = skb_push(skb, mac_len);
+	memset(mac, 0, mac_len);
+	bpf_compute_data_pointers(skb);
+
+	return 0;
+}
+
+__diag_pop()
+
+BTF_SET8_START(ip_frag_kfunc_set)
+BTF_ID_FLAGS(func, bpf_ip_check_defrag, KF_CHANGES_PKT)
+BTF_SET8_END(ip_frag_kfunc_set)
+
+static const struct btf_kfunc_id_set ip_frag_bpf_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set   = &ip_frag_kfunc_set,
+};
+
+int register_ip_frag_bpf(void)
+{
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS,
+					 &ip_frag_bpf_kfunc_set);
+}
-- 
2.39.1

Re: [PATCH bpf-next v2 3/8] bpf, net, frags: Add bpf_ip_check_defrag() kfunc

[Stanislav Fomichev]

On 02/27, Daniel Xu wrote:
> This kfunc is used to defragment IPv4 packets. The idea is that if you
> see a fragmented packet, you call this kfunc. If the kfunc returns 0,
> then the skb has been updated to contain the entire reassembled packet.

> If the kfunc returns an error (most likely -EINPROGRESS), then it means
> the skb is part of a yet-incomplete original packet. A reasonable
> response to -EINPROGRESS is to drop the packet, as the ip defrag
> infrastructure is already hanging onto the frag for future reassembly.

> Care has been taken to ensure the prog skb remains valid no matter what
> the underlying ip_check_defrag() call does. This is in contrast to
> ip_defrag(), which may consume the skb if the skb is part of a
> yet-incomplete original packet.

> So far this kfunc is only callable from TC clsact progs.

> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>   include/net/ip.h           | 11 +++++
>   net/ipv4/Makefile          |  1 +
>   net/ipv4/ip_fragment.c     |  2 +
>   net/ipv4/ip_fragment_bpf.c | 98 ++++++++++++++++++++++++++++++++++++++
>   4 files changed, 112 insertions(+)
>   create mode 100644 net/ipv4/ip_fragment_bpf.c

> diff --git a/include/net/ip.h b/include/net/ip.h
> index c3fffaa92d6e..f3796b1b5cac 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -680,6 +680,7 @@ enum ip_defrag_users {
>   	IP_DEFRAG_VS_FWD,
>   	IP_DEFRAG_AF_PACKET,
>   	IP_DEFRAG_MACVLAN,
> +	IP_DEFRAG_BPF,
>   };

>   /* Return true if the value of 'user' is between 'lower_bond'
> @@ -693,6 +694,16 @@ static inline bool ip_defrag_user_in_between(u32  
> user,
>   }

>   int ip_defrag(struct net *net, struct sk_buff *skb, u32 user);
> +
> +#ifdef CONFIG_DEBUG_INFO_BTF
> +int register_ip_frag_bpf(void);
> +#else
> +static inline int register_ip_frag_bpf(void)
> +{
> +	return 0;
> +}
> +#endif
> +
>   #ifdef CONFIG_INET
>   struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb,  
> u32 user);
>   #else
> diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
> index 880277c9fd07..950efb166d37 100644
> --- a/net/ipv4/Makefile
> +++ b/net/ipv4/Makefile
> @@ -65,6 +65,7 @@ obj-$(CONFIG_TCP_CONG_ILLINOIS) += tcp_illinois.o
>   obj-$(CONFIG_NET_SOCK_MSG) += tcp_bpf.o
>   obj-$(CONFIG_BPF_SYSCALL) += udp_bpf.o
>   obj-$(CONFIG_NETLABEL) += cipso_ipv4.o
> +obj-$(CONFIG_DEBUG_INFO_BTF) += ip_fragment_bpf.o

>   obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
>   		      xfrm4_output.o xfrm4_protocol.o
> diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
> index 959d2c4260ea..e3fda5203f09 100644
> --- a/net/ipv4/ip_fragment.c
> +++ b/net/ipv4/ip_fragment.c
> @@ -759,5 +759,7 @@ void __init ipfrag_init(void)
>   	if (inet_frags_init(&ip4_frags))
>   		panic("IP: failed to allocate ip4_frags cache\n");
>   	ip4_frags_ctl_register();
> +	if (register_ip_frag_bpf())
> +		panic("IP: bpf: failed to register ip_frag_bpf\n");
>   	register_pernet_subsys(&ip4_frags_ops);
>   }
> diff --git a/net/ipv4/ip_fragment_bpf.c b/net/ipv4/ip_fragment_bpf.c
> new file mode 100644
> index 000000000000..a9e5908ed216
> --- /dev/null
> +++ b/net/ipv4/ip_fragment_bpf.c
> @@ -0,0 +1,98 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Unstable ipv4 fragmentation helpers for TC-BPF hook
> + *
> + * These are called from SCHED_CLS BPF programs. Note that it is allowed  
> to
> + * break compatibility for these functions since the interface they are  
> exposed
> + * through to BPF programs is explicitly unstable.
> + */
> +
> +#include <linux/bpf.h>
> +#include <linux/btf_ids.h>
> +#include <linux/ip.h>
> +#include <linux/filter.h>
> +#include <linux/netdevice.h>
> +#include <net/ip.h>
> +#include <net/sock.h>
> +
> +__diag_push();
> +__diag_ignore_all("-Wmissing-prototypes",
> +		  "Global functions as their definitions will be in ip_fragment BTF");
> +
> +/* bpf_ip_check_defrag - Defragment an ipv4 packet
> + *
> + * This helper takes an skb as input. If this skb successfully  
> reassembles
> + * the original packet, the skb is updated to contain the original,  
> reassembled
> + * packet.
> + *
> + * Otherwise (on error or incomplete reassembly), the input skb remains
> + * unmodified.
> + *
> + * Parameters:
> + * @ctx		- Pointer to program context (skb)
> + * @netns	- Child network namespace id. If value is a negative signed
> + *		  32-bit integer, the netns of the device in the skb is used.
> + *
> + * Return:
> + * 0 on successfully reassembly or non-fragmented packet. Negative value  
> on
> + * error or incomplete reassembly.
> + */
> +int bpf_ip_check_defrag(struct __sk_buff *ctx, u64 netns)

Needs a __bpf_kfunc tag as well?

> +{
> +	struct sk_buff *skb = (struct sk_buff *)ctx;
> +	struct sk_buff *skb_cpy, *skb_out;
> +	struct net *caller_net;
> +	struct net *net;
> +	int mac_len;
> +	void *mac;
> +

[..]

> +	if (unlikely(!((s32)netns < 0 || netns <= S32_MAX)))
> +		return -EINVAL;

Can you explain what it does? Is it checking for -1 explicitly? Not sure
it works :-/

Maybe we can spell out the cases explicitly?

if (unlikely(
	     ((s32)netns < 0 && netns != S32_MAX) || /* -1 */
	     netns > U32_MAX /* higher 4 bytes */
	    )
	return -EINVAL;

> +
> +	caller_net = skb->dev ? dev_net(skb->dev) : sock_net(skb->sk);
> +	if ((s32)netns < 0) {
> +		net = caller_net;
> +	} else {
> +		net = get_net_ns_by_id(caller_net, netns);
> +		if (unlikely(!net))
> +			return -EINVAL;
> +	}
> +
> +	mac_len = skb->mac_len;
> +	skb_cpy = skb_copy(skb, GFP_ATOMIC);
> +	if (!skb_cpy)
> +		return -ENOMEM;
> +
> +	skb_out = ip_check_defrag(net, skb_cpy, IP_DEFRAG_BPF);
> +	if (IS_ERR(skb_out))
> +		return PTR_ERR(skb_out);
> +
> +	skb_morph(skb, skb_out);
> +	kfree_skb(skb_out);
> +
> +	/* ip_check_defrag() does not maintain mac header, so push empty header
> +	 * in so prog sees the correct layout. The empty mac header will be
> +	 * later pulled from cls_bpf.
> +	 */
> +	mac = skb_push(skb, mac_len);
> +	memset(mac, 0, mac_len);
> +	bpf_compute_data_pointers(skb);
> +
> +	return 0;
> +}
> +
> +__diag_pop()
> +
> +BTF_SET8_START(ip_frag_kfunc_set)
> +BTF_ID_FLAGS(func, bpf_ip_check_defrag, KF_CHANGES_PKT)
> +BTF_SET8_END(ip_frag_kfunc_set)
> +
> +static const struct btf_kfunc_id_set ip_frag_bpf_kfunc_set = {
> +	.owner = THIS_MODULE,
> +	.set   = &ip_frag_kfunc_set,
> +};
> +
> +int register_ip_frag_bpf(void)
> +{
> +	return register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS,
> +					 &ip_frag_bpf_kfunc_set);
> +}
> --
> 2.39.1

Re: [PATCH bpf-next v2 3/8] bpf, net, frags: Add bpf_ip_check_defrag() kfunc

[Daniel Xu]

Hi Stanislav,

On Tue, Feb 28, 2023 at 11:37:16AM -0800, Stanislav Fomichev wrote:
> On 02/27, Daniel Xu wrote:
> > This kfunc is used to defragment IPv4 packets. The idea is that if you
> > see a fragmented packet, you call this kfunc. If the kfunc returns 0,
> > then the skb has been updated to contain the entire reassembled packet.
> 
> > If the kfunc returns an error (most likely -EINPROGRESS), then it means
> > the skb is part of a yet-incomplete original packet. A reasonable
> > response to -EINPROGRESS is to drop the packet, as the ip defrag
> > infrastructure is already hanging onto the frag for future reassembly.
> 
> > Care has been taken to ensure the prog skb remains valid no matter what
> > the underlying ip_check_defrag() call does. This is in contrast to
> > ip_defrag(), which may consume the skb if the skb is part of a
> > yet-incomplete original packet.
> 
> > So far this kfunc is only callable from TC clsact progs.
> 
> > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> > ---
> >   include/net/ip.h           | 11 +++++
> >   net/ipv4/Makefile          |  1 +
> >   net/ipv4/ip_fragment.c     |  2 +
> >   net/ipv4/ip_fragment_bpf.c | 98 ++++++++++++++++++++++++++++++++++++++
> >   4 files changed, 112 insertions(+)
> >   create mode 100644 net/ipv4/ip_fragment_bpf.c
> 
> > diff --git a/include/net/ip.h b/include/net/ip.h
> > index c3fffaa92d6e..f3796b1b5cac 100644
> > --- a/include/net/ip.h
> > +++ b/include/net/ip.h
> > @@ -680,6 +680,7 @@ enum ip_defrag_users {
> >   	IP_DEFRAG_VS_FWD,
> >   	IP_DEFRAG_AF_PACKET,
> >   	IP_DEFRAG_MACVLAN,
> > +	IP_DEFRAG_BPF,
> >   };
> 
> >   /* Return true if the value of 'user' is between 'lower_bond'
> > @@ -693,6 +694,16 @@ static inline bool ip_defrag_user_in_between(u32
> > user,
> >   }
> 
> >   int ip_defrag(struct net *net, struct sk_buff *skb, u32 user);
> > +
> > +#ifdef CONFIG_DEBUG_INFO_BTF
> > +int register_ip_frag_bpf(void);
> > +#else
> > +static inline int register_ip_frag_bpf(void)
> > +{
> > +	return 0;
> > +}
> > +#endif
> > +
> >   #ifdef CONFIG_INET
> >   struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb,
> > u32 user);
> >   #else
> > diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
> > index 880277c9fd07..950efb166d37 100644
> > --- a/net/ipv4/Makefile
> > +++ b/net/ipv4/Makefile
> > @@ -65,6 +65,7 @@ obj-$(CONFIG_TCP_CONG_ILLINOIS) += tcp_illinois.o
> >   obj-$(CONFIG_NET_SOCK_MSG) += tcp_bpf.o
> >   obj-$(CONFIG_BPF_SYSCALL) += udp_bpf.o
> >   obj-$(CONFIG_NETLABEL) += cipso_ipv4.o
> > +obj-$(CONFIG_DEBUG_INFO_BTF) += ip_fragment_bpf.o
> 
> >   obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
> >   		      xfrm4_output.o xfrm4_protocol.o
> > diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
> > index 959d2c4260ea..e3fda5203f09 100644
> > --- a/net/ipv4/ip_fragment.c
> > +++ b/net/ipv4/ip_fragment.c
> > @@ -759,5 +759,7 @@ void __init ipfrag_init(void)
> >   	if (inet_frags_init(&ip4_frags))
> >   		panic("IP: failed to allocate ip4_frags cache\n");
> >   	ip4_frags_ctl_register();
> > +	if (register_ip_frag_bpf())
> > +		panic("IP: bpf: failed to register ip_frag_bpf\n");
> >   	register_pernet_subsys(&ip4_frags_ops);
> >   }
> > diff --git a/net/ipv4/ip_fragment_bpf.c b/net/ipv4/ip_fragment_bpf.c
> > new file mode 100644
> > index 000000000000..a9e5908ed216
> > --- /dev/null
> > +++ b/net/ipv4/ip_fragment_bpf.c
> > @@ -0,0 +1,98 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/* Unstable ipv4 fragmentation helpers for TC-BPF hook
> > + *
> > + * These are called from SCHED_CLS BPF programs. Note that it is
> > allowed to
> > + * break compatibility for these functions since the interface they are
> > exposed
> > + * through to BPF programs is explicitly unstable.
> > + */
> > +
> > +#include <linux/bpf.h>
> > +#include <linux/btf_ids.h>
> > +#include <linux/ip.h>
> > +#include <linux/filter.h>
> > +#include <linux/netdevice.h>
> > +#include <net/ip.h>
> > +#include <net/sock.h>
> > +
> > +__diag_push();
> > +__diag_ignore_all("-Wmissing-prototypes",
> > +		  "Global functions as their definitions will be in ip_fragment BTF");
> > +
> > +/* bpf_ip_check_defrag - Defragment an ipv4 packet
> > + *
> > + * This helper takes an skb as input. If this skb successfully
> > reassembles
> > + * the original packet, the skb is updated to contain the original,
> > reassembled
> > + * packet.
> > + *
> > + * Otherwise (on error or incomplete reassembly), the input skb remains
> > + * unmodified.
> > + *
> > + * Parameters:
> > + * @ctx		- Pointer to program context (skb)
> > + * @netns	- Child network namespace id. If value is a negative signed
> > + *		  32-bit integer, the netns of the device in the skb is used.
> > + *
> > + * Return:
> > + * 0 on successfully reassembly or non-fragmented packet. Negative
> > value on
> > + * error or incomplete reassembly.
> > + */
> > +int bpf_ip_check_defrag(struct __sk_buff *ctx, u64 netns)
> 
> Needs a __bpf_kfunc tag as well?

Ack.

> > +{
> > +	struct sk_buff *skb = (struct sk_buff *)ctx;
> > +	struct sk_buff *skb_cpy, *skb_out;
> > +	struct net *caller_net;
> > +	struct net *net;
> > +	int mac_len;
> > +	void *mac;
> > +
> 
> [..]
> 
> > +	if (unlikely(!((s32)netns < 0 || netns <= S32_MAX)))
> > +		return -EINVAL;
> 
> Can you explain what it does? Is it checking for -1 explicitly? Not sure
> it works :-/
> 
> Maybe we can spell out the cases explicitly?
> if (unlikely(
> 	     ((s32)netns < 0 && netns != S32_MAX) || /* -1 */
> 	     netns > U32_MAX /* higher 4 bytes */
> 	    )
> 	return -EINVAL;
> 

I copied this from net/core/filter.c:__bpf_skc_lookup:

	if (unlikely(flags || !((s32)netns_id < 0 || netns_id <= S32_MAX)))
		goto out;

The semantics are a bit odd, but I thought it'd be good to maintain
consistency. I believe the code correctly checks what the docs describe:

        @netns	- Child network namespace id. If value is a negative signed
                  32-bit integer, the netns of the device in the skb is used.

I can pull out the logic into a helper for v3.

[...]


Thanks,
Daniel

Re: [PATCH bpf-next v2 3/8] bpf, net, frags: Add bpf_ip_check_defrag() kfunc

[Stanislav Fomichev]

On Tue, Feb 28, 2023 at 2:00 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> Hi Stanislav,
>
> On Tue, Feb 28, 2023 at 11:37:16AM -0800, Stanislav Fomichev wrote:
> > On 02/27, Daniel Xu wrote:
> > > This kfunc is used to defragment IPv4 packets. The idea is that if you
> > > see a fragmented packet, you call this kfunc. If the kfunc returns 0,
> > > then the skb has been updated to contain the entire reassembled packet.
> >
> > > If the kfunc returns an error (most likely -EINPROGRESS), then it means
> > > the skb is part of a yet-incomplete original packet. A reasonable
> > > response to -EINPROGRESS is to drop the packet, as the ip defrag
> > > infrastructure is already hanging onto the frag for future reassembly.
> >
> > > Care has been taken to ensure the prog skb remains valid no matter what
> > > the underlying ip_check_defrag() call does. This is in contrast to
> > > ip_defrag(), which may consume the skb if the skb is part of a
> > > yet-incomplete original packet.
> >
> > > So far this kfunc is only callable from TC clsact progs.
> >
> > > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> > > ---
> > >   include/net/ip.h           | 11 +++++
> > >   net/ipv4/Makefile          |  1 +
> > >   net/ipv4/ip_fragment.c     |  2 +
> > >   net/ipv4/ip_fragment_bpf.c | 98 ++++++++++++++++++++++++++++++++++++++
> > >   4 files changed, 112 insertions(+)
> > >   create mode 100644 net/ipv4/ip_fragment_bpf.c
> >
> > > diff --git a/include/net/ip.h b/include/net/ip.h
> > > index c3fffaa92d6e..f3796b1b5cac 100644
> > > --- a/include/net/ip.h
> > > +++ b/include/net/ip.h
> > > @@ -680,6 +680,7 @@ enum ip_defrag_users {
> > >     IP_DEFRAG_VS_FWD,
> > >     IP_DEFRAG_AF_PACKET,
> > >     IP_DEFRAG_MACVLAN,
> > > +   IP_DEFRAG_BPF,
> > >   };
> >
> > >   /* Return true if the value of 'user' is between 'lower_bond'
> > > @@ -693,6 +694,16 @@ static inline bool ip_defrag_user_in_between(u32
> > > user,
> > >   }
> >
> > >   int ip_defrag(struct net *net, struct sk_buff *skb, u32 user);
> > > +
> > > +#ifdef CONFIG_DEBUG_INFO_BTF
> > > +int register_ip_frag_bpf(void);
> > > +#else
> > > +static inline int register_ip_frag_bpf(void)
> > > +{
> > > +   return 0;
> > > +}
> > > +#endif
> > > +
> > >   #ifdef CONFIG_INET
> > >   struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb,
> > > u32 user);
> > >   #else
> > > diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
> > > index 880277c9fd07..950efb166d37 100644
> > > --- a/net/ipv4/Makefile
> > > +++ b/net/ipv4/Makefile
> > > @@ -65,6 +65,7 @@ obj-$(CONFIG_TCP_CONG_ILLINOIS) += tcp_illinois.o
> > >   obj-$(CONFIG_NET_SOCK_MSG) += tcp_bpf.o
> > >   obj-$(CONFIG_BPF_SYSCALL) += udp_bpf.o
> > >   obj-$(CONFIG_NETLABEL) += cipso_ipv4.o
> > > +obj-$(CONFIG_DEBUG_INFO_BTF) += ip_fragment_bpf.o
> >
> > >   obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
> > >                   xfrm4_output.o xfrm4_protocol.o
> > > diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
> > > index 959d2c4260ea..e3fda5203f09 100644
> > > --- a/net/ipv4/ip_fragment.c
> > > +++ b/net/ipv4/ip_fragment.c
> > > @@ -759,5 +759,7 @@ void __init ipfrag_init(void)
> > >     if (inet_frags_init(&ip4_frags))
> > >             panic("IP: failed to allocate ip4_frags cache\n");
> > >     ip4_frags_ctl_register();
> > > +   if (register_ip_frag_bpf())
> > > +           panic("IP: bpf: failed to register ip_frag_bpf\n");
> > >     register_pernet_subsys(&ip4_frags_ops);
> > >   }
> > > diff --git a/net/ipv4/ip_fragment_bpf.c b/net/ipv4/ip_fragment_bpf.c
> > > new file mode 100644
> > > index 000000000000..a9e5908ed216
> > > --- /dev/null
> > > +++ b/net/ipv4/ip_fragment_bpf.c
> > > @@ -0,0 +1,98 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/* Unstable ipv4 fragmentation helpers for TC-BPF hook
> > > + *
> > > + * These are called from SCHED_CLS BPF programs. Note that it is
> > > allowed to
> > > + * break compatibility for these functions since the interface they are
> > > exposed
> > > + * through to BPF programs is explicitly unstable.
> > > + */
> > > +
> > > +#include <linux/bpf.h>
> > > +#include <linux/btf_ids.h>
> > > +#include <linux/ip.h>
> > > +#include <linux/filter.h>
> > > +#include <linux/netdevice.h>
> > > +#include <net/ip.h>
> > > +#include <net/sock.h>
> > > +
> > > +__diag_push();
> > > +__diag_ignore_all("-Wmissing-prototypes",
> > > +             "Global functions as their definitions will be in ip_fragment BTF");
> > > +
> > > +/* bpf_ip_check_defrag - Defragment an ipv4 packet
> > > + *
> > > + * This helper takes an skb as input. If this skb successfully
> > > reassembles
> > > + * the original packet, the skb is updated to contain the original,
> > > reassembled
> > > + * packet.
> > > + *
> > > + * Otherwise (on error or incomplete reassembly), the input skb remains
> > > + * unmodified.
> > > + *
> > > + * Parameters:
> > > + * @ctx            - Pointer to program context (skb)
> > > + * @netns  - Child network namespace id. If value is a negative signed
> > > + *           32-bit integer, the netns of the device in the skb is used.
> > > + *
> > > + * Return:
> > > + * 0 on successfully reassembly or non-fragmented packet. Negative
> > > value on
> > > + * error or incomplete reassembly.
> > > + */
> > > +int bpf_ip_check_defrag(struct __sk_buff *ctx, u64 netns)
> >
> > Needs a __bpf_kfunc tag as well?
>
> Ack.
>
> > > +{
> > > +   struct sk_buff *skb = (struct sk_buff *)ctx;
> > > +   struct sk_buff *skb_cpy, *skb_out;
> > > +   struct net *caller_net;
> > > +   struct net *net;
> > > +   int mac_len;
> > > +   void *mac;
> > > +
> >
> > [..]
> >
> > > +   if (unlikely(!((s32)netns < 0 || netns <= S32_MAX)))
> > > +           return -EINVAL;
> >
> > Can you explain what it does? Is it checking for -1 explicitly? Not sure
> > it works :-/
> >
> > Maybe we can spell out the cases explicitly?
> > if (unlikely(
> >            ((s32)netns < 0 && netns != S32_MAX) || /* -1 */
> >            netns > U32_MAX /* higher 4 bytes */
> >           )
> >       return -EINVAL;
> >
>
> I copied this from net/core/filter.c:__bpf_skc_lookup:
>
>         if (unlikely(flags || !((s32)netns_id < 0 || netns_id <= S32_MAX)))
>                 goto out;
>
> The semantics are a bit odd, but I thought it'd be good to maintain
> consistency. I believe the code correctly checks what the docs describe:
>
>         @netns  - Child network namespace id. If value is a negative signed
>                   32-bit integer, the netns of the device in the skb is used.
>
> I can pull out the logic into a helper for v3.
>
> [...]

Ah, so this comes from commit f71c6143c203 ("bpf: Support sk lookup in
netns with id 0") which explicitly treats everything <0 as
current_netns, makes sense.
In this case agreed, let's keep for consistency. Up to you on whether
to pull it out in the helper or keep as is.

>
>
> Thanks,
> Daniel

[PATCH bpf-next v2 4/8] net: ipv6: Factor ipv6_frag_rcv() to take netns and user

[Daniel Xu]

Factor _ipv6_frag_rcv() out of ipv6_frag_rcv() such that the former
takes a netns and user field.

We do this so that the BPF interface for ipv6 defrag can have the same
semantics as ipv4 defrag (see ip_check_defrag()).

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 include/net/ipv6.h    |  1 +
 net/ipv6/reassembly.c | 16 +++++++++++-----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index 7332296eca44..9bbdf82ca6c0 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -1238,6 +1238,7 @@ int inet6_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
 extern const struct proto_ops inet6_stream_ops;
 extern const struct proto_ops inet6_dgram_ops;
 extern const struct proto_ops inet6_sockraw_ops;
+int _ipv6_frag_rcv(struct net *net, struct sk_buff *skb, u32 user);
 
 struct group_source_req;
 struct group_filter;
diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
index 5bc8a28e67f9..5100430eb982 100644
--- a/net/ipv6/reassembly.c
+++ b/net/ipv6/reassembly.c
@@ -81,13 +81,13 @@ static void ip6_frag_expire(struct timer_list *t)
 }
 
 static struct frag_queue *
-fq_find(struct net *net, __be32 id, const struct ipv6hdr *hdr, int iif)
+fq_find(struct net *net, __be32 id, const struct ipv6hdr *hdr, int iif, u32 user)
 {
 	struct frag_v6_compare_key key = {
 		.id = id,
 		.saddr = hdr->saddr,
 		.daddr = hdr->daddr,
-		.user = IP6_DEFRAG_LOCAL_DELIVER,
+		.user = user,
 		.iif = iif,
 	};
 	struct inet_frag_queue *q;
@@ -324,12 +324,11 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *skb,
 	return -1;
 }
 
-static int ipv6_frag_rcv(struct sk_buff *skb)
+int _ipv6_frag_rcv(struct net *net, struct sk_buff *skb, u32 user)
 {
 	struct frag_hdr *fhdr;
 	struct frag_queue *fq;
 	const struct ipv6hdr *hdr = ipv6_hdr(skb);
-	struct net *net = dev_net(skb_dst(skb)->dev);
 	u8 nexthdr;
 	int iif;
 
@@ -377,7 +376,7 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
 	}
 
 	iif = skb->dev ? skb->dev->ifindex : 0;
-	fq = fq_find(net, fhdr->identification, hdr, iif);
+	fq = fq_find(net, fhdr->identification, hdr, iif, user);
 	if (fq) {
 		u32 prob_offset = 0;
 		int ret;
@@ -410,6 +409,13 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
 	return -1;
 }
 
+static int ipv6_frag_rcv(struct sk_buff *skb)
+{
+	struct net *net = dev_net(skb_dst(skb)->dev);
+
+	return _ipv6_frag_rcv(net, skb, IP6_DEFRAG_LOCAL_DELIVER);
+}
+
 static const struct inet6_protocol frag_protocol = {
 	.handler	=	ipv6_frag_rcv,
 	.flags		=	INET6_PROTO_NOPOLICY,
-- 
2.39.1

[PATCH bpf-next v2 5/8] bpf: net: ipv6: Add bpf_ipv6_frag_rcv() kfunc

[Daniel Xu]

This helper is used to defragment IPv6 packets. Similar to the previous
bpf_ip_check_defrag() kfunc, this kfunc:

* Returns 0 on defrag + skb update success
* Returns < 0 on error
* Takes care to ensure ctx (skb) remains valid no matter what the
  underlying call to _ipv6_frag_rcv() does
* Is only callable from TC clsact progs

Please see bpf_ip_check_defrag() commit for more details / suggestions.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 include/net/ipv6_frag.h   |   1 +
 include/net/transp_v6.h   |   1 +
 net/ipv6/Makefile         |   1 +
 net/ipv6/af_inet6.c       |   4 ++
 net/ipv6/reassembly_bpf.c | 143 ++++++++++++++++++++++++++++++++++++++
 5 files changed, 150 insertions(+)
 create mode 100644 net/ipv6/reassembly_bpf.c

diff --git a/include/net/ipv6_frag.h b/include/net/ipv6_frag.h
index 7321ffe3a108..cf4763cd3886 100644
--- a/include/net/ipv6_frag.h
+++ b/include/net/ipv6_frag.h
@@ -15,6 +15,7 @@ enum ip6_defrag_users {
 	__IP6_DEFRAG_CONNTRACK_OUT	= IP6_DEFRAG_CONNTRACK_OUT + USHRT_MAX,
 	IP6_DEFRAG_CONNTRACK_BRIDGE_IN,
 	__IP6_DEFRAG_CONNTRACK_BRIDGE_IN = IP6_DEFRAG_CONNTRACK_BRIDGE_IN + USHRT_MAX,
+	IP6_DEFRAG_BPF,
 };
 
 /*
diff --git a/include/net/transp_v6.h b/include/net/transp_v6.h
index d27b1caf3753..244123a74349 100644
--- a/include/net/transp_v6.h
+++ b/include/net/transp_v6.h
@@ -20,6 +20,7 @@ int ipv6_exthdrs_init(void);
 void ipv6_exthdrs_exit(void);
 int ipv6_frag_init(void);
 void ipv6_frag_exit(void);
+int register_ipv6_reassembly_bpf(void);
 
 /* transport protocols */
 int pingv6_init(void);
diff --git a/net/ipv6/Makefile b/net/ipv6/Makefile
index 3036a45e8a1e..6e90ff1d20c0 100644
--- a/net/ipv6/Makefile
+++ b/net/ipv6/Makefile
@@ -26,6 +26,7 @@ ipv6-$(CONFIG_IPV6_SEG6_LWTUNNEL) += seg6_iptunnel.o seg6_local.o
 ipv6-$(CONFIG_IPV6_SEG6_HMAC) += seg6_hmac.o
 ipv6-$(CONFIG_IPV6_RPL_LWTUNNEL) += rpl_iptunnel.o
 ipv6-$(CONFIG_IPV6_IOAM6_LWTUNNEL) += ioam6_iptunnel.o
+ipv6-$(CONFIG_DEBUG_INFO_BTF) += reassembly_bpf.o
 
 obj-$(CONFIG_INET6_AH) += ah6.o
 obj-$(CONFIG_INET6_ESP) += esp6.o
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index 38689bedfce7..39663de75fbd 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -1174,6 +1174,10 @@ static int __init inet6_init(void)
 	if (err)
 		goto ipv6_frag_fail;
 
+	err = register_ipv6_reassembly_bpf();
+	if (err)
+		goto ipv6_frag_fail;
+
 	/* Init v6 transport protocols. */
 	err = udpv6_init();
 	if (err)
diff --git a/net/ipv6/reassembly_bpf.c b/net/ipv6/reassembly_bpf.c
new file mode 100644
index 000000000000..c6c804d4f636
--- /dev/null
+++ b/net/ipv6/reassembly_bpf.c
@@ -0,0 +1,143 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Unstable ipv6 fragmentation helpers for TC-BPF hook
+ *
+ * These are called from SCHED_CLS BPF programs. Note that it is allowed to
+ * break compatibility for these functions since the interface they are exposed
+ * through to BPF programs is explicitly unstable.
+ */
+
+#include <linux/bpf.h>
+#include <linux/btf_ids.h>
+#include <linux/filter.h>
+#include <linux/netdevice.h>
+#include <net/ipv6.h>
+#include <net/ipv6_frag.h>
+#include <net/ipv6_stubs.h>
+
+static int set_dst(struct sk_buff *skb, struct net *net)
+{
+	const struct ipv6hdr *ip6h = ipv6_hdr(skb);
+	struct dst_entry *dst;
+
+	struct flowi6 fl6 = {
+		.flowi6_flags = FLOWI_FLAG_ANYSRC,
+		.flowi6_mark  = skb->mark,
+		.flowlabel    = ip6_flowinfo(ip6h),
+		.flowi6_iif   = skb->skb_iif,
+		.flowi6_proto = ip6h->nexthdr,
+		.daddr	      = ip6h->daddr,
+		.saddr	      = ip6h->saddr,
+	};
+
+	dst = ipv6_stub->ipv6_dst_lookup_flow(net, NULL, &fl6, NULL);
+	if (IS_ERR(dst))
+		return PTR_ERR(dst);
+
+	skb_dst_set(skb, dst);
+
+	return 0;
+}
+
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "Global functions as their definitions will be in reassembly BTF");
+
+/* bpf_ipv6_frag_rcv - Defragment an ipv6 packet
+ *
+ * This helper takes an skb as input. If this skb successfully reassembles
+ * the original packet, the skb is updated to contain the original, reassembled
+ * packet.
+ *
+ * Otherwise (on error or incomplete reassembly), the input skb remains
+ * unmodified.
+ *
+ * Parameters:
+ * @ctx		- Pointer to program context (skb)
+ * @netns	- Child network namespace id. If value is a negative signed
+ *		  32-bit integer, the netns of the device in the skb is used.
+ *
+ * Return:
+ * 0 on successfully reassembly or non-fragmented packet. Negative value on
+ * error or incomplete reassembly.
+ */
+int bpf_ipv6_frag_rcv(struct __sk_buff *ctx, u64 netns)
+{
+	struct sk_buff *skb = (struct sk_buff *)ctx;
+	struct sk_buff *skb_cpy;
+	struct net *caller_net;
+	unsigned int foff;
+	struct net *net;
+	int mac_len;
+	void *mac;
+	int err;
+
+	if (unlikely(!((s32)netns < 0 || netns <= S32_MAX)))
+		return -EINVAL;
+
+	caller_net = skb->dev ? dev_net(skb->dev) : sock_net(skb->sk);
+	if ((s32)netns < 0) {
+		net = caller_net;
+	} else {
+		net = get_net_ns_by_id(caller_net, netns);
+		if (unlikely(!net))
+			return -EINVAL;
+	}
+
+	err = set_dst(skb, net);
+	if (err < 0)
+		return err;
+
+	mac_len = skb->mac_len;
+	skb_cpy = skb_copy(skb, GFP_ATOMIC);
+	if (!skb_cpy)
+		return -ENOMEM;
+
+	/* _ipv6_frag_rcv() expects skb->transport_header to be set to start of
+	 * the frag header and nhoff to be set.
+	 */
+	err = ipv6_find_hdr(skb_cpy, &foff, NEXTHDR_FRAGMENT, NULL, NULL);
+	if (err < 0)
+		return err;
+	skb_set_transport_header(skb_cpy, foff);
+	IP6CB(skb_cpy)->nhoff = offsetof(struct ipv6hdr, nexthdr);
+
+	/* inet6_protocol handlers return >0 on success, 0 on out of band
+	 * consumption, <0 on error. We never expect to see 0 here.
+	 */
+	err = _ipv6_frag_rcv(net, skb_cpy, IP6_DEFRAG_BPF);
+	if (err < 0)
+		return err;
+	else if (err == 0)
+		return -EINVAL;
+
+	skb_morph(skb, skb_cpy);
+	kfree_skb(skb_cpy);
+
+	/* _ipv6_frag_rcv() does not maintain mac header, so push empty header
+	 * in so prog sees the correct layout. The empty mac header will be
+	 * later pulled from cls_bpf.
+	 */
+	skb->mac_len = mac_len;
+	mac = skb_push(skb, mac_len);
+	memset(mac, 0, mac_len);
+	bpf_compute_data_pointers(skb);
+
+	return 0;
+}
+
+__diag_pop()
+
+BTF_SET8_START(ipv6_reassembly_kfunc_set)
+BTF_ID_FLAGS(func, bpf_ipv6_frag_rcv, KF_CHANGES_PKT)
+BTF_SET8_END(ipv6_reassembly_kfunc_set)
+
+static const struct btf_kfunc_id_set ipv6_reassembly_bpf_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set   = &ipv6_reassembly_kfunc_set,
+};
+
+int register_ipv6_reassembly_bpf(void)
+{
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS,
+					 &ipv6_reassembly_bpf_kfunc_set);
+}
-- 
2.39.1

Re: [PATCH bpf-next v2 5/8] bpf: net: ipv6: Add bpf_ipv6_frag_rcv() kfunc

[kernel test robot]

Hi Daniel,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on bpf-next/master]

url:    https://github.com/intel-lab-lkp/linux/commits/Daniel-Xu/ip-frags-Return-actual-error-codes-from-ip_check_defrag/20230228-035449
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
patch link:    https://lore.kernel.org/r/bce083a4293eefb048a700b5a6086e8d8c957700.1677526810.git.dxu%40dxuuu.xyz
patch subject: [PATCH bpf-next v2 5/8] bpf: net: ipv6: Add bpf_ipv6_frag_rcv() kfunc
config: i386-defconfig (https://download.01.org/0day-ci/archive/20230228/202302281646.GYE1qnGb-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-8) 11.3.0
reproduce (this is a W=1 build):
        # https://github.com/intel-lab-lkp/linux/commit/be4610312351d4a658435bd4649a3a830322396d
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Daniel-Xu/ip-frags-Return-actual-error-codes-from-ip_check_defrag/20230228-035449
        git checkout be4610312351d4a658435bd4649a3a830322396d
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Link: https://lore.kernel.org/oe-kbuild-all/202302281646.GYE1qnGb-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: net/ipv6/af_inet6.o: in function `inet6_init':
>> af_inet6.c:(.init.text+0x22a): undefined reference to `register_ipv6_reassembly_bpf'

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests

Re: [PATCH bpf-next v2 5/8] bpf: net: ipv6: Add bpf_ipv6_frag_rcv() kfunc

[kernel test robot]

Hi Daniel,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on bpf-next/master]

url:    https://github.com/intel-lab-lkp/linux/commits/Daniel-Xu/ip-frags-Return-actual-error-codes-from-ip_check_defrag/20230228-035449
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
patch link:    https://lore.kernel.org/r/bce083a4293eefb048a700b5a6086e8d8c957700.1677526810.git.dxu%40dxuuu.xyz
patch subject: [PATCH bpf-next v2 5/8] bpf: net: ipv6: Add bpf_ipv6_frag_rcv() kfunc
config: i386-debian-10.3 (https://download.01.org/0day-ci/archive/20230228/202302281707.5vUL3boJ-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-8) 11.3.0
reproduce (this is a W=1 build):
        # https://github.com/intel-lab-lkp/linux/commit/be4610312351d4a658435bd4649a3a830322396d
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Daniel-Xu/ip-frags-Return-actual-error-codes-from-ip_check_defrag/20230228-035449
        git checkout be4610312351d4a658435bd4649a3a830322396d
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Link: https://lore.kernel.org/oe-kbuild-all/202302281707.5vUL3boJ-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: net/ipv6/af_inet6.o: in function `inet6_init':
>> net/ipv6/af_inet6.c:1177: undefined reference to `register_ipv6_reassembly_bpf'


vim +1177 net/ipv6/af_inet6.c

  1061	
  1062	static int __init inet6_init(void)
  1063	{
  1064		struct list_head *r;
  1065		int err = 0;
  1066	
  1067		sock_skb_cb_check_size(sizeof(struct inet6_skb_parm));
  1068	
  1069		/* Register the socket-side information for inet6_create.  */
  1070		for (r = &inetsw6[0]; r < &inetsw6[SOCK_MAX]; ++r)
  1071			INIT_LIST_HEAD(r);
  1072	
  1073		raw_hashinfo_init(&raw_v6_hashinfo);
  1074	
  1075		if (disable_ipv6_mod) {
  1076			pr_info("Loaded, but administratively disabled, reboot required to enable\n");
  1077			goto out;
  1078		}
  1079	
  1080		err = proto_register(&tcpv6_prot, 1);
  1081		if (err)
  1082			goto out;
  1083	
  1084		err = proto_register(&udpv6_prot, 1);
  1085		if (err)
  1086			goto out_unregister_tcp_proto;
  1087	
  1088		err = proto_register(&udplitev6_prot, 1);
  1089		if (err)
  1090			goto out_unregister_udp_proto;
  1091	
  1092		err = proto_register(&rawv6_prot, 1);
  1093		if (err)
  1094			goto out_unregister_udplite_proto;
  1095	
  1096		err = proto_register(&pingv6_prot, 1);
  1097		if (err)
  1098			goto out_unregister_raw_proto;
  1099	
  1100		/* We MUST register RAW sockets before we create the ICMP6,
  1101		 * IGMP6, or NDISC control sockets.
  1102		 */
  1103		err = rawv6_init();
  1104		if (err)
  1105			goto out_unregister_ping_proto;
  1106	
  1107		/* Register the family here so that the init calls below will
  1108		 * be able to create sockets. (?? is this dangerous ??)
  1109		 */
  1110		err = sock_register(&inet6_family_ops);
  1111		if (err)
  1112			goto out_sock_register_fail;
  1113	
  1114		/*
  1115		 *	ipngwg API draft makes clear that the correct semantics
  1116		 *	for TCP and UDP is to consider one TCP and UDP instance
  1117		 *	in a host available by both INET and INET6 APIs and
  1118		 *	able to communicate via both network protocols.
  1119		 */
  1120	
  1121		err = register_pernet_subsys(&inet6_net_ops);
  1122		if (err)
  1123			goto register_pernet_fail;
  1124		err = ip6_mr_init();
  1125		if (err)
  1126			goto ipmr_fail;
  1127		err = icmpv6_init();
  1128		if (err)
  1129			goto icmp_fail;
  1130		err = ndisc_init();
  1131		if (err)
  1132			goto ndisc_fail;
  1133		err = igmp6_init();
  1134		if (err)
  1135			goto igmp_fail;
  1136	
  1137		err = ipv6_netfilter_init();
  1138		if (err)
  1139			goto netfilter_fail;
  1140		/* Create /proc/foo6 entries. */
  1141	#ifdef CONFIG_PROC_FS
  1142		err = -ENOMEM;
  1143		if (raw6_proc_init())
  1144			goto proc_raw6_fail;
  1145		if (udplite6_proc_init())
  1146			goto proc_udplite6_fail;
  1147		if (ipv6_misc_proc_init())
  1148			goto proc_misc6_fail;
  1149		if (if6_proc_init())
  1150			goto proc_if6_fail;
  1151	#endif
  1152		err = ip6_route_init();
  1153		if (err)
  1154			goto ip6_route_fail;
  1155		err = ndisc_late_init();
  1156		if (err)
  1157			goto ndisc_late_fail;
  1158		err = ip6_flowlabel_init();
  1159		if (err)
  1160			goto ip6_flowlabel_fail;
  1161		err = ipv6_anycast_init();
  1162		if (err)
  1163			goto ipv6_anycast_fail;
  1164		err = addrconf_init();
  1165		if (err)
  1166			goto addrconf_fail;
  1167	
  1168		/* Init v6 extension headers. */
  1169		err = ipv6_exthdrs_init();
  1170		if (err)
  1171			goto ipv6_exthdrs_fail;
  1172	
  1173		err = ipv6_frag_init();
  1174		if (err)
  1175			goto ipv6_frag_fail;
  1176	
> 1177		err = register_ipv6_reassembly_bpf();
  1178		if (err)
  1179			goto ipv6_frag_fail;
  1180	
  1181		/* Init v6 transport protocols. */
  1182		err = udpv6_init();
  1183		if (err)
  1184			goto udpv6_fail;
  1185	
  1186		err = udplitev6_init();
  1187		if (err)
  1188			goto udplitev6_fail;
  1189	
  1190		err = udpv6_offload_init();
  1191		if (err)
  1192			goto udpv6_offload_fail;
  1193	
  1194		err = tcpv6_init();
  1195		if (err)
  1196			goto tcpv6_fail;
  1197	
  1198		err = ipv6_packet_init();
  1199		if (err)
  1200			goto ipv6_packet_fail;
  1201	
  1202		err = pingv6_init();
  1203		if (err)
  1204			goto pingv6_fail;
  1205	
  1206		err = calipso_init();
  1207		if (err)
  1208			goto calipso_fail;
  1209	
  1210		err = seg6_init();
  1211		if (err)
  1212			goto seg6_fail;
  1213	
  1214		err = rpl_init();
  1215		if (err)
  1216			goto rpl_fail;
  1217	
  1218		err = ioam6_init();
  1219		if (err)
  1220			goto ioam6_fail;
  1221	
  1222		err = igmp6_late_init();
  1223		if (err)
  1224			goto igmp6_late_err;
  1225	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Edward Cree]

On 27/02/2023 19:51, Daniel Xu wrote:
> However, when policy is enforced through BPF, the prog is run before the
> kernel reassembles fragmented packets. This leaves BPF developers in a
> awkward place: implement reassembly (possibly poorly) or use a stateless
> method as described above.

Just out of curiosity - what stops BPF progs using the middle ground of
 stateful validation?  I'm thinking of something like:
First-frag: run the usual checks on L4 headers etc, if we PASS then save
 IPID and maybe expected next frag-offset into a map.  But don't try to
 stash the packet contents anywhere for later reassembly, just PASS it.
Subsequent frags: look up the IPID in the map.  If we find it, validate
 and update the frag-offset in the map; if this is the last fragment then
 delete the map entry.  If the frag-offset was bogus or the IPID wasn't
 found in the map, DROP; otherwise PASS.
(If re-ordering is prevalent then use something more sophisticated than
 just expected next frag-offset, but the principle is the same. And of
 course you might want to put in timers for expiry etc.)
So this avoids the need to stash the packet data and modify/consume SKBs,
 because you're not actually doing reassembly; the down-side is that the
 BPF program can't so easily make decisions about the application-layer
 contents of the fragmented datagram, but for the common case (we just
 care about the 5-tuple) it's simple enough.
But I haven't actually tried it, so maybe there's some obvious reason why
 it can't work this way.

-ed

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Daniel Xu]

Hi Ed,

Thanks for giving this a look.

On Mon, Feb 27, 2023 at 08:38:41PM +0000, Edward Cree wrote:
> On 27/02/2023 19:51, Daniel Xu wrote:
> > However, when policy is enforced through BPF, the prog is run before the
> > kernel reassembles fragmented packets. This leaves BPF developers in a
> > awkward place: implement reassembly (possibly poorly) or use a stateless
> > method as described above.
> 
> Just out of curiosity - what stops BPF progs using the middle ground of
>  stateful validation?  I'm thinking of something like:
> First-frag: run the usual checks on L4 headers etc, if we PASS then save
>  IPID and maybe expected next frag-offset into a map.  But don't try to
>  stash the packet contents anywhere for later reassembly, just PASS it.
> Subsequent frags: look up the IPID in the map.  If we find it, validate
>  and update the frag-offset in the map; if this is the last fragment then
>  delete the map entry.  If the frag-offset was bogus or the IPID wasn't
>  found in the map, DROP; otherwise PASS.
> (If re-ordering is prevalent then use something more sophisticated than
>  just expected next frag-offset, but the principle is the same. And of
>  course you might want to put in timers for expiry etc.)
> So this avoids the need to stash the packet data and modify/consume SKBs,
>  because you're not actually doing reassembly; the down-side is that the
>  BPF program can't so easily make decisions about the application-layer
>  contents of the fragmented datagram, but for the common case (we just
>  care about the 5-tuple) it's simple enough.
> But I haven't actually tried it, so maybe there's some obvious reason why
>  it can't work this way.

I don't believe full L4 headers are required in the first fragment.
Sufficiently sneaky attackers can, I think, send a byte at a time to
subvert your proposed algorithm. Storing skb data seems inevitable here.
Someone can correct me if I'm wrong here.

Reordering like you mentioned is another attack vector. Perhaps there
are more sophisticated semi-stateful algorithms that can solve the
problem, but it leads me to my next point.

A semi-stateful method like you are proposing is concerning to me from a
reliability and correctness stand point. Such a method can suffer from
impedance mismatches with the rest of the system. For example, whatever
map sizes you choose should probably be aligned with sysfs conntrack
values otherwise you may get some very interesting and unexpected pkt
drops. I think cilium had a talk about debugging a related conntrack
issue in the same vein a while ago. Furthermore, the debugging and
troubleshooting facilities will be different (counters, logs, etc).

Unless someone has had lots of experience writing an ip stack from
the ground up, I suspect there are quite a few more unknown-unknowns
here. What I find valuable about this patch series is that we can
leverage the well understood and battle hardened kernel facilities. So
avoid all the correctness and security issues that the kernel has spent
20+ years fixing. And make it trivial for the next person that comes
along to do the right thing.

Hopefully this all makes sense.

Thanks,
Daniel

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Edward Cree]

On 27/02/2023 22:04, Daniel Xu wrote:
> I don't believe full L4 headers are required in the first fragment.
> Sufficiently sneaky attackers can, I think, send a byte at a time to
> subvert your proposed algorithm. Storing skb data seems inevitable here.
> Someone can correct me if I'm wrong here.

My thinking was that legitimate traffic would never do this and thus if
 your first fragment doesn't have enough data to make a determination
 then you just DROP the packet.

> What I find valuable about this patch series is that we can
> leverage the well understood and battle hardened kernel facilities. So
> avoid all the correctness and security issues that the kernel has spent
> 20+ years fixing.

I can certainly see the argument here.  I guess it's a question of are
 you more worried about the DoS from tricking the validator into thinking
 good fragments are bad (the reverse is irrelevant because if you can
 trick a validator into thinking your bad fragment belongs to a previously
 seen good packet, then you can equally trick a reassembler into stitching
 your bad fragment into that packet), or are you more worried about the
 DoS from tying lots of memory down in the reassembly cache.
Even with reordering handling, a data structure to record which ranges of
 a packet have been seen takes much less memory than storing the complete
 fragment bodies.  (Just a simple bitmap of 8-byte blocks — the resolution
 of iph->frag_off — reduces size by a factor of 64, not counting all the
 overhead of a struct sk_buff for each fragment in the queue.  Or you
 could re-use the rbtree-based code from the reassembler, just with a
 freshly allocated node containing only offset & length, instead of the
 whole SKB.)
And having a BPF helper effectively consume the skb is awkward, as you
 noted; someone is likely to decide that skb_copy() is too slow, try to
 add ctx invalidation, and thereby create a whole new swathe of potential
 correctness and security issues.
Plus, imagine trying to support this in a hardware-offload XDP device.
 They'd have to reimplement the entire frag cache, which is a much bigger
 attack surface than just a frag validator, and they couldn't leverage
 the battle-hardened kernel implementation.

> And make it trivial for the next person that comes
> along to do the right thing.

Fwiw the validator approach could *also* be a helper, it doesn't have to
 be something the BPF developer writes for themselves.

But if after thinking about the possibility you still prefer your way, I
 won't try to stop you — I just wanted to ensure it had been considered.

-ed

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Daniel Xu]

Hi Ed,

Had some trouble with email yesterday (forgot to renew domain
registration) and this reply might not have made it out. Apologies
if it's a repost.

On Mon, Feb 27, 2023 at 10:58:47PM +0000, Edward Cree wrote:
> On 27/02/2023 22:04, Daniel Xu wrote:
> > I don't believe full L4 headers are required in the first fragment.
> > Sufficiently sneaky attackers can, I think, send a byte at a time to
> > subvert your proposed algorithm. Storing skb data seems inevitable here.
> > Someone can correct me if I'm wrong here.
> 
> My thinking was that legitimate traffic would never do this and thus if
>  your first fragment doesn't have enough data to make a determination
>  then you just DROP the packet.

Right, that would be practical. I had some discussion with coworkers and
the other option on the table is to drop all fragments. At least for us
in the cloud, fragments are heavily frowned upon (where are they not..)
anyways.

> > What I find valuable about this patch series is that we can
> > leverage the well understood and battle hardened kernel facilities. So
> > avoid all the correctness and security issues that the kernel has spent
> > 20+ years fixing.
> 
> I can certainly see the argument here.  I guess it's a question of are
>  you more worried about the DoS from tricking the validator into thinking
>  good fragments are bad (the reverse is irrelevant because if you can
>  trick a validator into thinking your bad fragment belongs to a previously
>  seen good packet, then you can equally trick a reassembler into stitching
>  your bad fragment into that packet), or are you more worried about the
>  DoS from tying lots of memory down in the reassembly cache.

Equal balance of concerns on my side. Ideally there are no dropping of
valid packets and DoS is very hard to achieve.

> Even with reordering handling, a data structure to record which ranges of
>  a packet have been seen takes much less memory than storing the complete
>  fragment bodies.  (Just a simple bitmap of 8-byte blocks — the resolution
>  of iph->frag_off — reduces size by a factor of 64, not counting all the
>  overhead of a struct sk_buff for each fragment in the queue.  Or you
>  could re-use the rbtree-based code from the reassembler, just with a
>  freshly allocated node containing only offset & length, instead of the
>  whole SKB.)

Yeah, now that you say that, it doesn't sound too bad on space side. But
I do wonder -- how much code and complexity is that going to be? For
example I think ipv6 frags have a 60s reassembly timeout which adds more
stuff to consider. And probably even more I've already forgotten.

B/c at least on the kernel side, this series is 80% code for tests. And
the kfunc wrappers are not very invasive at all.  Plus it's wrapping
infra that hasn't changed much for decades.


> And having a BPF helper effectively consume the skb is awkward, as you
>  noted; someone is likely to decide that skb_copy() is too slow, try to
>  add ctx invalidation, and thereby create a whole new swathe of potential
>  correctness and security issues.

Yep. I did try that. While the verifier bits weren't too tricky, there
are a lot of infra concerns to solve:

* https://github.com/danobi/linux/commit/35a66af8d54cca647b0adfc7c1da7105d2603dde
* https://github.com/danobi/linux/commit/e8c86ea75e2ca8f0631632d54ef763381308711e
* https://github.com/danobi/linux/commit/972bcf769f41fbfa7f84ce00faf06b5b57bc6f7a

But FWIW, fragmented packets are kinda a corner case anyways. I don't
think it would be resonable to expect high perf when packets are in
play.

> Plus, imagine trying to support this in a hardware-offload XDP device.
>  They'd have to reimplement the entire frag cache, which is a much bigger
>  attack surface than just a frag validator, and they couldn't leverage
>  the battle-hardened kernel implementation.

Hmm, well this helper is restricted to TC progs for now. I don't quite
see a path to enabling for XDP as there would have to be at a minimum
quite a few allocations to handle frags. So not sure XDP is a factor at
the moment.

> 
> > And make it trivial for the next person that comes
> > along to do the right thing.
> 
> Fwiw the validator approach could *also* be a helper, it doesn't have to
>  be something the BPF developer writes for themselves.
> 
> But if after thinking about the possibility you still prefer your way, I
>  won't try to stop you — I just wanted to ensure it had been considered.

Thank you for the discussion. The thought had come to mind originally,
but I shied away after seeing some of the reassembly details. Would be
interested in hearing more from other folks.


Thanks,
Daniel

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Alexei Starovoitov]

On Mon, Feb 27, 2023 at 12:51:02PM -0700, Daniel Xu wrote:
> === Context ===
> 
> In the context of a middlebox, fragmented packets are tricky to handle.
> The full 5-tuple of a packet is often only available in the first
> fragment which makes enforcing consistent policy difficult. There are
> really only two stateless options, neither of which are very nice:
> 
> 1. Enforce policy on first fragment and accept all subsequent fragments.
>    This works but may let in certain attacks or allow data exfiltration.
> 
> 2. Enforce policy on first fragment and drop all subsequent fragments.
>    This does not really work b/c some protocols may rely on
>    fragmentation. For example, DNS may rely on oversized UDP packets for
>    large responses.
> 
> So stateful tracking is the only sane option. RFC 8900 [0] calls this
> out as well in section 6.3:
> 
>     Middleboxes [...] should process IP fragments in a manner that is
>     consistent with [RFC0791] and [RFC8200]. In many cases, middleboxes
>     must maintain state in order to achieve this goal.
> 
> === BPF related bits ===
> 
> However, when policy is enforced through BPF, the prog is run before the
> kernel reassembles fragmented packets. This leaves BPF developers in a
> awkward place: implement reassembly (possibly poorly) or use a stateless
> method as described above.
> 
> Fortunately, the kernel has robust support for fragmented IP packets.
> This patchset wraps the existing defragmentation facilities in kfuncs so
> that BPF progs running on middleboxes can reassemble fragmented packets
> before applying policy.
> 
> === Patchset details ===
> 
> This patchset is (hopefully) relatively straightforward from BPF perspective.
> One thing I'd like to call out is the skb_copy()ing of the prog skb. I
> did this to maintain the invariant that the ctx remains valid after prog
> has run. This is relevant b/c ip_defrag() and ip_check_defrag() may
> consume the skb if the skb is a fragment.

Instead of doing all that with extra skb copy can you hook bpf prog after
the networking stack already handled ip defrag?
What kind of middle box are you doing? Why does it have to run at TC layer?

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Alexei Starovoitov]

On Mon, Feb 27, 2023 at 5:57 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> Hi Alexei,
>
> On Mon, Feb 27, 2023 at 03:03:38PM -0800, Alexei Starovoitov wrote:
> > On Mon, Feb 27, 2023 at 12:51:02PM -0700, Daniel Xu wrote:
> > > === Context ===
> > >
> > > In the context of a middlebox, fragmented packets are tricky to handle.
> > > The full 5-tuple of a packet is often only available in the first
> > > fragment which makes enforcing consistent policy difficult. There are
> > > really only two stateless options, neither of which are very nice:
> > >
> > > 1. Enforce policy on first fragment and accept all subsequent fragments.
> > >    This works but may let in certain attacks or allow data exfiltration.
> > >
> > > 2. Enforce policy on first fragment and drop all subsequent fragments.
> > >    This does not really work b/c some protocols may rely on
> > >    fragmentation. For example, DNS may rely on oversized UDP packets for
> > >    large responses.
> > >
> > > So stateful tracking is the only sane option. RFC 8900 [0] calls this
> > > out as well in section 6.3:
> > >
> > >     Middleboxes [...] should process IP fragments in a manner that is
> > >     consistent with [RFC0791] and [RFC8200]. In many cases, middleboxes
> > >     must maintain state in order to achieve this goal.
> > >
> > > === BPF related bits ===
> > >
> > > However, when policy is enforced through BPF, the prog is run before the
> > > kernel reassembles fragmented packets. This leaves BPF developers in a
> > > awkward place: implement reassembly (possibly poorly) or use a stateless
> > > method as described above.
> > >
> > > Fortunately, the kernel has robust support for fragmented IP packets.
> > > This patchset wraps the existing defragmentation facilities in kfuncs so
> > > that BPF progs running on middleboxes can reassemble fragmented packets
> > > before applying policy.
> > >
> > > === Patchset details ===
> > >
> > > This patchset is (hopefully) relatively straightforward from BPF perspective.
> > > One thing I'd like to call out is the skb_copy()ing of the prog skb. I
> > > did this to maintain the invariant that the ctx remains valid after prog
> > > has run. This is relevant b/c ip_defrag() and ip_check_defrag() may
> > > consume the skb if the skb is a fragment.
> >
> > Instead of doing all that with extra skb copy can you hook bpf prog after
> > the networking stack already handled ip defrag?
> > What kind of middle box are you doing? Why does it have to run at TC layer?
>
> Unless I'm missing something, the only other relevant hooks would be
> socket hooks, right?
>
> Unfortunately I don't think my use case can do that. We are running the
> kernel as a router, so no sockets are involved.

Are you using bpf_fib_lookup and populating kernel routing
table and doing everything on your own including neigh ?

Have you considered to skb redirect to another netdev that does ip defrag?
Like macvlan does it under some conditions. This can be generalized.

Recently Florian proposed to allow calling bpf progs from all existing
netfilter hooks.
You can pretend to local deliver and hook in NF_INET_LOCAL_IN ?
I feel it would be so much cleaner if stack does ip_defrag normally.
The general issue of skb ownership between bpf prog and defrag logic
isn't really solved with skb_copy. It's still an issue.

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Daniel Borkmann]

On 2/28/23 5:56 AM, Alexei Starovoitov wrote:
> On Mon, Feb 27, 2023 at 5:57 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>> On Mon, Feb 27, 2023 at 03:03:38PM -0800, Alexei Starovoitov wrote:
>>> On Mon, Feb 27, 2023 at 12:51:02PM -0700, Daniel Xu wrote:
>>>> === Context ===
>>>>
>>>> In the context of a middlebox, fragmented packets are tricky to handle.
>>>> The full 5-tuple of a packet is often only available in the first
>>>> fragment which makes enforcing consistent policy difficult. There are
>>>> really only two stateless options, neither of which are very nice:
>>>>
>>>> 1. Enforce policy on first fragment and accept all subsequent fragments.
>>>>     This works but may let in certain attacks or allow data exfiltration.
>>>>
>>>> 2. Enforce policy on first fragment and drop all subsequent fragments.
>>>>     This does not really work b/c some protocols may rely on
>>>>     fragmentation. For example, DNS may rely on oversized UDP packets for
>>>>     large responses.
>>>>
>>>> So stateful tracking is the only sane option. RFC 8900 [0] calls this
>>>> out as well in section 6.3:
>>>>
>>>>      Middleboxes [...] should process IP fragments in a manner that is
>>>>      consistent with [RFC0791] and [RFC8200]. In many cases, middleboxes
>>>>      must maintain state in order to achieve this goal.
>>>>
>>>> === BPF related bits ===
>>>>
>>>> However, when policy is enforced through BPF, the prog is run before the
>>>> kernel reassembles fragmented packets. This leaves BPF developers in a
>>>> awkward place: implement reassembly (possibly poorly) or use a stateless
>>>> method as described above.
>>>>
>>>> Fortunately, the kernel has robust support for fragmented IP packets.
>>>> This patchset wraps the existing defragmentation facilities in kfuncs so
>>>> that BPF progs running on middleboxes can reassemble fragmented packets
>>>> before applying policy.
>>>>
>>>> === Patchset details ===
>>>>
>>>> This patchset is (hopefully) relatively straightforward from BPF perspective.
>>>> One thing I'd like to call out is the skb_copy()ing of the prog skb. I
>>>> did this to maintain the invariant that the ctx remains valid after prog
>>>> has run. This is relevant b/c ip_defrag() and ip_check_defrag() may
>>>> consume the skb if the skb is a fragment.
>>>
>>> Instead of doing all that with extra skb copy can you hook bpf prog after
>>> the networking stack already handled ip defrag?
>>> What kind of middle box are you doing? Why does it have to run at TC layer?
>>
>> Unless I'm missing something, the only other relevant hooks would be
>> socket hooks, right?
>>
>> Unfortunately I don't think my use case can do that. We are running the
>> kernel as a router, so no sockets are involved.
> 
> Are you using bpf_fib_lookup and populating kernel routing
> table and doing everything on your own including neigh ?
> 
> Have you considered to skb redirect to another netdev that does ip defrag?
> Like macvlan does it under some conditions. This can be generalized.
> 
> Recently Florian proposed to allow calling bpf progs from all existing
> netfilter hooks.
> You can pretend to local deliver and hook in NF_INET_LOCAL_IN ?
> I feel it would be so much cleaner if stack does ip_defrag normally.
> The general issue of skb ownership between bpf prog and defrag logic
> isn't really solved with skb_copy. It's still an issue.

I do like this series and we would also use it for Cilium case, so +1 on the
tc BPF integration. Today we have in Cilium what Ed [0] hinted in his earlier
mail where we extract information from first fragment and store the meta data
in a BPF map for subsequent packets based on ipid [1], but limitations apply
e.g. service load-balancing won't work. Redirecting to a different device
or moving higher up the stack is cumbersome since we then need to go and
recirculate back into tc BPF layer where all the business logic is located and
handling the regular (non-fragmented) path, too. Wrt skb ownership, can you
elaborate what is a concrete issue exactly? Anything that comes to mind with
this approach that could crash the kernel?

   [0] https://lore.kernel.org/bpf/cf49a091-9b14-05b8-6a79-00e56f3019e1@gmail.com/
   [1] https://github.com/cilium/cilium/pull/10264

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Daniel Xu]

Hi Alexei,

On Mon, Feb 27, 2023 at 08:56:38PM -0800, Alexei Starovoitov wrote:
> On Mon, Feb 27, 2023 at 5:57 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> >
> > Hi Alexei,
> >
> > On Mon, Feb 27, 2023 at 03:03:38PM -0800, Alexei Starovoitov wrote:
> > > On Mon, Feb 27, 2023 at 12:51:02PM -0700, Daniel Xu wrote:
> > > > === Context ===
> > > >
> > > > In the context of a middlebox, fragmented packets are tricky to handle.
> > > > The full 5-tuple of a packet is often only available in the first
> > > > fragment which makes enforcing consistent policy difficult. There are
> > > > really only two stateless options, neither of which are very nice:
> > > >
> > > > 1. Enforce policy on first fragment and accept all subsequent fragments.
> > > >    This works but may let in certain attacks or allow data exfiltration.
> > > >
> > > > 2. Enforce policy on first fragment and drop all subsequent fragments.
> > > >    This does not really work b/c some protocols may rely on
> > > >    fragmentation. For example, DNS may rely on oversized UDP packets for
> > > >    large responses.
> > > >
> > > > So stateful tracking is the only sane option. RFC 8900 [0] calls this
> > > > out as well in section 6.3:
> > > >
> > > >     Middleboxes [...] should process IP fragments in a manner that is
> > > >     consistent with [RFC0791] and [RFC8200]. In many cases, middleboxes
> > > >     must maintain state in order to achieve this goal.
> > > >
> > > > === BPF related bits ===
> > > >
> > > > However, when policy is enforced through BPF, the prog is run before the
> > > > kernel reassembles fragmented packets. This leaves BPF developers in a
> > > > awkward place: implement reassembly (possibly poorly) or use a stateless
> > > > method as described above.
> > > >
> > > > Fortunately, the kernel has robust support for fragmented IP packets.
> > > > This patchset wraps the existing defragmentation facilities in kfuncs so
> > > > that BPF progs running on middleboxes can reassemble fragmented packets
> > > > before applying policy.
> > > >
> > > > === Patchset details ===
> > > >
> > > > This patchset is (hopefully) relatively straightforward from BPF perspective.
> > > > One thing I'd like to call out is the skb_copy()ing of the prog skb. I
> > > > did this to maintain the invariant that the ctx remains valid after prog
> > > > has run. This is relevant b/c ip_defrag() and ip_check_defrag() may
> > > > consume the skb if the skb is a fragment.
> > >
> > > Instead of doing all that with extra skb copy can you hook bpf prog after
> > > the networking stack already handled ip defrag?
> > > What kind of middle box are you doing? Why does it have to run at TC layer?
> >
> > Unless I'm missing something, the only other relevant hooks would be
> > socket hooks, right?
> >
> > Unfortunately I don't think my use case can do that. We are running the
> > kernel as a router, so no sockets are involved.
> 
> Are you using bpf_fib_lookup and populating kernel routing
> table and doing everything on your own including neigh ?

We're currently not doing any routing things in BPF yet. All the routing
manipulation has been done in iptables / netfilter so far. I'm not super
familiar with routing stuff but from what I understand there is some
relatively complicated stuff going on with BGP and ipsec tunnels at the
moment. Not sure if that answers your question.

> Have you considered to skb redirect to another netdev that does ip defrag?
> Like macvlan does it under some conditions. This can be generalized.

I had not considered that yet. Are you suggesting adding a new
passthrough netdev thing that'll defrags? I looked at the macvlan driver
and it looks like it defrags to handle some multicast corner case.

> Recently Florian proposed to allow calling bpf progs from all existing
> netfilter hooks.
> You can pretend to local deliver and hook in NF_INET_LOCAL_IN ?

Does that work for forwarding cases? I'm reading through [0] and it
seems to suggest that it'll only defrag for locally destined packets:

    If the destination IP address is matches with
    local NIC's IP address, the dst_input() function will brings the packets
    into the ip_local_deliver(), which will defrag the packet and pass it
    to the NF_IP_LOCAL_IN hook

Faking local delivery seems kinda ugly -- maybe I don't know any clean
ways.

[...]

[0]: https://kernelnewbies.org/Networking?action=AttachFile&do=get&target=hacking_the_wholism_of_linux_net.txt


Thanks,
Daniel

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Alexei Starovoitov]

On Tue, Feb 28, 2023 at 3:17 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> > Have you considered to skb redirect to another netdev that does ip defrag?
> > Like macvlan does it under some conditions. This can be generalized.
>
> I had not considered that yet. Are you suggesting adding a new
> passthrough netdev thing that'll defrags? I looked at the macvlan driver
> and it looks like it defrags to handle some multicast corner case.

Something like that. A netdev that bpf prog can redirect too.
It will consume ip frags and eventually will produce reassembled skb.

The kernel ip_defrag logic has timeouts, counters, rhashtable
with thresholds, etc. All of them are per netns.
Just another ip_defrag_user will still share rhashtable
with its limits. The kernel can even do icmp_send().
ip_defrag is not a kfunc. It's a big block with plenty of kernel
wide side effects.
I really don't think we can alloc_skb, copy_skb, and ip_defrag it.
It messes with the stack too much.
It's also not clear to me when skb is reassembled and how bpf sees it.
"redirect into reassembling netdev" and attaching bpf prog to consume
that skb is much cleaner imo.
May be there are other ways to use ip_defrag, but certainly not like
synchronous api helper.

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Daniel Xu]

Hi Alexei,

(cc netfilter maintainers)

On Mon, Mar 06, 2023 at 08:17:20PM -0800, Alexei Starovoitov wrote:
> On Tue, Feb 28, 2023 at 3:17 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> >
> > > Have you considered to skb redirect to another netdev that does ip defrag?
> > > Like macvlan does it under some conditions. This can be generalized.
> >
> > I had not considered that yet. Are you suggesting adding a new
> > passthrough netdev thing that'll defrags? I looked at the macvlan driver
> > and it looks like it defrags to handle some multicast corner case.
> 
> Something like that. A netdev that bpf prog can redirect too.
> It will consume ip frags and eventually will produce reassembled skb.
> 
> The kernel ip_defrag logic has timeouts, counters, rhashtable
> with thresholds, etc. All of them are per netns.
> Just another ip_defrag_user will still share rhashtable
> with its limits. The kernel can even do icmp_send().
> ip_defrag is not a kfunc. It's a big block with plenty of kernel
> wide side effects.
> I really don't think we can alloc_skb, copy_skb, and ip_defrag it.
> It messes with the stack too much.
> It's also not clear to me when skb is reassembled and how bpf sees it.
> "redirect into reassembling netdev" and attaching bpf prog to consume
> that skb is much cleaner imo.
> May be there are other ways to use ip_defrag, but certainly not like
> synchronous api helper.

I was giving the virtual netdev idea some thought this morning and I
thought I'd give the netfilter approach a deeper look.

From my reading (I'll run some tests later) it looks like netfilter
will defrag all ipv4/ipv6 packets in any netns with conntrack enabled.
It appears to do so in NF_INET_PRE_ROUTING.

Unfortunately that does run after tc hooks. But fortunately with the
new BPF netfilter hooks I think we can make defrag work outside of BPF
kfuncs like you want. And the NF_IP_FORWARD hook works well for my
router use case.

One thing we would need though are (probably kfunc) wrappers around
nf_defrag_ipv4_enable() and nf_defrag_ipv6_enable() to ensure BPF progs
are not transitively depending on defrag support from other netfilter
modules.

The exact mechanism would probably need some thinking, as the above
functions kinda rely on module_init() and module_exit() semantics. We
cannot make the prog bump the refcnt every time it runs -- it would
overflow.  And it would be nice to automatically free the refcnt when
prog is unloaded. 

Once the netfilter prog type series lands I can get that discussion
started. Unless Daniel feels strongly that we should continue with
the approach in this patchset, I am leaning towards dropping in favor
of netfilter approach.

Thanks,
Daniel

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Florian Westphal]

Daniel Xu <dxu@dxuuu.xyz> wrote:
> From my reading (I'll run some tests later) it looks like netfilter
> will defrag all ipv4/ipv6 packets in any netns with conntrack enabled.
> It appears to do so in NF_INET_PRE_ROUTING.

Yes, and output.

> One thing we would need though are (probably kfunc) wrappers around
> nf_defrag_ipv4_enable() and nf_defrag_ipv6_enable() to ensure BPF progs
> are not transitively depending on defrag support from other netfilter
> modules.
>
> The exact mechanism would probably need some thinking, as the above
> functions kinda rely on module_init() and module_exit() semantics. We
> cannot make the prog bump the refcnt every time it runs -- it would
> overflow.  And it would be nice to automatically free the refcnt when
> prog is unloaded.

Probably add a flag attribute that is evaluated at BPF_LINK time, so
progs can say they need defrag enabled.  Same could be used to request
conntrack enablement.

Will need some glue on netfilter side to handle DEFRAG=m, but we already
have plenty of those.

Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

[Alexei Starovoitov]

On Tue, Mar 7, 2023 at 12:11 PM Florian Westphal <fw@strlen.de> wrote:
>
> Daniel Xu <dxu@dxuuu.xyz> wrote:
> > From my reading (I'll run some tests later) it looks like netfilter
> > will defrag all ipv4/ipv6 packets in any netns with conntrack enabled.
> > It appears to do so in NF_INET_PRE_ROUTING.
>
> Yes, and output.
>
> > One thing we would need though are (probably kfunc) wrappers around
> > nf_defrag_ipv4_enable() and nf_defrag_ipv6_enable() to ensure BPF progs
> > are not transitively depending on defrag support from other netfilter
> > modules.
> >
> > The exact mechanism would probably need some thinking, as the above
> > functions kinda rely on module_init() and module_exit() semantics. We
> > cannot make the prog bump the refcnt every time it runs -- it would
> > overflow.  And it would be nice to automatically free the refcnt when
> > prog is unloaded.
>
> Probably add a flag attribute that is evaluated at BPF_LINK time, so
> progs can say they need defrag enabled.  Same could be used to request
> conntrack enablement.
>
> Will need some glue on netfilter side to handle DEFRAG=m, but we already
> have plenty of those.

All makes perfect sense to me.
It's cleaner than a special netdevice.
ipv4_conntrack_defrag() is pretty neat. I didn't know about it.
If we can reuse it as-is that would be ideal.
Conceptually it fits perfectly.
If we cannot reuse it (for whatever unlikely reason) I would
argue that TC hook should gain similar functionality.