[PATCH 5.15 81/84] bpf: Derive source IP addr via bpf_*_fib_lookup()

Greg Kroah-Hartman
2024-03-04T21:56:03Z

From: Martynas Pumputis <m@lambda.lt>

commit dab4e1f06cabb6834de14264394ccab197007302 upstream.

Extend the bpf_fib_lookup() helper by making it to return the source
IPv4/IPv6 address if the BPF_FIB_LOOKUP_SRC flag is set.

For example, the following snippet can be used to derive the desired
source IP address:

struct bpf_fib_lookup p = { .ipv4_dst = ip4->daddr };

ret = bpf_skb_fib_lookup(skb, p, sizeof(p),
BPF_FIB_LOOKUP_SRC | BPF_FIB_LOOKUP_SKIP_NEIGH);
if (ret != BPF_FIB_LKUP_RET_SUCCESS)
return TC_ACT_SHOT;

/* the p.ipv4_src now contains the source address */

The inability to derive the proper source address may cause malfunctions
in BPF-based dataplanes for hosts containing netdevs with more than one
routable IP address or for multi-homed hosts.

For example, Cilium implements packet masquerading in BPF. If an
egressing netdev to which the Cilium's BPF prog is attached has
multiple IP addresses, then only one [hardcoded] IP address can be used for
masquerading. This breaks connectivity if any other IP address should have
been selected instead, for example, when a public and private addresses
are attached to the same egress interface.

The change was tested with Cilium [1].

Nikolay Aleksandrov helped to figure out the IPv6 addr selection.

[1]: https://github.com/cilium/cilium/pull/28283

Signed-off-by: Martynas Pumputis <m@lambda.lt>
Link: https://lore.kernel.org/r/20231007081415.33502-2-m@lambda.lt
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/net/ipv6_stubs.h | 5 +++++
include/uapi/linux/bpf.h | 10 ++++++++++
net/core/filter.c | 18 +++++++++++++++++-
net/ipv6/af_inet6.c | 1 +
tools/include/uapi/linux/bpf.h | 10 ++++++++++
5 files changed, 43 insertions(+), 1 deletion(-)

--- a/include/net/ipv6_stubs.h
+++ b/include/net/ipv6_stubs.h
@@ -81,6 +81,11 @@ struct ipv6_bpf_stub {
const struct in6_addr *daddr, __be16 dport,
int dif, int sdif, struct udp_table *tbl,
struct sk_buff *skb);
+ int (*ipv6_dev_get_saddr)(struct net *net,
+ const struct net_device *dst_dev,
+ const struct in6_addr *daddr,
+ unsigned int prefs,
+ struct in6_addr *saddr);
};
extern const struct ipv6_bpf_stub *ipv6_bpf_stub __read_mostly;

--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3023,6 +3023,11 @@ union bpf_attr {
* and *params*->smac will not be set as output. A common
* use case is to call **bpf_redirect_neigh**\ () after
* doing **bpf_fib_lookup**\ ().
+ * **BPF_FIB_LOOKUP_SRC**
+ * Derive and set source IP addr in *params*->ipv{4,6}_src
+ * for the nexthop. If the src addr cannot be derived,
+ * **BPF_FIB_LKUP_RET_NO_SRC_ADDR** is returned. In this
+ * case, *params*->dmac and *params*->smac are not set either.
*
* *ctx* is either **struct xdp_md** for XDP programs or
* **struct sk_buff** tc cls_act programs.
@@ -6051,6 +6056,7 @@ enum {
BPF_FIB_LOOKUP_OUTPUT = (1U << 1),
BPF_FIB_LOOKUP_SKIP_NEIGH = (1U << 2),
BPF_FIB_LOOKUP_TBID = (1U << 3),
+ BPF_FIB_LOOKUP_SRC = (1U << 4),
};

enum {
@@ -6063,6 +6069,7 @@ enum {
BPF_FIB_LKUP_RET_UNSUPP_LWT, /* fwd requires encapsulation */
BPF_FIB_LKUP_RET_NO_NEIGH, /* no neighbor entry for nh */
BPF_FIB_LKUP_RET_FRAG_NEEDED, /* fragmentation required to fwd */
+ BPF_FIB_LKUP_RET_NO_SRC_ADDR, /* failed to derive IP src addr */
};

struct bpf_fib_lookup {
@@ -6097,6 +6104,9 @@ struct bpf_fib_lookup {
__u32 rt_metric;
};

+ /* input: source address to consider for lookup
+ * output: source address result from lookup
+ */
union {
__be32 ipv4_src;
__u32 ipv6_src[4]; /* in6_addr; network order */
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5504,6 +5504,9 @@ static int bpf_ipv4_fib_lookup(struct ne
params->rt_metric = res.fi->fib_priority;
params->ifindex = dev->ifindex;

+ if (flags & BPF_FIB_LOOKUP_SRC)
+ params->ipv4_src = fib_result_prefsrc(net, &res);
+
/* xdp and cls_bpf programs are run in RCU-bh so
* rcu_read_lock_bh is not needed here
*/
@@ -5646,6 +5649,18 @@ static int bpf_ipv6_fib_lookup(struct ne
params->rt_metric = res.f6i->fib6_metric;
params->ifindex = dev->ifindex;

+ if (flags & BPF_FIB_LOOKUP_SRC) {
+ if (res.f6i->fib6_prefsrc.plen) {
+ *src = res.f6i->fib6_prefsrc.addr;
+ } else {
+ err = ipv6_bpf_stub->ipv6_dev_get_saddr(net, dev,
+ &fl6.daddr, 0,
+ src);
+ if (err)
+ return BPF_FIB_LKUP_RET_NO_SRC_ADDR;
+ }
+ }
+
if (flags & BPF_FIB_LOOKUP_SKIP_NEIGH)
goto set_fwd_params;

@@ -5664,7 +5679,8 @@ set_fwd_params:
#endif

#define BPF_FIB_LOOKUP_MASK (BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT | \
- BPF_FIB_LOOKUP_SKIP_NEIGH | BPF_FIB_LOOKUP_TBID)
+ BPF_FIB_LOOKUP_SKIP_NEIGH | BPF_FIB_LOOKUP_TBID | \
+ BPF_FIB_LOOKUP_SRC)

BPF_CALL_4(bpf_xdp_fib_lookup, struct xdp_buff *, ctx,
struct bpf_fib_lookup *, params, int, plen, u32, flags)
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -1061,6 +1061,7 @@ static const struct ipv6_stub ipv6_stub_
static const struct ipv6_bpf_stub ipv6_bpf_stub_impl = {
.inet6_bind = __inet6_bind,
.udp6_lib_lookup = __udp6_lib_lookup,
+ .ipv6_dev_get_saddr = ipv6_dev_get_saddr,
};

static int __init inet6_init(void)
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3023,6 +3023,11 @@ union bpf_attr {
* and *params*->smac will not be set as output. A common
* use case is to call **bpf_redirect_neigh**\ () after
* doing **bpf_fib_lookup**\ ().
+ * **BPF_FIB_LOOKUP_SRC**
+ * Derive and set source IP addr in *params*->ipv{4,6}_src
+ * for the nexthop. If the src addr cannot be derived,
+ * **BPF_FIB_LKUP_RET_NO_SRC_ADDR** is returned. In this
+ * case, *params*->dmac and *params*->smac are not set either.
*
* *ctx* is either **struct xdp_md** for XDP programs or
* **struct sk_buff** tc cls_act programs.
@@ -6051,6 +6056,7 @@ enum {
BPF_FIB_LOOKUP_OUTPUT = (1U << 1),
BPF_FIB_LOOKUP_SKIP_NEIGH = (1U << 2),
BPF_FIB_LOOKUP_TBID = (1U << 3),
+ BPF_FIB_LOOKUP_SRC = (1U << 4),
};

enum {
@@ -6063,6 +6069,7 @@ enum {
BPF_FIB_LKUP_RET_UNSUPP_LWT, /* fwd requires encapsulation */
BPF_FIB_LKUP_RET_NO_NEIGH, /* no neighbor entry for nh */
BPF_FIB_LKUP_RET_FRAG_NEEDED, /* fragmentation required to fwd */
+ BPF_FIB_LKUP_RET_NO_SRC_ADDR, /* failed to derive IP src addr */
};

struct bpf_fib_lookup {
@@ -6097,6 +6104,9 @@ struct bpf_fib_lookup {
__u32 rt_metric;
};

+ /* input: source address to consider for lookup
+ * output: source address result from lookup
+ */
union {
__be32 ipv4_src;
__u32 ipv6_src[4]; /* in6_addr; network order */