bpf: make direct packet write unclone more robust

Given this seems to be quite fragile and can easily slip through the
cracks, lets make direct packet write more robust by requiring that
future program types which allow for such write must provide a prologue
callback. In case of XDP and sk_msg it's noop, thus add a generic noop
handler there. The latter starts out with NULL data/data_end unconditionally
when sg pages are shared.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -5709,7 +5709,11 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 	bool is_narrower_load;
 	u32 target_size;
 
-	if (ops->gen_prologue) {
+	if (ops->gen_prologue || env->seen_direct_write) {
+		if (!ops->gen_prologue) {
+			verbose(env, "bpf verifier is misconfigured\n");
+			return -EINVAL;
+		}
 		cnt = ops->gen_prologue(insn_buf, env->seen_direct_write,
 					env->prog);
 		if (cnt >= ARRAY_SIZE(insn_buf)) {

net/core/filter.c

@@ -5644,6 +5644,15 @@ static bool sock_filter_is_valid_access(int off, int size,
 					       prog->expected_attach_type);
 }
 
+static int bpf_noop_prologue(struct bpf_insn *insn_buf, bool direct_write,
+			     const struct bpf_prog *prog)
+{
+	/* Neither direct read nor direct write requires any preliminary
+	 * action.
+	 */
+	return 0;
+}
+
 static int bpf_unclone_prologue(struct bpf_insn *insn_buf, bool direct_write,
 				const struct bpf_prog *prog, int drop_verdict)
 {
@@ -7210,6 +7219,7 @@ const struct bpf_verifier_ops xdp_verifier_ops = {
 	.get_func_proto		= xdp_func_proto,
 	.is_valid_access	= xdp_is_valid_access,
 	.convert_ctx_access	= xdp_convert_ctx_access,
+	.gen_prologue		= bpf_noop_prologue,
 };
 
 const struct bpf_prog_ops xdp_prog_ops = {
@@ -7308,6 +7318,7 @@ const struct bpf_verifier_ops sk_msg_verifier_ops = {
 	.get_func_proto		= sk_msg_func_proto,
 	.is_valid_access	= sk_msg_is_valid_access,
 	.convert_ctx_access	= sk_msg_convert_ctx_access,
+	.gen_prologue		= bpf_noop_prologue,
 };
 
 const struct bpf_prog_ops sk_msg_prog_ops = {