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

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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>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Martynas Pumputis
2023-10-07 10:14:14 +02:00
committed by Greg Kroah-Hartman
parent 980278aca1
commit b96f500dbb
5 changed files with 43 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

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

@@ -3257,6 +3257,11 @@ 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.
@@ -6956,6 +6961,7 @@ 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 {
@@ -6968,6 +6974,7 @@ 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 {
@@ -7002,6 +7009,9 @@ 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 */