[nft v2 PATCH 2/3] nftables: add input flag NFT_CTX_INPUT_NO_DNS to avoid blocking getaddrinfo()

[Thomas Haller <thaller@redhat.com>]

getaddrinfo() blocks while trying to resolve the name. Blocking the
caller of the library is in many cases undesirable. Also, while
reconfiguring the firewall, it's not clear that resolving names via
the network works or makes sense.

Add a new input flag NFT_CTX_INPUT_NO_DNS to opt-out from getaddrinfo()
and only accept plain IP addresses.

We could also use AI_NUMERICHOST with getaddrinfo() instead of
inet_pton(). But it seems we can do a better job of generating an error
message, when we try to parse via inet_pton(). Then our error message
can clearly indicate that the string is not a valid IP address.

Signed-off-by: Thomas Haller <thaller@redhat.com>
---
 doc/libnftables.adoc           | 10 ++++-
 include/datatype.h             |  1 +
 include/nftables/libnftables.h |  4 ++
 src/datatype.c                 | 68 ++++++++++++++++++++--------------
 src/evaluate.c                 | 16 +++++++-
 5 files changed, 68 insertions(+), 31 deletions(-)

diff --git a/doc/libnftables.adoc b/doc/libnftables.adoc
index 96a580469ee0..77f3a0fd5659 100644
--- a/doc/libnftables.adoc
+++ b/doc/libnftables.adoc
@@ -84,7 +84,15 @@ The *nft_ctx_set_dry_run*() function sets the dry-run setting in 'ctx' to the va
 === nft_ctx_input_get_flags() and nft_ctx_input_set_flags()
 The flags setting controls the input format.
 
-Currently not flags are implemented.
+----
+enum {
+        NFT_CTX_INPUT_NO_DNS = (1 << 0),
+};
+----
+
+NFT_CTX_INPUT_NO_DNS::
+	Avoid resolving IP addresses with blocking getaddrinfo(). In that case,
+	only plain IP addresses are accepted.
 
 The *nft_ctx_input_get_flags*() function returns the input flags setting's value in 'ctx'.
 
diff --git a/include/datatype.h b/include/datatype.h
index 4b59790b67f9..be5c6d1b4011 100644
--- a/include/datatype.h
+++ b/include/datatype.h
@@ -182,6 +182,7 @@ struct datatype *dtype_clone(const struct datatype *orig_dtype);
 
 struct parse_ctx {
 	struct symbol_tables	*tbl;
+	const struct input_ctx	*input;
 };
 
 extern struct error_record *symbol_parse(struct parse_ctx *ctx,
diff --git a/include/nftables/libnftables.h b/include/nftables/libnftables.h
index 7fb621be1f12..2f5f079efff0 100644
--- a/include/nftables/libnftables.h
+++ b/include/nftables/libnftables.h
@@ -48,6 +48,10 @@ enum nft_optimize_flags {
 uint32_t nft_ctx_get_optimize(struct nft_ctx *ctx);
 void nft_ctx_set_optimize(struct nft_ctx *ctx, uint32_t flags);
 
+enum {
+	NFT_CTX_INPUT_NO_DNS		= (1 << 0),
+};
+
 unsigned int nft_ctx_input_get_flags(struct nft_ctx *ctx);
 void nft_ctx_input_set_flags(struct nft_ctx *ctx, unsigned int flags);
 
diff --git a/src/datatype.c b/src/datatype.c
index da802a18bccd..8629a38da56a 100644
--- a/src/datatype.c
+++ b/src/datatype.c
@@ -599,27 +599,33 @@ static struct error_record *ipaddr_type_parse(struct parse_ctx *ctx,
 					      const struct expr *sym,
 					      struct expr **res)
 {
-	struct addrinfo *ai, hints = { .ai_family = AF_INET,
-				       .ai_socktype = SOCK_DGRAM};
-	struct in_addr *addr;
-	int err;
+	struct in_addr addr;
 
-	err = getaddrinfo(sym->identifier, NULL, &hints, &ai);
-	if (err != 0)
-		return error(&sym->location, "Could not resolve hostname: %s",
-			     gai_strerror(err));
+	if (ctx->input->flags & NFT_CTX_INPUT_NO_DNS) {
+		if (inet_pton(AF_INET, sym->identifier, &addr) != 1)
+			return error(&sym->location, "Invalid IPv4 address");
+	} else {
+		struct addrinfo *ai, hints = { .ai_family = AF_INET,
+					       .ai_socktype = SOCK_DGRAM};
+		int err;
 
-	if (ai->ai_next != NULL) {
+		err = getaddrinfo(sym->identifier, NULL, &hints, &ai);
+		if (err != 0)
+			return error(&sym->location, "Could not resolve hostname: %s",
+				     gai_strerror(err));
+
+		if (ai->ai_next != NULL) {
+			freeaddrinfo(ai);
+			return error(&sym->location,
+				     "Hostname resolves to multiple addresses");
+		}
+		addr = ((struct sockaddr_in *)ai->ai_addr)->sin_addr;
 		freeaddrinfo(ai);
-		return error(&sym->location,
-			     "Hostname resolves to multiple addresses");
 	}
 
-	addr = &((struct sockaddr_in *)ai->ai_addr)->sin_addr;
 	*res = constant_expr_alloc(&sym->location, &ipaddr_type,
 				   BYTEORDER_BIG_ENDIAN,
-				   sizeof(*addr) * BITS_PER_BYTE, addr);
-	freeaddrinfo(ai);
+				   sizeof(addr) * BITS_PER_BYTE, &addr);
 	return NULL;
 }
 
@@ -658,27 +664,33 @@ static struct error_record *ip6addr_type_parse(struct parse_ctx *ctx,
 					       const struct expr *sym,
 					       struct expr **res)
 {
-	struct addrinfo *ai, hints = { .ai_family = AF_INET6,
-				       .ai_socktype = SOCK_DGRAM};
-	struct in6_addr *addr;
-	int err;
+	struct in6_addr addr;
 
-	err = getaddrinfo(sym->identifier, NULL, &hints, &ai);
-	if (err != 0)
-		return error(&sym->location, "Could not resolve hostname: %s",
-			     gai_strerror(err));
+	if (ctx->input->flags & NFT_CTX_INPUT_NO_DNS) {
+		if (inet_pton(AF_INET6, sym->identifier, &addr) != 1)
+			return error(&sym->location, "Invalid IPv6 address");
+	} else {
+		struct addrinfo *ai, hints = { .ai_family = AF_INET6,
+					       .ai_socktype = SOCK_DGRAM};
+		int err;
 
-	if (ai->ai_next != NULL) {
+		err = getaddrinfo(sym->identifier, NULL, &hints, &ai);
+		if (err != 0)
+			return error(&sym->location, "Could not resolve hostname: %s",
+				     gai_strerror(err));
+
+		if (ai->ai_next != NULL) {
+			freeaddrinfo(ai);
+			return error(&sym->location,
+				     "Hostname resolves to multiple addresses");
+		}
+		addr = ((struct sockaddr_in6 *)ai->ai_addr)->sin6_addr;
 		freeaddrinfo(ai);
-		return error(&sym->location,
-			     "Hostname resolves to multiple addresses");
 	}
 
-	addr = &((struct sockaddr_in6 *)ai->ai_addr)->sin6_addr;
 	*res = constant_expr_alloc(&sym->location, &ip6addr_type,
 				   BYTEORDER_BIG_ENDIAN,
-				   sizeof(*addr) * BITS_PER_BYTE, addr);
-	freeaddrinfo(ai);
+				   sizeof(addr) * BITS_PER_BYTE, &addr);
 	return NULL;
 }
 
diff --git a/src/evaluate.c b/src/evaluate.c
index 33e4ac93e89a..a904714acd48 100644
--- a/src/evaluate.c
+++ b/src/evaluate.c
@@ -46,6 +46,14 @@ struct proto_ctx *eval_proto_ctx(struct eval_ctx *ctx)
 	return &ctx->_pctx[idx];
 }
 
+static void parse_ctx_init(struct parse_ctx *parse_ctx, const struct eval_ctx *ctx)
+{
+	*parse_ctx = (struct parse_ctx) {
+		.tbl	= &ctx->nft->output.tbl,
+		.input	= &ctx->nft->input,
+	};
+}
+
 static int expr_evaluate(struct eval_ctx *ctx, struct expr **expr);
 
 static const char * const byteorder_names[] = {
@@ -277,12 +285,14 @@ static int flowtable_not_found(struct eval_ctx *ctx, const struct location *loc,
  */
 static int expr_evaluate_symbol(struct eval_ctx *ctx, struct expr **expr)
 {
-	struct parse_ctx parse_ctx = { .tbl = &ctx->nft->output.tbl, };
+	struct parse_ctx parse_ctx;
 	struct error_record *erec;
 	struct table *table;
 	struct set *set;
 	struct expr *new;
 
+	parse_ctx_init(&parse_ctx, ctx);
+
 	switch ((*expr)->symtype) {
 	case SYMBOL_VALUE:
 		datatype_set(*expr, ctx->ectx.dtype);
@@ -3432,10 +3442,12 @@ static int stmt_evaluate_reject_default(struct eval_ctx *ctx,
 
 static int stmt_evaluate_reject_icmp(struct eval_ctx *ctx, struct stmt *stmt)
 {
-	struct parse_ctx parse_ctx = { .tbl = &ctx->nft->output.tbl, };
+	struct parse_ctx parse_ctx;
 	struct error_record *erec;
 	struct expr *code;
 
+	parse_ctx_init(&parse_ctx, ctx);
+
 	erec = symbol_parse(&parse_ctx, stmt->reject.expr, &code);
 	if (erec != NULL) {
 		erec_queue(erec, ctx->msgs);
-- 
2.41.0