Re: [PATCH bpf-next v2 1/3] bpf: sock ops: add netns ino and dev in bpf context

From: Y Song
Date: Thu Apr 18 2019 - 17:31:56 EST

On Thu, Apr 18, 2019 at 8:58 AM Alban Crequy <alban.crequy@xxxxxxxxx> wrote:
>
> From: Alban Crequy <alban@xxxxxxxxxx>
>
> sockops programs can now access the network namespace inode and device
> via (struct bpf_sock_ops)->netns_ino and ->netns_dev. This can be useful
> to apply different policies on different network namespaces.
>
> In the unlikely case where network namespaces are not compiled in
> (CONFIG_NET_NS=n), the verifier will not allow access to ->netns_*.
>
> The generated BPF bytecode for netns_ino is loading the correct inode
> number at the time of execution.
>
> However, the generated BPF bytecode for netns_dev is loading an
> immediate value determined at BPF-load-time by looking at the initial
> network namespace. In practice, this works because all netns currently
> use the same virtual device. If this was to change, this code would need
> to be updated too.
>
> Signed-off-by: Alban Crequy <alban@xxxxxxxxxx>
>
> ---
>
> Changes since v1:
> - add netns_dev (review from Alexei)
> ---
> include/uapi/linux/bpf.h | 2 ++
> net/core/filter.c | 70 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 72 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index eaf2d3284248..f4f841dde42c 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3213,6 +3213,8 @@ struct bpf_sock_ops {
> __u32 sk_txhash;
> __u64 bytes_received;
> __u64 bytes_acked;
> + __u64 netns_dev;
> + __u64 netns_ino;

Maybe we can define netns_dev as __u32?
__u64 netns_ino;
__u32 netns_dev;

There is a hole at the end which can be used if the next
field to be added in the future is a __u32.

>From
static inline u32 new_encode_dev(dev_t dev)
{
unsigned major = MAJOR(dev);
unsigned minor = MINOR(dev);
return (minor & 0xff) | (major << 8) | ((minor & ~0xff) << 12);
}

device num is encoded in a u32.

> };
>
> /* Definitions for bpf_sock_ops_cb_flags */
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 1833926a63fc..93e3429603d7 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -75,6 +75,8 @@
> #include <net/seg6_local.h>
> #include <net/lwtunnel.h>
> #include <net/ipv6_stubs.h>
> +#include <linux/kdev_t.h>
> +#include <linux/proc_ns.h>
>
> /**
> * sk_filter_trim_cap - run a packet through a socket filter
> @@ -6774,6 +6776,15 @@ static bool sock_ops_is_valid_access(int off, int size,
> }
> } else {
> switch (off) {
> + case offsetof(struct bpf_sock_ops, netns_dev):
> + case offsetof(struct bpf_sock_ops, netns_ino):
> +#ifdef CONFIG_NET_NS
> + if (size != sizeof(__u64))
> + return false;

for netns_dev, looks like we have encoding like
(minor & 0xff) | (major << 8) | ((minor & ~0xff) << 12)

it is possible user may access like:
u16 dev = sk->netns_dev;
the compiler may translate into a 2-byte load
or even
u8 major = (sk->netns_dev >> 8) & 0xf
the compiler may translate to a byte load of &sk->netns_dev + 1 or 3
depends on endianness.
I suggest we relax the access pattern now to avoid patch later.

> +#else
> + return false;
> +#endif
> + break;
> case bpf_ctx_range_till(struct bpf_sock_ops, bytes_received,
> bytes_acked):
> if (size != sizeof(__u64))
> @@ -7660,6 +7671,11 @@ static u32 sock_addr_convert_ctx_access(enum bpf_access_type type,
> return insn - insn_buf;
> }
>
> +static struct ns_common *sockops_netns_cb(void *private_data)
> +{
> + return &init_net.ns;
> +}
> +
> static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
> const struct bpf_insn *si,
> struct bpf_insn *insn_buf,
> @@ -7668,6 +7684,10 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
> {
> struct bpf_insn *insn = insn_buf;
> int off;
> + struct inode *ns_inode;
> + struct path ns_path;
> + __u64 netns_dev;

Although there are a few discrepancies, I suggest to use u64 for
kernel as __u64 is typically
used in user space.

> + void *res;
>
> /* Helper macro for adding read access to tcp_sock or sock fields. */
> #define SOCK_OPS_GET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ) \
> @@ -7914,6 +7934,56 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
> SOCK_OPS_GET_OR_SET_FIELD(sk_txhash, sk_txhash,
> struct sock, type);
> break;
> +
> + case offsetof(struct bpf_sock_ops, netns_dev):
> +#ifdef CONFIG_NET_NS
> + /* We get the netns_dev at BPF-load-time and not at
> + * BPF-exec-time. We assume that netns_dev is a constant.
> + */
> + res = ns_get_path_cb(&ns_path, sockops_netns_cb, NULL);
> + if (IS_ERR(res)) {
> + netns_dev = 0;
> + } else {
> + ns_inode = ns_path.dentry->d_inode;
> + netns_dev = new_encode_dev(ns_inode->i_sb->s_dev);
> + }
> +#else
> + netns_dev = 0;

The #else branch is not needed. During is_valid_access check, if CONFIG_NET_NS
is not defined, the program should have been rejected.

> +#endif
> + *insn++ = BPF_MOV64_IMM(si->dst_reg, netns_dev);
> + break;
> +
> + case offsetof(struct bpf_sock_ops, netns_ino):
> +#ifdef CONFIG_NET_NS
> + /* Loading: sk_ops->sk->__sk_common.skc_net.net->ns.inum
> + * Type: (struct bpf_sock_ops_kern *)
> + * ->(struct sock *)
> + * ->(struct sock_common)
> + * .possible_net_t
> + * .(struct net *)
> + * ->(struct ns_common)
> + * .(unsigned int)
> + */
> + BUILD_BUG_ON(offsetof(struct sock, __sk_common) != 0);
> + BUILD_BUG_ON(offsetof(possible_net_t, net) != 0);
> + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
> + struct bpf_sock_ops_kern, sk),
> + si->dst_reg, si->src_reg,
> + offsetof(struct bpf_sock_ops_kern, sk));
> + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
> + possible_net_t, net),
> + si->dst_reg, si->dst_reg,
> + offsetof(struct sock_common, skc_net));
> + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
> + struct ns_common, inum),
> + si->dst_reg, si->dst_reg,
> + offsetof(struct net, ns) +
> + offsetof(struct ns_common, inum));
> +#else
> + *insn++ = BPF_MOV64_IMM(si->dst_reg, 0);

The same as above, this else branch is not needed.

> +#endif
> + break;
> +
> }
> return insn - insn_buf;
> }
> --
> 2.20.1
>