netlink: fix stack buffer overrun when emitting ranged expressions

Included bogon input generates following Sanitizer splat:

AddressSanitizer: dynamic-stack-buffer-overflow on address 0x7...
WRITE of size 2 at 0x7fffffffcbe4 thread T0
    #0 0x0000003a68b8 in __asan_memset (src/nft+0x3a68b8) (BuildId: 3678ff51a5405c77e3e0492b9a985910efee73b8)
    #1 0x0000004eb603 in __mpz_export_data src/gmputil.c:108:2
    #2 0x0000004eb603 in netlink_export_pad src/netlink.c:256:2
    #3 0x0000004eb603 in netlink_gen_range src/netlink.c:471:2
    #4 0x0000004ea250 in __netlink_gen_data src/netlink.c:523:10
    #5 0x0000004e8ee3 in alloc_nftnl_setelem src/netlink.c:205:3
    #6 0x0000004d4541 in mnl_nft_setelem_batch src/mnl.c:1816:11

Problem is that the range end is emitted to the buffer at the *padded*
location (rounded up to next register size), but buffer sizing is
based of the expression length, not the padded length.

Also extend the test script: Capture stderr and if we see
AddressSanitizer warning, make it fail.

Same bug as the one fixed in 600b84631410 ("netlink: fix stack buffer overflow with sub-reg sized prefixes"),
just in a different function.

Apply same fix: no dynamic array + add a range check.

Joint work with Pablo Neira Ayuso.

Signed-off-by: Florian Westphal <fw@strlen.de>

1 files changed, 35 insertions, 15 deletions

diff --git a/src/netlink.c b/src/netlink.c
index 8d7377e4..dfb7f4d1 100644
--- a/src/netlink.c
+++ b/src/netlink.c
@@ -330,11 +330,15 @@ static void nft_data_memcpy(struct nft_data_linearize *nld,
 static void netlink_gen_concat_key(const struct expr *expr,
 				    struct nft_data_linearize *nld)
 {
-	unsigned int len = expr->len / BITS_PER_BYTE, offset = 0;
-	unsigned char data[len];
+	unsigned int len = netlink_padded_len(expr->len) / BITS_PER_BYTE;
+	unsigned char data[NFT_MAX_EXPR_LEN_BYTES];
+	unsigned int offset = 0;
 	const struct expr *i;
 
-	memset(data, 0, len);
+	if (len > sizeof(data))
+		BUG("Value export of %u bytes would overflow", len);
+
+	memset(data, 0, sizeof(data));
 
 	list_for_each_entry(i, &expr->expressions, list)
 		offset += __netlink_gen_concat_key(expr->flags, i, data + offset);
@@ -373,11 +377,15 @@ static int __netlink_gen_concat_data(int end, const struct expr *i,
 static void __netlink_gen_concat_expand(const struct expr *expr,
 				        struct nft_data_linearize *nld)
 {
-	unsigned int len = div_round_up(expr->len, BITS_PER_BYTE) * 2, offset = 0;
-	unsigned char data[len];
+	unsigned int len = (netlink_padded_len(expr->len) / BITS_PER_BYTE) * 2;
+	unsigned char data[NFT_MAX_EXPR_LEN_BYTES];
+	unsigned int offset = 0;
 	const struct expr *i;
 
-	memset(data, 0, len);
+	if (len > sizeof(data))
+		BUG("Value export of %u bytes would overflow", len);
+
+	memset(data, 0, sizeof(data));
 
 	list_for_each_entry(i, &expr->expressions, list)
 		offset += __netlink_gen_concat_data(false, i, data + offset);
@@ -391,11 +399,15 @@ static void __netlink_gen_concat_expand(const struct expr *expr,
 static void __netlink_gen_concat(const struct expr *expr,
 				 struct nft_data_linearize *nld)
 {
-	unsigned int len = expr->len / BITS_PER_BYTE, offset = 0;
-	unsigned char data[len];
+	unsigned int len = netlink_padded_len(expr->len) / BITS_PER_BYTE;
+	unsigned char data[NFT_MAX_EXPR_LEN_BYTES];
+	unsigned int offset = 0;
 	const struct expr *i;
 
-	memset(data, 0, len);
+	if (len > sizeof(data))
+		BUG("Value export of %u bytes would overflow", len);
+
+	memset(data, 0, sizeof(data));
 
 	list_for_each_entry(i, &expr->expressions, list)
 		offset += __netlink_gen_concat_data(expr->flags, i, data + offset);
@@ -462,11 +474,14 @@ static void netlink_gen_verdict(const struct expr *expr,
 static void netlink_gen_range(const struct expr *expr,
 			      struct nft_data_linearize *nld)
 {
-	unsigned int len = div_round_up(expr->left->len, BITS_PER_BYTE) * 2;
-	unsigned char data[len];
-	unsigned int offset = 0;
+	unsigned int len = (netlink_padded_len(expr->left->len) / BITS_PER_BYTE) * 2;
+	unsigned char data[NFT_MAX_EXPR_LEN_BYTES];
+	unsigned int offset;
+
+	if (len > sizeof(data))
+		BUG("Value export of %u bytes would overflow", len);
 
-	memset(data, 0, len);
+	memset(data, 0, sizeof(data));
 	offset = netlink_export_pad(data, expr->left->value, expr->left);
 	netlink_export_pad(data + offset, expr->right->value, expr->right);
 	nft_data_memcpy(nld, data, len);
@@ -1215,10 +1230,15 @@ static struct expr *range_expr_reduce(struct expr *range)
 static struct expr *netlink_parse_interval_elem(const struct set *set,
 						struct expr *expr)
 {
-	unsigned int len = div_round_up(expr->len, BITS_PER_BYTE);
+	unsigned int len = netlink_padded_len(expr->len) / BITS_PER_BYTE;
 	const struct datatype *dtype = set->data->dtype;
 	struct expr *range, *left, *right;
-	char data[len];
+	char data[NFT_MAX_EXPR_LEN_BYTES];
+
+	if (len > sizeof(data))
+		BUG("Value export of %u bytes would overflow", len);
+
+	memset(data, 0, sizeof(data));
 
 	mpz_export_data(data, expr->value, dtype->byteorder, len);
 	left = constant_expr_alloc(&internal_location, dtype,