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bpf: Derive source IP addr via bpf_*_fib_lookup()

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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>
This commit is contained in:
Martynas Pumputis 2023-10-07 10:14:14 +02:00 committed by Martin KaFai Lau
parent 1be84ca53c
commit dab4e1f06c
5 changed files with 43 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
include/net/ipv6_stubs.h
View File

@ -85,6 +85,11 @@ struct ipv6_bpf_stub {
sockptr_t optval, unsigned int optlen);
int (*ipv6_getsockopt)(struct sock *sk, int level, int optname,
sockptr_t optval, sockptr_t optlen);
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;

10
include/uapi/linux/bpf.h
View File

@ -3264,6 +3264,11 @@ 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.
@ -6964,6 +6969,7 @@ 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 {
@ -6976,6 +6982,7 @@ 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 {
@ -7010,6 +7017,9 @@ 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 */

18
net/core/filter.c
View File

@ -5850,6 +5850,9 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
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
*/
@ -5992,6 +5995,18 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
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;
@ -6010,7 +6025,8 @@ 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)

1
net/ipv6/af_inet6.c
View File

@ -1061,6 +1061,7 @@ static const struct ipv6_bpf_stub ipv6_bpf_stub_impl = {
.udp6_lib_lookup = __udp6_lib_lookup,
.ipv6_setsockopt = do_ipv6_setsockopt,
.ipv6_getsockopt = do_ipv6_getsockopt,
.ipv6_dev_get_saddr = ipv6_dev_get_saddr,
};
static int __init inet6_init(void)

10
tools/include/uapi/linux/bpf.h
View File

@ -3264,6 +3264,11 @@ 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.
@ -6964,6 +6969,7 @@ 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 {
@ -6976,6 +6982,7 @@ 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 {
@ -7010,6 +7017,9 @@ 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 */