bpf: remove mark access for SK_SKB program types

The skb->mark field is a union with reserved_tailroom which is used
in the TCP code paths from stream memory allocation. Allowing SK_SKB
programs to set this field creates a conflict with future code
optimizations, such as "gifting" the skb to the egress path instead
of creating a new skb and doing a memcpy.

Because we do not have a released version of SK_SKB yet lets just
remove it for now. A more appropriate scratch pad to use at the
socket layer is dev_scratch, but lets add that in future kernels
when needed.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -3684,7 +3684,6 @@ static bool sk_skb_is_valid_access(int off, int size,
 {
 	if (type == BPF_WRITE) {
 		switch (off) {
-		case bpf_ctx_range(struct __sk_buff, mark):
 		case bpf_ctx_range(struct __sk_buff, tc_index):
 		case bpf_ctx_range(struct __sk_buff, priority):
 			break;
@@ -3694,6 +3693,7 @@ static bool sk_skb_is_valid_access(int off, int size,
 	}
 
 	switch (off) {
+	case bpf_ctx_range(struct __sk_buff, mark):
 	case bpf_ctx_range(struct __sk_buff, tc_classid):
 		return false;
 	case bpf_ctx_range(struct __sk_buff, data):

tools/testing/selftests/bpf/test_verifier.c

@@ -1130,15 +1130,27 @@ static struct bpf_test tests[] = {
 		.errstr = "invalid bpf_context access",
 	},
 	{
-		"check skb->mark is writeable by SK_SKB",
+		"invalid access of skb->mark for SK_SKB",
+		.insns = {
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, mark)),
+			BPF_EXIT_INSN(),
+		},
+		.result =  REJECT,
+		.prog_type = BPF_PROG_TYPE_SK_SKB,
+		.errstr = "invalid bpf_context access",
+	},
+	{
+		"check skb->mark is not writeable by SK_SKB",
 		.insns = {
 			BPF_MOV64_IMM(BPF_REG_0, 0),
 			BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_0,
 				    offsetof(struct __sk_buff, mark)),
 			BPF_EXIT_INSN(),
 		},
-		.result = ACCEPT,
+		.result =  REJECT,
 		.prog_type = BPF_PROG_TYPE_SK_SKB,
+		.errstr = "invalid bpf_context access",
 	},
 	{
 		"check skb->tc_index is writeable by SK_SKB",