tools/ocaml/xenstored: Fix path length validation

Currently, oxenstored checks the length of paths against 1024, then
prepends "/local/domain/$DOMID/" to relative paths.  This allows a domU
to create paths which can't subsequently be read by anyone, even dom0.
This also interferes with listing directories, etc.

Define a new oxenstored.conf entry: quota-path-max, defaulting to 1024
as before.  For paths that begin with "/local/domain/$DOMID/" check the
relative path length against this quota. For all other paths check the
entire path length.

This ensures that if the domid changes (and thus the length of a prefix
changes) a path that used to be valid stays valid (e.g. after a
live-migration).  It also ensures that regardless how the client tries
to access a path (domid-relative or absolute) it will get consistent
results, since the limit is always applied on the final canonicalized
path.

Delete the unused Domain.get_path to avoid it being confused with
Connection.get_path (which differs by a trailing slash only).

Rewrite Util.path_validate to apply the appropriate length restriction
based on whether the path is relative or not.  Remove the check for
connection_path being absolute, because it is not guest controlled data.

This is part of XSA-323.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>

tools/ocaml/libs/xb/partial.ml

@@ -28,6 +28,7 @@ external header_of_string_internal: string -> int * int * int * int
          = "stub_header_of_string"
 
 let xenstore_payload_max = 4096 (* xen/include/public/io/xs_wire.h *)
+let xenstore_rel_path_max = 2048 (* xen/include/public/io/xs_wire.h *)
 
 let of_string s =
 	let tid, rid, opint, dlen = header_of_string_internal s in

tools/ocaml/libs/xb/partial.mli

@@ -9,6 +9,7 @@ external header_size : unit -> int = "stub_header_size"
 external header_of_string_internal : string -> int * int * int * int
   = "stub_header_of_string"
 val xenstore_payload_max : int
+val xenstore_rel_path_max : int
 val of_string : string -> pkt
 val append : pkt -> string -> int -> unit
 val to_complete : pkt -> int

tools/ocaml/xenstored/define.ml

@@ -32,6 +32,8 @@ let conflict_rate_limit_is_aggregate = ref true
 
 let domid_self = 0x7FF0
 
+let path_max = ref Xenbus.Partial.xenstore_rel_path_max
+
 exception Not_a_directory of string
 exception Not_a_value of string
 exception Already_exist

tools/ocaml/xenstored/domain.ml

@@ -38,7 +38,6 @@ type t =
 }
 
 let is_dom0 d = d.id = 0
-let get_path dom = "/local/domain/" ^ (sprintf "%u" dom.id)
 let get_id domain = domain.id
 let get_interface d = d.interface
 let get_mfn d = d.mfn

tools/ocaml/xenstored/oxenstored.conf.in

@@ -61,6 +61,7 @@ quota-maxsize = 2048
 quota-maxwatch = 100
 quota-transaction = 10
 quota-maxrequests = 1024
+quota-path-max = 1024
 
 # Activate filed base backend
 persistent = false

tools/ocaml/xenstored/utils.ml

@@ -93,7 +93,7 @@ let read_file_single_integer filename =
 let path_validate path connection_path =
 	let len = String.length path in
 
-	if len = 0 || len > 1024 then raise Define.Invalid_path;
+	if len = 0 then raise Define.Invalid_path;
 
 	let abs_path =
 		match String.get path 0 with
@@ -101,4 +101,17 @@ let path_validate path connection_path =
 		| _   -> connection_path ^ path
 	in
 
+	(* Regardless whether client specified absolute or relative path,
+	   canonicalize it (above) and, for domain-relative paths, check the
+	   length of the relative part.
+
+	   This prevents paths becoming invalid across migrate when the length
+	   of the domid changes in @param connection_path.
+	 *)
+	let len = String.length abs_path in
+	let on_absolute _ _ = len in
+	let on_relative _ offset = len - offset in
+	let len = Scanf.ksscanf abs_path on_absolute "/local/domain/%d/%n" on_relative in
+	if len > !Define.path_max then raise Define.Invalid_path;
+
 	abs_path

tools/ocaml/xenstored/xenstored.ml

@@ -102,6 +102,7 @@ let parse_config filename =
 		("quota-maxentity", Config.Set_int Quota.maxent);
 		("quota-maxsize", Config.Set_int Quota.maxsize);
 		("quota-maxrequests", Config.Set_int Define.maxrequests);
+		("quota-path-max", Config.Set_int Define.path_max);
 		("test-eagain", Config.Set_bool Transaction.test_eagain);
 		("persistent", Config.Set_bool Disk.enable);
 		("xenstored-log-file", Config.String Logging.set_xenstored_log_destination);