tools/oxenstored: Avoid allocating invalid transaction ids

The transaction id of 0 is reserved, meaning "not in a transaction".  It is up
to the xenstored server to allocate transaction ids.  While oxenstored starts
its ids at 1, but insufficient care is taken with truncation cases.

A 32bit oxenstored has an int with 31 bits of width, meaning that the
transaction id will wrap around to 0 after 2 billion transactions.

A 64bit oxenstored has an int with 63 bits of width, meaning that once 4
billion transactions are used, the allocated id will be truncated when written
into the uin32_t field in the ring.  This causes the client to reply with the
truncated id, breaking any further attempt to use any transactions.

Limit all transaction ids to the range between 1 and 0x7ffffffe.  This is the
best which can be done without making oxenstored depend on Stdint or Cstruct,
yet still work for 32bit builds.

Also check that the proposed new transaction id isn't currently in use.  For
the first 2 billion transactions there is no chance of a collision, and after
that, the chance is at most 20 (the default open transaction quota) in 2
billion.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: David Scott <dave@recoil.org>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

diff --git a/tools/ocaml/xenstored/connection.ml b/tools/ocaml/xenstored/connection.ml
index b18336f8cd5e0f5872f681ba2a7ed288c97d2be2..0b47009da29340ab93a162234218b7585f7d35c0 100644 (file)
--- a/tools/ocaml/xenstored/connection.ml
+++ b/tools/ocaml/xenstored/connection.ml
@@ -216,14 +216,23 @@ let fire_watch watch path =
        let data = Utils.join_by_null [ new_path; watch.token; "" ] in
        send_reply watch.con Transaction.none 0 Xenbus.Xb.Op.Watchevent data
 
-let find_next_tid con =
-       let ret = con.next_tid in con.next_tid <- con.next_tid + 1; ret
+(* Search for a valid unused transaction id. *)
+let rec valid_transaction_id con proposed_id =
+       (* Clip proposed_id to the range [1, 0x7ffffffe] *)
+       let id = if proposed_id <= 0 || proposed_id >= 0x7fffffff then 1 else proposed_id in
+
+       if Hashtbl.mem con.transactions id then (
+               (* Outstanding transaction with this id.  Try the next. *)
+               valid_transaction_id con (id + 1)
+       ) else
+               id
 
 let start_transaction con store =
        if !Define.maxtransaction > 0 && not (is_dom0 con)
        && Hashtbl.length con.transactions > !Define.maxtransaction then
                raise Quota.Transaction_opened;
-       let id = find_next_tid con in
+       let id = valid_transaction_id con con.next_tid in
+       con.next_tid <- id + 1;
        let ntrans = Transaction.make id store in
        Hashtbl.add con.transactions id ntrans;
        Logging.start_transaction ~tid:id ~con:(get_domstr con);