ksmbd: prevent out of share access

Because of .., files outside the share directory
could be accessed. To prevent this, normalize
the given path and remove all . and ..
components.

In addition to the usual large set of regression tests (smbtorture
and xfstests), ran various tests on this to specifically check
path name validation including libsmb2 tests to verify path
normalization:

 ./examples/smb2-ls-async smb://172.30.1.15/homes2/../
 ./examples/smb2-ls-async smb://172.30.1.15/homes2/foo/../
 ./examples/smb2-ls-async smb://172.30.1.15/homes2/foo/../../
 ./examples/smb2-ls-async smb://172.30.1.15/homes2/foo/../
 ./examples/smb2-ls-async smb://172.30.1.15/homes2/foo/..bar/
 ./examples/smb2-ls-async smb://172.30.1.15/homes2/foo/bar../
 ./examples/smb2-ls-async smb://172.30.1.15/homes2/foo/bar..
 ./examples/smb2-ls-async smb://172.30.1.15/homes2/foo/bar../../../../

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/ksmbd/misc.c

@@ -191,19 +191,77 @@ int get_nlink(struct kstat *st)
 	return nlink;
 }
 
-void ksmbd_conv_path_to_unix(char *path)
+char *ksmbd_conv_path_to_unix(char *path)
 {
+	size_t path_len, remain_path_len, out_path_len;
+	char *out_path, *out_next;
+	int i, pre_dotdot_cnt = 0, slash_cnt = 0;
+	bool is_last;
+
 	strreplace(path, '\\', '/');
-}
+	path_len = strlen(path);
+	remain_path_len = path_len;
+	if (path_len == 0)
+		return ERR_PTR(-EINVAL);
 
-void ksmbd_strip_last_slash(char *path)
-{
-	int len = strlen(path);
+	out_path = kzalloc(path_len + 2, GFP_KERNEL);
+	if (!out_path)
+		return ERR_PTR(-ENOMEM);
+	out_path_len = 0;
+	out_next = out_path;
+
+	do {
+		char *name = path + path_len - remain_path_len;
+		char *next = strchrnul(name, '/');
+		size_t name_len = next - name;
+
+		is_last = !next[0];
+		if (name_len == 2 && name[0] == '.' && name[1] == '.') {
+			pre_dotdot_cnt++;
+			/* handle the case that path ends with "/.." */
+			if (is_last)
+				goto follow_dotdot;
+		} else {
+			if (pre_dotdot_cnt) {
+follow_dotdot:
+				slash_cnt = 0;
+				for (i = out_path_len - 1; i >= 0; i--) {
+					if (out_path[i] == '/' &&
+					    ++slash_cnt == pre_dotdot_cnt + 1)
+						break;
+				}
+
+				if (i < 0 &&
+				    slash_cnt != pre_dotdot_cnt) {
+					kfree(out_path);
+					return ERR_PTR(-EINVAL);
+				}
+
+				out_next = &out_path[i+1];
+				*out_next = '\0';
+				out_path_len = i + 1;
 
-	while (len && path[len - 1] == '/') {
-		path[len - 1] = '\0';
-		len--;
-	}
+			}
+
+			if (name_len != 0 &&
+			    !(name_len == 1 && name[0] == '.') &&
+			    !(name_len == 2 && name[0] == '.' && name[1] == '.')) {
+				next[0] = '\0';
+				sprintf(out_next, "%s/", name);
+				out_next += name_len + 1;
+				out_path_len += name_len + 1;
+				next[0] = '/';
+			}
+			pre_dotdot_cnt = 0;
+		}
+
+		remain_path_len -= name_len + 1;
+	} while (!is_last);
+
+	if (out_path_len > 0)
+		out_path[out_path_len-1] = '\0';
+	path[path_len] = '\0';
+	return out_path;
 }
 
 void ksmbd_conv_path_to_windows(char *path)

fs/ksmbd/misc.h

@@ -16,8 +16,7 @@ int ksmbd_validate_filename(char *filename);
 int parse_stream_name(char *filename, char **stream_name, int *s_type);
 char *convert_to_nt_pathname(char *filename, char *sharepath);
 int get_nlink(struct kstat *st);
-void ksmbd_conv_path_to_unix(char *path);
-void ksmbd_strip_last_slash(char *path);
+char *ksmbd_conv_path_to_unix(char *path);
 void ksmbd_conv_path_to_windows(char *path);
 char *ksmbd_extract_sharename(char *treename);
 char *convert_to_unix_name(struct ksmbd_share_config *share, char *name);

fs/ksmbd/smb2pdu.c

@@ -634,7 +634,7 @@ static char *
 smb2_get_name(struct ksmbd_share_config *share, const char *src,
 	      const int maxlen, struct nls_table *local_nls)
 {
-	char *name, *unixname;
+	char *name, *norm_name, *unixname;
 
 	name = smb_strndup_from_utf16(src, maxlen, 1, local_nls);
 	if (IS_ERR(name)) {
@@ -643,11 +643,15 @@ smb2_get_name(struct ksmbd_share_config *share, const char *src,
 	}
 
 	/* change it to absolute unix name */
-	ksmbd_conv_path_to_unix(name);
-	ksmbd_strip_last_slash(name);
-
-	unixname = convert_to_unix_name(share, name);
+	norm_name = ksmbd_conv_path_to_unix(name);
+	if (IS_ERR(norm_name)) {
+		kfree(name);
+		return norm_name;
+	}
 	kfree(name);
+
+	unixname = convert_to_unix_name(share, norm_name);
+	kfree(norm_name);
 	if (!unixname) {
 		pr_err("can not convert absolute name\n");
 		return ERR_PTR(-ENOMEM);