bpf: Allow rewriting to ports under ip_unprivileged_port_start

At the moment, BPF_CGROUP_INET{4,6}_BIND hooks can rewrite user_port
to the privileged ones (< ip_unprivileged_port_start), but it will
be rejected later on in the __inet_bind or __inet6_bind.

Let's add another return value to indicate that CAP_NET_BIND_SERVICE
check should be ignored. Use the same idea as we currently use
in cgroup/egress where bit #1 indicates CN. Instead, for
cgroup/bind{4,6}, bit #1 indicates that CAP_NET_BIND_SERVICE should
be bypassed.

v5:
- rename flags to be less confusing (Andrey Ignatov)
- rework BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY to work on flags
  and accept BPF_RET_SET_CN (no behavioral changes)

v4:
- Add missing IPv6 support (Martin KaFai Lau)

v3:
- Update description (Martin KaFai Lau)
- Fix capability restore in selftest (Martin KaFai Lau)

v2:
- Switch to explicit return code (Martin KaFai Lau)

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrey Ignatov <rdna@fb.com>
Link: https://lore.kernel.org/bpf/20210127193140.3170382-1-sdf@google.com

1 files changed, 6 insertions, 2 deletions

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index da649f20d6b2..cdf3c7e611d9 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1055,6 +1055,8 @@ EXPORT_SYMBOL(__cgroup_bpf_run_filter_sk);
  * @uaddr: sockaddr struct provided by user
  * @type: The type of program to be exectuted
  * @t_ctx: Pointer to attach type specific context
+ * @flags: Pointer to u32 which contains higher bits of BPF program
+ *         return value (OR'ed together).
  *
  * socket is expected to be of type INET or INET6.
  *
@@ -1064,7 +1066,8 @@ EXPORT_SYMBOL(__cgroup_bpf_run_filter_sk);
 int __cgroup_bpf_run_filter_sock_addr(struct sock *sk,
 				      struct sockaddr *uaddr,
 				      enum bpf_attach_type type,
-				      void *t_ctx)
+				      void *t_ctx,
+				      u32 *flags)
 {
 	struct bpf_sock_addr_kern ctx = {
 		.sk = sk,
@@ -1087,7 +1090,8 @@ int __cgroup_bpf_run_filter_sock_addr(struct sock *sk,
 	}
 
 	cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
-	ret = BPF_PROG_RUN_ARRAY(cgrp->bpf.effective[type], &ctx, BPF_PROG_RUN);
+	ret = BPF_PROG_RUN_ARRAY_FLAGS(cgrp->bpf.effective[type], &ctx,
+				       BPF_PROG_RUN, flags);
 
 	return ret == 1 ? 0 : -EPERM;
 }