[PATCH v2 1/2] tools/ocaml/xenstored: trim txhistory on xenbus reconnect

["Edwin Török" <edvin.torok@citrix.com>]

There is a global history, containing transactions from the past 0.05s, which get trimmed whenever any transaction commits or aborts.
Destroying a domain will cause xenopsd to perform some transactions deleting the tree, so that is fine.
But I think that a domain can abuse the xenbus reconnect facility to cause a large history to be
recorded - provided that noone does any transactions on the system inbetween, which may be difficult
to achieve given squeezed's constant pinging.

The theoretical situation is like this:
- a domain starts a transaction, creates as large a tree as it can, commits it. Then repeatedly:
    - start a transaction, do nothing with it, start a transaction, delete part of the large tree, write some new unique data there, don't commit
    - cause a xenbus reconnect (I think this can be done by writing something to the ring). This causes all transactions/watches for the connection to be cleared, but NOT the history, there were no commits, so nobody trimmed the history, i.e. it the history can contain transactions from more than just 0.05s
    - loop back and start more transactions, you can keep this up indefinitely without hitting quotas

Now there is a periodic History.trim running every 0.05s, so I don't think you can do much damage
with it.
But lets be safe an trim the transaction history anyway on reconnect.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
---
Changed since V1:
* post publicly now that the XSA is out (not a security issue)
---
 tools/ocaml/xenstored/connection.ml | 2 +-
 tools/ocaml/xenstored/history.ml    | 4 ++++
 tools/ocaml/xenstored/process.ml    | 4 ++--
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/tools/ocaml/xenstored/connection.ml b/tools/ocaml/xenstored/connection.ml
index eb23c3af7a..1cf24beafd 100644
--- a/tools/ocaml/xenstored/connection.ml
+++ b/tools/ocaml/xenstored/connection.ml
@@ -47,7 +47,7 @@ let mark_as_bad con =
 
 let initial_next_tid = 1
 
-let reconnect con =
+let do_reconnect con =
 	Xenbus.Xb.reconnect con.xb;
 	(* dom is the same *)
 	Hashtbl.clear con.transactions;
diff --git a/tools/ocaml/xenstored/history.ml b/tools/ocaml/xenstored/history.ml
index f39565bff5..3899353da8 100644
--- a/tools/ocaml/xenstored/history.ml
+++ b/tools/ocaml/xenstored/history.ml
@@ -53,6 +53,10 @@ let end_transaction txn con tid commit =
 	trim ~txn ();
 	success
 
+let reconnect con =
+	trim ();
+	Connection.do_reconnect con
+
 let push (x: history_record) =
 	let dom = x.con.Connection.dom in
 	match dom with
diff --git a/tools/ocaml/xenstored/process.ml b/tools/ocaml/xenstored/process.ml
index dd50456ad5..da8e9cdb26 100644
--- a/tools/ocaml/xenstored/process.ml
+++ b/tools/ocaml/xenstored/process.ml
@@ -705,7 +705,7 @@ let do_input store cons doms con =
 			Connection.do_input con
 		with Xenbus.Xb.Reconnect ->
 			info "%s requests a reconnect" (Connection.get_domstr con);
-			Connection.reconnect con;
+			History.reconnect con;
 			info "%s reconnection complete" (Connection.get_domstr con);
 			false
 		| Failure exp ->
@@ -744,7 +744,7 @@ let do_output _store _cons _doms con =
 			ignore (Connection.do_output con)
 		with Xenbus.Xb.Reconnect ->
 			info "%s requests a reconnect" (Connection.get_domstr con);
-			Connection.reconnect con;
+			History.reconnect con;
 			info "%s reconnection complete" (Connection.get_domstr con)
 	)
 
-- 
2.29.2