bpf: allow BPF read access to qdisc pkt_len

The pkt_len field in qdisc_skb_cb stores the skb length as it will
appear on the wire after segmentation. For byte accounting, this value
is more accurate than skb->len. It is computed on entry to the TC
layer, so only valid there.

Allow read access to this field from BPF tc classifier and action
programs. The implementation is analogous to tc_classid, aside from
restricting to read access.

To distinguish it from skb->len and self-describe export as wire_len.

Changes v1->v2
  - Rename pkt_len to wire_len

Signed-off-by: Petar Penkov <ppenkov@google.com>
Signed-off-by: Vlad Dumitrescu <vladum@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 8050caea74955e0d2b2034d653a3b8f42995aa73..0183b8e70a9e6dfa963713305242964bdf04219b 100644 (file)
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2497,6 +2497,7 @@ struct __sk_buff {
        __u32 data_meta;
        struct bpf_flow_keys *flow_keys;
        __u64 tstamp;
+       __u32 wire_len;
 };
 
 struct bpf_tunnel_key {

diff --git a/net/core/filter.c b/net/core/filter.c
index bd0df75dc7b6e42097786fc6e73ea650104fa09a..3d54af4c363d8fb6988313a445a060938b0ab478 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5773,6 +5773,7 @@ static bool sk_filter_is_valid_access(int off, int size,
        case bpf_ctx_range(struct __sk_buff, flow_keys):
        case bpf_ctx_range_till(struct __sk_buff, family, local_port):
        case bpf_ctx_range(struct __sk_buff, tstamp):
+       case bpf_ctx_range(struct __sk_buff, wire_len):
                return false;
        }
 
@@ -5797,6 +5798,7 @@ static bool cg_skb_is_valid_access(int off, int size,
        case bpf_ctx_range(struct __sk_buff, tc_classid):
        case bpf_ctx_range(struct __sk_buff, data_meta):
        case bpf_ctx_range(struct __sk_buff, flow_keys):
+       case bpf_ctx_range(struct __sk_buff, wire_len):
                return false;
        case bpf_ctx_range(struct __sk_buff, data):
        case bpf_ctx_range(struct __sk_buff, data_end):
@@ -5843,6 +5845,7 @@ static bool lwt_is_valid_access(int off, int size,
        case bpf_ctx_range(struct __sk_buff, data_meta):
        case bpf_ctx_range(struct __sk_buff, flow_keys):
        case bpf_ctx_range(struct __sk_buff, tstamp):
+       case bpf_ctx_range(struct __sk_buff, wire_len):
                return false;
        }
 
@@ -6273,6 +6276,7 @@ static bool sk_skb_is_valid_access(int off, int size,
        case bpf_ctx_range(struct __sk_buff, data_meta):
        case bpf_ctx_range(struct __sk_buff, flow_keys):
        case bpf_ctx_range(struct __sk_buff, tstamp):
+       case bpf_ctx_range(struct __sk_buff, wire_len):
                return false;
        }
 
@@ -6360,6 +6364,7 @@ static bool flow_dissector_is_valid_access(int off, int size,
        case bpf_ctx_range(struct __sk_buff, data_meta):
        case bpf_ctx_range_till(struct __sk_buff, family, local_port):
        case bpf_ctx_range(struct __sk_buff, tstamp):
+       case bpf_ctx_range(struct __sk_buff, wire_len):
                return false;
        }
 
@@ -6685,6 +6690,17 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
                                              bpf_target_off(struct sk_buff,
                                                             tstamp, 8,
                                                             target_size));
+               break;
+
+       case offsetof(struct __sk_buff, wire_len):
+               BUILD_BUG_ON(FIELD_SIZEOF(struct qdisc_skb_cb, pkt_len) != 4);
+
+               off = si->off;
+               off -= offsetof(struct __sk_buff, wire_len);
+               off += offsetof(struct sk_buff, cb);
+               off += offsetof(struct qdisc_skb_cb, pkt_len);
+               *target_size = 4;
+               *insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg, off);
        }
 
        return insn - insn_buf;

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 8050caea74955e0d2b2034d653a3b8f42995aa73..0183b8e70a9e6dfa963713305242964bdf04219b 100644 (file)
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2497,6 +2497,7 @@ struct __sk_buff {
        __u32 data_meta;
        struct bpf_flow_keys *flow_keys;
        __u64 tstamp;
+       __u32 wire_len;
 };
 
 struct bpf_tunnel_key {

diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index c3b038f26ecef94ff7d5a779cf22183f57ba5570..b4b4a3f9363961494b8ec4db008bca4ea7586d02 100644 (file)
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -14033,6 +14033,38 @@ static struct bpf_test tests[] = {
                .result_unpriv = REJECT,
                .result = ACCEPT,
        },
+       {
+               "check wire_len is not readable by sockets",
+               .insns = {
+                       BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+                                   offsetof(struct __sk_buff, wire_len)),
+                       BPF_EXIT_INSN(),
+               },
+               .errstr = "invalid bpf_context access",
+               .result = REJECT,
+       },
+       {
+               "check wire_len is readable by tc classifier",
+               .insns = {
+                       BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+                                   offsetof(struct __sk_buff, wire_len)),
+                       BPF_EXIT_INSN(),
+               },
+               .prog_type = BPF_PROG_TYPE_SCHED_CLS,
+               .result = ACCEPT,
+       },
+       {
+               "check wire_len is not writable by tc classifier",
+               .insns = {
+                       BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_1,
+                                   offsetof(struct __sk_buff, wire_len)),
+                       BPF_EXIT_INSN(),
+               },
+               .prog_type = BPF_PROG_TYPE_SCHED_CLS,
+               .errstr = "invalid bpf_context access",
+               .errstr_unpriv = "R1 leaks addr",
+               .result = REJECT,
+       },
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)