[RFC bpf-next v3 1/8] flow_dissector: allow access only to a subset of __sk_buff fields

[Stanislav Fomichev <sdf@google.com>]

Use whitelist instead of a blacklist and allow only a small set of
fields that might be relevant in the context of flow dissector:
* protocol
* vlan_present
* vlan_tci
* vlan_proto

This is required for the eth_get_headlen case where we have only a
chunk of data to dissect (i.e. trying to read the other skb fields
doesn't make sense).

Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 net/core/filter.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 647c63a7b25b..62a7a2f50200 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6613,14 +6613,8 @@ static bool flow_dissector_is_valid_access(int off, int size,
 					   const struct bpf_prog *prog,
 					   struct bpf_insn_access_aux *info)
 {
-	if (type == BPF_WRITE) {
-		switch (off) {
-		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
-			break;
-		default:
-			return false;
-		}
-	}
+	if (type == BPF_WRITE)
+		return false;
 
 	switch (off) {
 	case bpf_ctx_range(struct __sk_buff, data):
@@ -6632,11 +6626,12 @@ static bool flow_dissector_is_valid_access(int off, int size,
 	case bpf_ctx_range_ptr(struct __sk_buff, flow_keys):
 		info->reg_type = PTR_TO_FLOW_KEYS;
 		break;
-	case bpf_ctx_range(struct __sk_buff, tc_classid):
-	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):
+	case bpf_ctx_range(struct __sk_buff, protocol):
+	case bpf_ctx_range(struct __sk_buff, vlan_present):
+	case bpf_ctx_range(struct __sk_buff, vlan_tci):
+	case bpf_ctx_range(struct __sk_buff, vlan_proto):
+		break;
+	default:
 		return false;
 	}
 
-- 
2.21.0.392.gf8f6787159e-goog