On Fri, May 24, 2019 at 7:48 PM Y Song <ys114321@xxxxxxxxx> wrote:
>
> On Fri, May 24, 2019 at 9:01 AM Iago López 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 López Galeiras <iago@xxxxxxxxxx>
> > Signed-off-by: Alban Crequy <alban@xxxxxxxxxx>
> > Signed-off-by: Iago López 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 López Galeiras
Kinvolk GmbH | Adalbertstr. 6a, 10999 Berlin
Geschäftsführer/Directors: Alban Crequy, Chris Kühl, Iago López Galeiras
Registergericht/Court of registration: Amtsgericht Charlottenburg
Registernummer/Registration number: HRB 171414 B
Ust-ID-Nummer/VAT ID number: DE302207000
