CVE-2020-10745: ndr/dns-utils: prepare for NBT compatibility

NBT has a funny thing where it sometimes needs to send a trailing dot as
part of the last component, because the string representation is a user
name. In DNS, "example.com", and "example.com." are the same, both
having three components ("example", "com", ""); in NBT, we want to treat
them differently, with the second form having the three components
("example", "com.", "").

This retains the logic of e6e2ec0.

Also DNS compression cannot be turned off for NBT.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14378

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>

librpc/ndr/ndr_dns.c

@@ -163,7 +163,8 @@ _PUBLIC_ enum ndr_err_code ndr_push_dns_string(struct ndr_push *ndr,
 	return ndr_push_dns_string_list(ndr,
 					&ndr->dns_string_list,
 					ndr_flags,
-					s);
+					s,
+					false);
 }
 
 _PUBLIC_ enum ndr_err_code ndr_pull_dns_txt_record(struct ndr_pull *ndr, int ndr_flags, struct dns_txt_record *r)

librpc/ndr/ndr_dns_utils.c

@@ -9,9 +9,32 @@
 enum ndr_err_code ndr_push_dns_string_list(struct ndr_push *ndr,
 					   struct ndr_token_list *string_list,
 					   int ndr_flags,
-					   const char *s)
+					   const char *s,
+					   bool is_nbt)
 {
 	const char *start = s;
+	bool use_compression;
+	size_t max_length;
+	if (is_nbt) {
+		use_compression = true;
+		/*
+		 * Max length is longer in NBT/Wins, because Windows counts
+		 * the semi-decompressed size of the netbios name (16 bytes)
+		 * rather than the wire size of 32, which is what you'd expect
+		 * if it followed RFC1002 (it uses the short form in
+		 * [MS-WINSRA]). In other words the maximum size of the
+		 * "scope" is 237, not 221.
+		 *
+		 * We make the size limit slightly larger than 255 + 16,
+		 * because the 237 scope limit is already enforced in the
+		 * winsserver code with a specific return value; bailing out
+		 * here would muck with that.
+		 */
+		max_length = 274;
+	} else {
+		use_compression = !(ndr->flags & LIBNDR_FLAG_NO_COMPRESSION);
+		max_length = 255;
+	}
 
 	if (!(ndr_flags & NDR_SCALARS)) {
 		return NDR_ERR_SUCCESS;
@@ -23,7 +46,7 @@ enum ndr_err_code ndr_push_dns_string_list(struct ndr_push *ndr,
 		size_t complen;
 		uint32_t offset;
 
-		if (!(ndr->flags & LIBNDR_FLAG_NO_COMPRESSION)) {
+		if (use_compression) {
 			/* see if we have pushed the remaining string already,
 			 * if so we use a label pointer to this string
 			 */
@@ -66,6 +89,14 @@ enum ndr_err_code ndr_push_dns_string_list(struct ndr_push *ndr,
 					      "(consecutive dots)");
 		}
 
+		if (is_nbt && s[complen] == '.' && s[complen + 1] == '\0') {
+			/* nbt names are sometimes usernames, and we need to
+			 * keep a trailing dot to ensure it is byte-identical,
+			 * (not just semantically identical given DNS
+			 * semantics). */
+			complen++;
+		}
+
 		compname = talloc_asprintf(ndr, "%c%*.*s",
 						(unsigned char)complen,
 						(unsigned char)complen,
@@ -75,7 +106,7 @@ enum ndr_err_code ndr_push_dns_string_list(struct ndr_push *ndr,
 		/* remember the current component + the rest of the string
 		 * so it can be reused later
 		 */
-		if (!(ndr->flags & LIBNDR_FLAG_NO_COMPRESSION)) {
+		if (use_compression) {
 			NDR_CHECK(ndr_token_store(ndr, string_list, s,
 						  ndr->offset));
 		}
@@ -89,9 +120,10 @@ enum ndr_err_code ndr_push_dns_string_list(struct ndr_push *ndr,
 		if (*s == '.') {
 			s++;
 		}
-		if (s - start > 255) {
+		if (s - start > max_length) {
 			return ndr_push_error(ndr, NDR_ERR_STRING,
-					      "name > 255 character long");
+					      "name > %zu character long",
+					      max_length);
 		}
 	}
 

librpc/ndr/ndr_dns_utils.h

@@ -2,4 +2,5 @@
 enum ndr_err_code ndr_push_dns_string_list(struct ndr_push *ndr,
 					   struct ndr_token_list *string_list,
 					   int ndr_flags,
-					   const char *s);
+					   const char *s,
+					   bool is_nbt);

librpc/ndr/ndr_nbt.c

@@ -25,6 +25,8 @@
 #include "includes.h"
 #include "../libcli/nbt/libnbt.h"
 #include "../libcli/netlogon/netlogon.h"
+#include "ndr_dns_utils.h"
+
 
 /* don't allow an unlimited number of name components */
 #define MAX_COMPONENTS 128
@@ -141,71 +143,11 @@ _PUBLIC_ enum ndr_err_code ndr_pull_nbt_string(struct ndr_pull *ndr, int ndr_fla
 */
 _PUBLIC_ enum ndr_err_code ndr_push_nbt_string(struct ndr_push *ndr, int ndr_flags, const char *s)
 {
-	if (!(ndr_flags & NDR_SCALARS)) {
-		return NDR_ERR_SUCCESS;
-	}
-
-	while (s && *s) {
-		enum ndr_err_code ndr_err;
-		char *compname;
-		size_t complen;
-		uint32_t offset;
-
-		/* see if we have pushed the remaining string already,
-		 * if so we use a label pointer to this string
-		 */
-		ndr_err = ndr_token_retrieve_cmp_fn(&ndr->nbt_string_list, s, &offset, (comparison_fn_t)strcmp, false);
-		if (NDR_ERR_CODE_IS_SUCCESS(ndr_err)) {
-			uint8_t b[2];
-
-			if (offset > 0x3FFF) {
-				return ndr_push_error(ndr, NDR_ERR_STRING,
-						      "offset for nbt string label pointer %u[%08X] > 0x00003FFF",
-						      offset, offset);
-			}
-
-			b[0] = 0xC0 | (offset>>8);
-			b[1] = (offset & 0xFF);
-
-			return ndr_push_bytes(ndr, b, 2);
-		}
-
-		complen = strcspn(s, ".");
-
-		/* we need to make sure the length fits into 6 bytes */
-		if (complen > 0x3F) {
-			return ndr_push_error(ndr, NDR_ERR_STRING,
-					      "component length %u[%08X] > 0x0000003F",
-					      (unsigned)complen, (unsigned)complen);
-		}
-
-		if (s[complen] == '.' && s[complen+1] == '\0') {
-			complen++;
-		}
-
-		compname = talloc_asprintf(ndr, "%c%*.*s",
-						(unsigned char)complen,
-						(unsigned char)complen,
-						(unsigned char)complen, s);
-		NDR_ERR_HAVE_NO_MEMORY(compname);
-
-		/* remember the current componemt + the rest of the string
-		 * so it can be reused later
-		 */
-		NDR_CHECK(ndr_token_store(ndr, &ndr->nbt_string_list, s, ndr->offset));
-
-		/* push just this component into the blob */
-		NDR_CHECK(ndr_push_bytes(ndr, (const uint8_t *)compname, complen+1));
-		talloc_free(compname);
-
-		s += complen;
-		if (*s == '.') s++;
-	}
-
-	/* if we reach the end of the string and have pushed the last component
-	 * without using a label pointer, we need to terminate the string
-	 */
-	return ndr_push_bytes(ndr, (const uint8_t *)"", 1);
+	return ndr_push_dns_string_list(ndr,
+					&ndr->dns_string_list,
+					ndr_flags,
+					s,
+					true);
 }
 
 

librpc/wscript_build

@@ -415,7 +415,7 @@ bld.SAMBA_SUBSYSTEM('NDR_NBT',
 
 bld.SAMBA_LIBRARY('ndr_nbt',
     source='gen_ndr/ndr_nbt.c ndr/ndr_nbt.c',
-    public_deps='ndr NDR_NBT_BUF NDR_SECURITY',
+    public_deps='ndr NDR_NBT_BUF NDR_SECURITY NDR_DNS',
     public_headers='gen_ndr/nbt.h gen_ndr/ndr_nbt.h ndr/ndr_nbt.h',
     header_path=[ ('gen_ndr*', 'gen_ndr'), ('ndr*', 'ndr')],
     pc_files='ndr_nbt.pc',
@@ -718,6 +718,5 @@ bld.SAMBA_BINARY('test_ndr_dns_nbt',
                       cmocka
                       ndr
                       ndr_nbt
-                      NDR_DNS
                       ''',
                  for_selftest=True)

selftest/knownfail.d/dns_packet

@@ -1 +0,0 @@
-samba.tests.dns_packet.samba.tests.dns_packet.TestNbtPackets.test_127_very_dotty_components