bpf: Add a bpf_sock pointer to __sk_buff and a bpf_sk_fullsock helper

In kernel, it is common to check "skb->sk && sk_fullsock(skb->sk)"
before accessing the fields in sock.  For example, in __netdev_pick_tx:

static u16 __netdev_pick_tx(struct net_device *dev, struct sk_buff *skb,
			    struct net_device *sb_dev)
{
	/* ... */

	struct sock *sk = skb->sk;

		if (queue_index != new_index && sk &&
		    sk_fullsock(sk) &&
		    rcu_access_pointer(sk->sk_dst_cache))
			sk_tx_queue_set(sk, new_index);

	/* ... */

	return queue_index;
}

This patch adds a "struct bpf_sock *sk" pointer to the "struct __sk_buff"
where a few of the convert_ctx_access() in filter.c has already been
accessing the skb->sk sock_common's fields,
e.g. sock_ops_convert_ctx_access().

"__sk_buff->sk" is a PTR_TO_SOCK_COMMON_OR_NULL in the verifier.
Some of the fileds in "bpf_sock" will not be directly
accessible through the "__sk_buff->sk" pointer.  It is limited
by the new "bpf_sock_common_is_valid_access()".
e.g. The existing "type", "protocol", "mark" and "priority" in bpf_sock
     are not allowed.

The newly added "struct bpf_sock *bpf_sk_fullsock(struct bpf_sock *sk)"
can be used to get a sk with all accessible fields in "bpf_sock".
This helper is added to both cg_skb and sched_(cls|act).

int cg_skb_foo(struct __sk_buff *skb) {
	struct bpf_sock *sk;

	sk = skb->sk;
	if (!sk)
		return 1;

	sk = bpf_sk_fullsock(sk);
	if (!sk)
		return 1;

	if (sk->family != AF_INET6 || sk->protocol != IPPROTO_TCP)
		return 1;

	/* some_traffic_shaping(); */

	return 1;
}

(1) The sk is read only

(2) There is no new "struct bpf_sock_common" introduced.

(3) Future kernel sock's members could be added to bpf_sock only
    instead of repeatedly adding at multiple places like currently
    in bpf_sock_ops_md, bpf_sock_addr_md, sk_reuseport_md...etc.

(4) After "sk = skb->sk", the reg holding sk is in type
    PTR_TO_SOCK_COMMON_OR_NULL.

(5) After bpf_sk_fullsock(), the return type will be in type
    PTR_TO_SOCKET_OR_NULL which is the same as the return type of
    bpf_sk_lookup_xxx().

    However, bpf_sk_fullsock() does not take refcnt.  The
    acquire_reference_state() is only depending on the return type now.
    To avoid it, a new is_acquire_function() is checked before calling
    acquire_reference_state().

(6) The WARN_ON in "release_reference_state()" is no longer an
    internal verifier bug.

    When reg->id is not found in state->refs[], it means the
    bpf_prog does something wrong like
    "bpf_sk_release(bpf_sk_fullsock(skb->sk))" where reference has
    never been acquired by calling "bpf_sk_fullsock(skb->sk)".

    A -EINVAL and a verbose are done instead of WARN_ON.  A test is
    added to the test_verifier in a later patch.

    Since the WARN_ON in "release_reference_state()" is no longer
    needed, "__release_reference_state()" is folded into
    "release_reference_state()" also.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -331,10 +331,17 @@ static bool type_is_pkt_pointer(enum bpf_reg_type type)
 	       type == PTR_TO_PACKET_META;
 }
 
+static bool type_is_sk_pointer(enum bpf_reg_type type)
+{
+	return type == PTR_TO_SOCKET ||
+		type == PTR_TO_SOCK_COMMON;
+}
+
 static bool reg_type_may_be_null(enum bpf_reg_type type)
 {
 	return type == PTR_TO_MAP_VALUE_OR_NULL ||
-	       type == PTR_TO_SOCKET_OR_NULL;
+	       type == PTR_TO_SOCKET_OR_NULL ||
+	       type == PTR_TO_SOCK_COMMON_OR_NULL;
 }
 
 static bool type_is_refcounted(enum bpf_reg_type type)
@@ -377,6 +384,12 @@ static bool is_release_function(enum bpf_func_id func_id)
 	return func_id == BPF_FUNC_sk_release;
 }
 
+static bool is_acquire_function(enum bpf_func_id func_id)
+{
+	return func_id == BPF_FUNC_sk_lookup_tcp ||
+		func_id == BPF_FUNC_sk_lookup_udp;
+}
+
 /* string representation of 'enum bpf_reg_type' */
 static const char * const reg_type_str[] = {
 	[NOT_INIT]		= "?",
@@ -392,6 +405,8 @@ static const char * const reg_type_str[] = {
 	[PTR_TO_FLOW_KEYS]	= "flow_keys",
 	[PTR_TO_SOCKET]		= "sock",
 	[PTR_TO_SOCKET_OR_NULL] = "sock_or_null",
+	[PTR_TO_SOCK_COMMON]	= "sock_common",
+	[PTR_TO_SOCK_COMMON_OR_NULL] = "sock_common_or_null",
 };
 
 static char slot_type_char[] = {
@@ -618,13 +633,10 @@ static int acquire_reference_state(struct bpf_verifier_env *env, int insn_idx)
 }
 
 /* release function corresponding to acquire_reference_state(). Idempotent. */
-static int __release_reference_state(struct bpf_func_state *state, int ptr_id)
+static int release_reference_state(struct bpf_func_state *state, int ptr_id)
 {
 	int i, last_idx;
 
-	if (!ptr_id)
-		return -EFAULT;
-
 	last_idx = state->acquired_refs - 1;
 	for (i = 0; i < state->acquired_refs; i++) {
 		if (state->refs[i].id == ptr_id) {
@@ -636,21 +648,7 @@ static int __release_reference_state(struct bpf_func_state *state, int ptr_id)
 			return 0;
 		}
 	}
-	return -EFAULT;
-}
-
-/* variation on the above for cases where we expect that there must be an
- * outstanding reference for the specified ptr_id.
- */
-static int release_reference_state(struct bpf_verifier_env *env, int ptr_id)
-{
-	struct bpf_func_state *state = cur_func(env);
-	int err;
-
-	err = __release_reference_state(state, ptr_id);
-	if (WARN_ON_ONCE(err != 0))
-		verbose(env, "verifier internal error: can't release reference\n");
-	return err;
+	return -EINVAL;
 }
 
 static int transfer_reference_state(struct bpf_func_state *dst,
@@ -1209,6 +1207,8 @@ static bool is_spillable_regtype(enum bpf_reg_type type)
 	case CONST_PTR_TO_MAP:
 	case PTR_TO_SOCKET:
 	case PTR_TO_SOCKET_OR_NULL:
+	case PTR_TO_SOCK_COMMON:
+	case PTR_TO_SOCK_COMMON_OR_NULL:
 		return true;
 	default:
 		return false;
@@ -1647,6 +1647,7 @@ static int check_sock_access(struct bpf_verifier_env *env, int insn_idx,
 	struct bpf_reg_state *regs = cur_regs(env);
 	struct bpf_reg_state *reg = &regs[regno];
 	struct bpf_insn_access_aux info = {};
+	bool valid;
 
 	if (reg->smin_value < 0) {
 		verbose(env, "R%d min value is negative, either use unsigned index or do a if (index >=0) check.\n",
@@ -1654,15 +1655,28 @@ static int check_sock_access(struct bpf_verifier_env *env, int insn_idx,
 		return -EACCES;
 	}
 
-	if (!bpf_sock_is_valid_access(off, size, t, &info)) {
-		verbose(env, "invalid bpf_sock access off=%d size=%d\n",
-			off, size);
-		return -EACCES;
+	switch (reg->type) {
+	case PTR_TO_SOCK_COMMON:
+		valid = bpf_sock_common_is_valid_access(off, size, t, &info);
+		break;
+	case PTR_TO_SOCKET:
+		valid = bpf_sock_is_valid_access(off, size, t, &info);
+		break;
+	default:
+		valid = false;
 	}
 
-	env->insn_aux_data[insn_idx].ctx_field_size = info.ctx_field_size;
 
-	return 0;
+	if (valid) {
+		env->insn_aux_data[insn_idx].ctx_field_size =
+			info.ctx_field_size;
+		return 0;
+	}
+
+	verbose(env, "R%d invalid %s access off=%d size=%d\n",
+		regno, reg_type_str[reg->type], off, size);
+
+	return -EACCES;
 }
 
 static bool __is_pointer_value(bool allow_ptr_leaks,
@@ -1688,8 +1702,14 @@ static bool is_ctx_reg(struct bpf_verifier_env *env, int regno)
 {
 	const struct bpf_reg_state *reg = reg_state(env, regno);
 
-	return reg->type == PTR_TO_CTX ||
-	       reg->type == PTR_TO_SOCKET;
+	return reg->type == PTR_TO_CTX;
+}
+
+static bool is_sk_reg(struct bpf_verifier_env *env, int regno)
+{
+	const struct bpf_reg_state *reg = reg_state(env, regno);
+
+	return type_is_sk_pointer(reg->type);
 }
 
 static bool is_pkt_reg(struct bpf_verifier_env *env, int regno)
@@ -1800,6 +1820,9 @@ static int check_ptr_alignment(struct bpf_verifier_env *env,
 	case PTR_TO_SOCKET:
 		pointer_desc = "sock ";
 		break;
+	case PTR_TO_SOCK_COMMON:
+		pointer_desc = "sock_common ";
+		break;
 	default:
 		break;
 	}
@@ -2003,11 +2026,14 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 			 * PTR_TO_PACKET[_META,_END]. In the latter
 			 * case, we know the offset is zero.
 			 */
-			if (reg_type == SCALAR_VALUE)
+			if (reg_type == SCALAR_VALUE) {
 				mark_reg_unknown(env, regs, value_regno);
-			else
+			} else {
 				mark_reg_known_zero(env, regs,
 						    value_regno);
+				if (reg_type_may_be_null(reg_type))
+					regs[value_regno].id = ++env->id_gen;
+			}
 			regs[value_regno].type = reg_type;
 		}
 
@@ -2053,9 +2079,10 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 		err = check_flow_keys_access(env, off, size);
 		if (!err && t == BPF_READ && value_regno >= 0)
 			mark_reg_unknown(env, regs, value_regno);
-	} else if (reg->type == PTR_TO_SOCKET) {
+	} else if (type_is_sk_pointer(reg->type)) {
 		if (t == BPF_WRITE) {
-			verbose(env, "cannot write into socket\n");
+			verbose(env, "R%d cannot write into %s\n",
+				regno, reg_type_str[reg->type]);
 			return -EACCES;
 		}
 		err = check_sock_access(env, insn_idx, regno, off, size, t);
@@ -2102,7 +2129,8 @@ static int check_xadd(struct bpf_verifier_env *env, int insn_idx, struct bpf_ins
 
 	if (is_ctx_reg(env, insn->dst_reg) ||
 	    is_pkt_reg(env, insn->dst_reg) ||
-	    is_flow_key_reg(env, insn->dst_reg)) {
+	    is_flow_key_reg(env, insn->dst_reg) ||
+	    is_sk_reg(env, insn->dst_reg)) {
 		verbose(env, "BPF_XADD stores into R%d %s is not allowed\n",
 			insn->dst_reg,
 			reg_type_str[reg_state(env, insn->dst_reg)->type]);
@@ -2369,6 +2397,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
 		err = check_ctx_reg(env, reg, regno);
 		if (err < 0)
 			return err;
+	} else if (arg_type == ARG_PTR_TO_SOCK_COMMON) {
+		expected_type = PTR_TO_SOCK_COMMON;
+		/* Any sk pointer can be ARG_PTR_TO_SOCK_COMMON */
+		if (!type_is_sk_pointer(type))
+			goto err_type;
 	} else if (arg_type == ARG_PTR_TO_SOCKET) {
 		expected_type = PTR_TO_SOCKET;
 		if (type != expected_type)
@@ -2783,7 +2816,7 @@ static int release_reference(struct bpf_verifier_env *env,
 	for (i = 0; i <= vstate->curframe; i++)
 		release_reg_references(env, vstate->frame[i], meta->ptr_id);
 
-	return release_reference_state(env, meta->ptr_id);
+	return release_reference_state(cur_func(env), meta->ptr_id);
 }
 
 static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
@@ -3049,8 +3082,11 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
 		}
 	} else if (is_release_function(func_id)) {
 		err = release_reference(env, &meta);
-		if (err)
+		if (err) {
+			verbose(env, "func %s#%d reference has not been acquired before\n",
+				func_id_name(func_id), func_id);
 			return err;
+		}
 	}
 
 	regs = cur_regs(env);
@@ -3099,12 +3135,19 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
 			regs[BPF_REG_0].id = ++env->id_gen;
 		}
 	} else if (fn->ret_type == RET_PTR_TO_SOCKET_OR_NULL) {
-		int id = acquire_reference_state(env, insn_idx);
-		if (id < 0)
-			return id;
 		mark_reg_known_zero(env, regs, BPF_REG_0);
 		regs[BPF_REG_0].type = PTR_TO_SOCKET_OR_NULL;
-		regs[BPF_REG_0].id = id;
+		if (is_acquire_function(func_id)) {
+			int id = acquire_reference_state(env, insn_idx);
+
+			if (id < 0)
+				return id;
+			/* For release_reference() */
+			regs[BPF_REG_0].id = id;
+		} else {
+			/* For mark_ptr_or_null_reg() */
+			regs[BPF_REG_0].id = ++env->id_gen;
+		}
 	} else {
 		verbose(env, "unknown return type %d of func %s#%d\n",
 			fn->ret_type, func_id_name(func_id), func_id);
@@ -3364,6 +3407,8 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	case PTR_TO_PACKET_END:
 	case PTR_TO_SOCKET:
 	case PTR_TO_SOCKET_OR_NULL:
+	case PTR_TO_SOCK_COMMON:
+	case PTR_TO_SOCK_COMMON_OR_NULL:
 		verbose(env, "R%d pointer arithmetic on %s prohibited\n",
 			dst, reg_type_str[ptr_reg->type]);
 		return -EACCES;
@@ -4597,6 +4642,8 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 			}
 		} else if (reg->type == PTR_TO_SOCKET_OR_NULL) {
 			reg->type = PTR_TO_SOCKET;
+		} else if (reg->type == PTR_TO_SOCK_COMMON_OR_NULL) {
+			reg->type = PTR_TO_SOCK_COMMON;
 		}
 		if (is_null || !(reg_is_refcounted(reg) ||
 				 reg_may_point_to_spin_lock(reg))) {
@@ -4621,7 +4668,7 @@ static void mark_ptr_or_null_regs(struct bpf_verifier_state *vstate, u32 regno,
 	int i, j;
 
 	if (reg_is_refcounted_or_null(&regs[regno]) && is_null)
-		__release_reference_state(state, id);
+		release_reference_state(state, id);
 
 	for (i = 0; i < MAX_BPF_REG; i++)
 		mark_ptr_or_null_reg(state, &regs[i], id, is_null);
@@ -5790,6 +5837,8 @@ static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
 	case PTR_TO_FLOW_KEYS:
 	case PTR_TO_SOCKET:
 	case PTR_TO_SOCKET_OR_NULL:
+	case PTR_TO_SOCK_COMMON:
+	case PTR_TO_SOCK_COMMON_OR_NULL:
 		/* Only valid matches are exact, which memcmp() above
 		 * would have accepted
 		 */
@@ -6110,6 +6159,8 @@ static bool reg_type_mismatch_ok(enum bpf_reg_type type)
 	case PTR_TO_CTX:
 	case PTR_TO_SOCKET:
 	case PTR_TO_SOCKET_OR_NULL:
+	case PTR_TO_SOCK_COMMON:
+	case PTR_TO_SOCK_COMMON_OR_NULL:
 		return false;
 	default:
 		return true;
@@ -7112,6 +7163,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 			convert_ctx_access = ops->convert_ctx_access;
 			break;
 		case PTR_TO_SOCKET:
+		case PTR_TO_SOCK_COMMON:
 			convert_ctx_access = bpf_sock_convert_ctx_access;
 			break;
 		default: