Re: [PATCH bpf-next v4 1/4] bpf: sock ops: add netns ino and dev in bpf context

[Iago LÃpez Galeiras]

On Fri, May 24, 2019 at 7:48 PM Y Song <ys114321@xxxxxxxxx> wrote:
>
> On Fri, May 24, 2019 at 9:01 AM Iago LÃpez Galeiras <iago@xxxxxxxxxx> 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 return netns_dev as usual and will
> > return 0 for netns_ino.
> >
> > 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.
> >
> > Co-authored-by: Iago LÃpez Galeiras <iago@xxxxxxxxxx>
> > Signed-off-by: Alban Crequy <alban@xxxxxxxxxx>
> > Signed-off-by: Iago LÃpez Galeiras <iago@xxxxxxxxxx>
> >
> > ---
> >
> > Changes since v1:
> > - add netns_dev (review from Alexei)
> >
> > Changes since v2:
> > - replace __u64 by u64 in kernel code (review from Y Song)
> > - remove unneeded #else branch: program would be rejected in
> >   is_valid_access (review from Y Song)
> > - allow partial reads (<u64) (review from Y Song)
> >
> > Changes since v3:
> > - return netns_dev unconditionally and set netns_ino to 0 if
> >   CONFIG_NET_NS is not enabled (review from Jakub Kicinski)
> > - use bpf_ctx_record_field_size and bpf_ctx_narrow_access_ok instead of
> >   manually deal with partial reads (review from Y Song)
> > - update commit message to reflect new code and remove note about
> >   partial reads since it was discussed in the review
> > - use bpf_ctx_range() and offsetofend()
> > ---
> >  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 63e0cf66f01a..e64066a09a5f 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3261,6 +3261,8 @@ struct bpf_sock_ops {
> >         __u32 sk_txhash;
> >         __u64 bytes_received;
> >         __u64 bytes_acked;
> > +       __u64 netns_dev;
> > +       __u64 netns_ino;
> >  };
> >
> >  /* Definitions for bpf_sock_ops_cb_flags */
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 55bfc941d17a..2b1552a8dd74 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -76,6 +76,8 @@
> >  #include <net/lwtunnel.h>
> >  #include <net/ipv6_stubs.h>
> >  #include <net/bpf_sk_storage.h>
> > +#include <linux/kdev_t.h>
> > +#include <linux/proc_ns.h>
> >
> >  /**
> >   *     sk_filter_trim_cap - run a packet through a socket filter
> > @@ -6822,6 +6824,18 @@ static bool sock_ops_is_valid_access(int off, int size,
> >                 }
> >         } else {
> >                 switch (off) {
> > +               case bpf_ctx_range(struct bpf_sock_ops, netns_dev):
> > +                       if (off >= offsetofend(struct bpf_sock_ops, netns_dev))
> > +                               return false;
>
> This is not needed.
> #define bpf_ctx_range(TYPE, MEMBER)
>          \
>         offsetof(TYPE, MEMBER) ... offsetofend(TYPE, MEMBER) - 1
> off should never be >= offsetofend(struct bpf_sock_ops, netns_dev).
>

Right, I'll remove it.

> > +
> > +                       bpf_ctx_record_field_size(info, sizeof(u64));
> > +                       if (!bpf_ctx_narrow_access_ok(off, size, sizeof(u64)))
> > +                               return false;
> > +                       break;
> > +               case offsetof(struct bpf_sock_ops, netns_ino):
> > +                       if (size != sizeof(u64))
> > +                               return false;
> > +                       break;
> >                 case bpf_ctx_range_till(struct bpf_sock_ops, bytes_received,
> >                                         bytes_acked):
> >                         if (size != sizeof(__u64))
> > @@ -7739,6 +7753,11 @@ 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,
> > @@ -7747,6 +7766,10 @@ 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;
> > +       void *res;
> >
> >  /* Helper macro for adding read access to tcp_sock or sock fields. */
> >  #define SOCK_OPS_GET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ)                        \
> > @@ -7993,6 +8016,53 @@ 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 bpf_ctx_range(struct bpf_sock_ops, netns_dev):
> > +               /* 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;
>
> What is the proper way to handle error here?
> If we indeed get an error and netns_dev = 0, do this mean bpf program
> should check netns_dev == 0 and special case it? Or since this is really
> a lower probability thing we can set to 0 and bpf program's logic does not
> need to specialize this one.
>
> At least, maybe we need a little documentation for the field in uapi header
> to point out this potential caveat?
>

As far as I can tell, this function can only error with ENOMEM when allocating
a new inode or dentry, which is very unlikely. I don't think bpf programs
should check for this. Also, for sockops programs worst case is usually that
the connection goes through the usual networking stack so it shouldn't be
dangerous.

I can add a comment like this to the field in the uapi header file:

    ...
    /*
     * netns_dev might be zero if there's an error getting it
     * when loading the BPF program. This is very unlikely.
     */
    __u64 netns_dev;
    ...

What do you think?




> > +               } else {
> > +                       ns_inode = ns_path.dentry->d_inode;
> > +                       netns_dev = new_encode_dev(ns_inode->i_sb->s_dev);
> > +               }
> > +               *target_size = 8;
> > +               *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);
> > +#endif
> > +               break;
> > +
> >         }
> >         return insn - insn_buf;
> >  }
> > --
> > 2.21.0
> >



--
Iago LÃpez Galeiras

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