netfilter: bpf: Support BPF_F_NETFILTER_IP_DEFRAG in netfilter link

This commit adds support for enabling IP defrag using pre-existing
netfilter defrag support. Basically all the flag does is bump a refcnt
while the link the active. Checks are also added to ensure the prog
requesting defrag support is run _after_ netfilter defrag hooks.

We also take care to avoid any issues w.r.t. module unloading -- while
defrag is active on a link, the module is prevented from unloading.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Reviewed-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/5cff26f97e55161b7d56b09ddcf5f8888a5add1d.1689970773.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 14fd26b..70da852 100644 (file)
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1188,6 +1188,11 @@ enum bpf_perf_event_type {
  */
 #define BPF_F_KPROBE_MULTI_RETURN      (1U << 0)
 
+/* link_create.netfilter.flags used in LINK_CREATE command for
+ * BPF_PROG_TYPE_NETFILTER to enable IP packet defragmentation.
+ */
+#define BPF_F_NETFILTER_IP_DEFRAG (1U << 0)
+
 /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
  * the following extensions:
  *

diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
index c36da56..8fe594b 100644 (file)
--- a/net/netfilter/nf_bpf_link.c
+++ b/net/netfilter/nf_bpf_link.c
@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/bpf.h>
 #include <linux/filter.h>
+#include <linux/kmod.h>
+#include <linux/module.h>
 #include <linux/netfilter.h>
 
 #include <net/netfilter/nf_bpf_link.h>
@@ -23,8 +25,88 @@ struct bpf_nf_link {
        struct nf_hook_ops hook_ops;
        struct net *net;
        u32 dead;
+       const struct nf_defrag_hook *defrag_hook;
 };
 
+static const struct nf_defrag_hook *
+get_proto_defrag_hook(struct bpf_nf_link *link,
+                     const struct nf_defrag_hook __rcu *global_hook,
+                     const char *mod)
+{
+       const struct nf_defrag_hook *hook;
+       int err;
+
+       /* RCU protects us from races against module unloading */
+       rcu_read_lock();
+       hook = rcu_dereference(global_hook);
+       if (!hook) {
+               rcu_read_unlock();
+               err = request_module(mod);
+               if (err)
+                       return ERR_PTR(err < 0 ? err : -EINVAL);
+
+               rcu_read_lock();
+               hook = rcu_dereference(global_hook);
+       }
+
+       if (hook && try_module_get(hook->owner)) {
+               /* Once we have a refcnt on the module, we no longer need RCU */
+               hook = rcu_pointer_handoff(hook);
+       } else {
+               WARN_ONCE(!hook, "%s has bad registration", mod);
+               hook = ERR_PTR(-ENOENT);
+       }
+       rcu_read_unlock();
+
+       if (!IS_ERR(hook)) {
+               err = hook->enable(link->net);
+               if (err) {
+                       module_put(hook->owner);
+                       hook = ERR_PTR(err);
+               }
+       }
+
+       return hook;
+}
+
+static int bpf_nf_enable_defrag(struct bpf_nf_link *link)
+{
+       const struct nf_defrag_hook __maybe_unused *hook;
+
+       switch (link->hook_ops.pf) {
+#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4)
+       case NFPROTO_IPV4:
+               hook = get_proto_defrag_hook(link, nf_defrag_v4_hook, "nf_defrag_ipv4");
+               if (IS_ERR(hook))
+                       return PTR_ERR(hook);
+
+               link->defrag_hook = hook;
+               return 0;
+#endif
+#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)
+       case NFPROTO_IPV6:
+               hook = get_proto_defrag_hook(link, nf_defrag_v6_hook, "nf_defrag_ipv6");
+               if (IS_ERR(hook))
+                       return PTR_ERR(hook);
+
+               link->defrag_hook = hook;
+               return 0;
+#endif
+       default:
+               return -EAFNOSUPPORT;
+       }
+}
+
+static void bpf_nf_disable_defrag(struct bpf_nf_link *link)
+{
+       const struct nf_defrag_hook *hook = link->defrag_hook;
+
+       if (!hook)
+               return;
+       hook->disable(link->net);
+       module_put(hook->owner);
+}
+
 static void bpf_nf_link_release(struct bpf_link *link)
 {
        struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link);
@@ -32,11 +114,11 @@ static void bpf_nf_link_release(struct bpf_link *link)
        if (nf_link->dead)
                return;
 
-       /* prevent hook-not-found warning splat from netfilter core when
-        * .detach was already called
-        */
-       if (!cmpxchg(&nf_link->dead, 0, 1))
+       /* do not double release in case .detach was already called */
+       if (!cmpxchg(&nf_link->dead, 0, 1)) {
                nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops);
+               bpf_nf_disable_defrag(nf_link);
+       }
 }
 
 static void bpf_nf_link_dealloc(struct bpf_link *link)
@@ -92,6 +174,8 @@ static const struct bpf_link_ops bpf_nf_link_lops = {
 
 static int bpf_nf_check_pf_and_hooks(const union bpf_attr *attr)
 {
+       int prio;
+
        switch (attr->link_create.netfilter.pf) {
        case NFPROTO_IPV4:
        case NFPROTO_IPV6:
@@ -102,19 +186,18 @@ static int bpf_nf_check_pf_and_hooks(const union bpf_attr *attr)
                return -EAFNOSUPPORT;
        }
 
-       if (attr->link_create.netfilter.flags)
+       if (attr->link_create.netfilter.flags & ~BPF_F_NETFILTER_IP_DEFRAG)
                return -EOPNOTSUPP;
 
-       /* make sure conntrack confirm is always last.
-        *
-        * In the future, if userspace can e.g. request defrag, then
-        * "defrag_requested && prio before NF_IP_PRI_CONNTRACK_DEFRAG"
-        * should fail.
-        */
-       switch (attr->link_create.netfilter.priority) {
-       case NF_IP_PRI_FIRST: return -ERANGE; /* sabotage_in and other warts */
-       case NF_IP_PRI_LAST: return -ERANGE; /* e.g. conntrack confirm */
-       }
+       /* make sure conntrack confirm is always last */
+       prio = attr->link_create.netfilter.priority;
+       if (prio == NF_IP_PRI_FIRST)
+               return -ERANGE;  /* sabotage_in and other warts */
+       else if (prio == NF_IP_PRI_LAST)
+               return -ERANGE;  /* e.g. conntrack confirm */
+       else if ((attr->link_create.netfilter.flags & BPF_F_NETFILTER_IP_DEFRAG) &&
+                prio <= NF_IP_PRI_CONNTRACK_DEFRAG)
+               return -ERANGE;  /* cannot use defrag if prog runs before nf_defrag */
 
        return 0;
 }
@@ -149,6 +232,7 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 
        link->net = net;
        link->dead = false;
+       link->defrag_hook = NULL;
 
        err = bpf_link_prime(&link->link, &link_primer);
        if (err) {
@@ -156,8 +240,17 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
                return err;
        }
 
+       if (attr->link_create.netfilter.flags & BPF_F_NETFILTER_IP_DEFRAG) {
+               err = bpf_nf_enable_defrag(link);
+               if (err) {
+                       bpf_link_cleanup(&link_primer);
+                       return err;
+               }
+       }
+
        err = nf_register_net_hook(net, &link->hook_ops);
        if (err) {
+               bpf_nf_disable_defrag(link);
                bpf_link_cleanup(&link_primer);
                return err;
        }

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 14fd26b..70da852 100644 (file)
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1188,6 +1188,11 @@ enum bpf_perf_event_type {
  */
 #define BPF_F_KPROBE_MULTI_RETURN      (1U << 0)
 
+/* link_create.netfilter.flags used in LINK_CREATE command for
+ * BPF_PROG_TYPE_NETFILTER to enable IP packet defragmentation.
+ */
+#define BPF_F_NETFILTER_IP_DEFRAG (1U << 0)
+
 /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
  * the following extensions:
  *