io_archive: add bounds limit for uname and gname tar header fields

Modify apk_resolve_[ug]id to take the user/groupname as a blob, so
proper length checking is done and honored.

==31584== Conditional jump or move depends on uninitialised value(s)
==31584==    at 0x5C8CA5: strlen (strlen.c:17)
==31584==    by 0x432575: APK_BLOB_STR (apk_blob.h:79)
==31584==    by 0x4350EB: apk_resolve_uid (io.c:1112)
==31584==    by 0x43696C: apk_tar_parse (io_archive.c:152)
==31584==    by 0x4271BC: apk_pkg_read (package.c:929)
==31584==    by 0x402D75: add_main (app_add.c:163)
==31584==    by 0x40D5FF: main (apk-static.c:516)

Fixes a potential crash (DoS) on a crafted TAR file. CVE-2021-30139.

Reported-by: Sören Tempel <soeren+git@soeren-tempel.net>
Reviewed-by: Ariadne Conill <ariadne@dereferenced.org>

src/apk_io.h

@@ -180,7 +180,7 @@ const char *apk_url_local_file(const char *url);
 void apk_id_cache_init(struct apk_id_cache *idc, int root_fd);
 void apk_id_cache_free(struct apk_id_cache *idc);
 void apk_id_cache_reset(struct apk_id_cache *idc);
-uid_t apk_resolve_uid(struct apk_id_cache *idc, const char *username, uid_t default_uid);
-uid_t apk_resolve_gid(struct apk_id_cache *idc, const char *groupname, uid_t default_gid);
+uid_t apk_resolve_uid(struct apk_id_cache *idc, apk_blob_t username, uid_t default_uid);
+uid_t apk_resolve_gid(struct apk_id_cache *idc, apk_blob_t groupname, uid_t default_gid);
 
 #endif

src/io.c

@@ -1114,7 +1114,7 @@ static FILE *fopenat(int dirfd, const char *pathname)
 	return f;
 }
 
-uid_t apk_resolve_uid(struct apk_id_cache *idc, const char *username, uid_t default_uid)
+uid_t apk_resolve_uid(struct apk_id_cache *idc, apk_blob_t username, uid_t default_uid)
 {
 #ifdef HAVE_FGETPWENT_R
 	char buf[1024];
@@ -1124,7 +1124,7 @@ uid_t apk_resolve_uid(struct apk_id_cache *idc, const char *username, uid_t defa
 	struct passwd *pwd;
 	FILE *in;
 
-	ci = resolve_cache_item(&idc->uid_cache, APK_BLOB_STR(username));
+	ci = resolve_cache_item(&idc->uid_cache, username);
 	if (ci == NULL) return default_uid;
 
 	if (ci->genid != idc->genid) {
@@ -1141,7 +1141,7 @@ uid_t apk_resolve_uid(struct apk_id_cache *idc, const char *username, uid_t defa
 #endif
 				if (pwd == NULL)
 					break;
-				if (strcmp(pwd->pw_name, username) == 0) {
+				if (apk_blob_compare(APK_BLOB_STR(pwd->pw_name), username) == 0) {
 					ci->uid = pwd->pw_uid;
 					break;
 				}
@@ -1156,7 +1156,7 @@ uid_t apk_resolve_uid(struct apk_id_cache *idc, const char *username, uid_t defa
 	return default_uid;
 }
 
-uid_t apk_resolve_gid(struct apk_id_cache *idc, const char *groupname, uid_t default_gid)
+uid_t apk_resolve_gid(struct apk_id_cache *idc, apk_blob_t groupname, uid_t default_gid)
 {
 #ifdef HAVE_FGETGRENT_R
 	char buf[1024];
@@ -1166,7 +1166,7 @@ uid_t apk_resolve_gid(struct apk_id_cache *idc, const char *groupname, uid_t def
 	struct group *grp;
 	FILE *in;
 
-	ci = resolve_cache_item(&idc->gid_cache, APK_BLOB_STR(groupname));
+	ci = resolve_cache_item(&idc->gid_cache, groupname);
 	if (ci == NULL) return default_gid;
 
 	if (ci->genid != idc->genid) {
@@ -1183,7 +1183,7 @@ uid_t apk_resolve_gid(struct apk_id_cache *idc, const char *groupname, uid_t def
 #endif
 				if (grp == NULL)
 					break;
-				if (strcmp(grp->gr_name, groupname) == 0) {
+				if (apk_blob_compare(APK_BLOB_STR(grp->gr_name), groupname) == 0) {
 					ci->gid = grp->gr_gid;
 					break;
 				}

src/io_archive.c

@@ -49,6 +49,7 @@ struct tar_header {
 	char padding[12];   /* 500-511 */
 };
 
+#define TAR_BLOB(s)	APK_BLOB_PTR_LEN(s, strnlen(s, sizeof(s)))
 #define GET_OCTAL(s)	get_octal(s, sizeof(s))
 #define PUT_OCTAL(s,v)	put_octal(s, sizeof(s), v)
 
@@ -149,8 +150,8 @@ int apk_tar_parse(struct apk_istream *is, apk_archive_entry_parser parser,
 
 		entry = (struct apk_file_info){
 			.size  = GET_OCTAL(buf.size),
-			.uid   = apk_resolve_uid(idc, buf.uname, GET_OCTAL(buf.uid)),
-			.gid   = apk_resolve_gid(idc, buf.gname, GET_OCTAL(buf.gid)),
+			.uid   = apk_resolve_uid(idc, TAR_BLOB(buf.uname), GET_OCTAL(buf.uid)),
+			.gid   = apk_resolve_gid(idc, TAR_BLOB(buf.gname), GET_OCTAL(buf.gid)),
 			.mode  = GET_OCTAL(buf.mode) & 07777,
 			.mtime = GET_OCTAL(buf.mtime),
 			.name  = entry.name,