[xen staging] tools/oxenstored: Trim txhistory on xenbus reconnect

[patchbot@xxxxxxx]

commit 2a47797d1f3b14aab4f0368ab833abd311f94a70
Author:     Edwin Török <edvin.torok@xxxxxxxxxx>
AuthorDate: Fri Jan 8 11:57:37 2021 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Jan 22 18:01:34 2021 +0000

    tools/oxenstored: Trim txhistory on xenbus reconnect
    
    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@xxxxxxxxxx>
    Acked-by: Christian Lindig <christian.lindig@xxxxxxxxxx>
---
 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 a36ec4a821..684f7a3861 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 584c03b332..27790d4a5c 100644
--- a/tools/ocaml/xenstored/process.ml
+++ b/tools/ocaml/xenstored/process.ml
@@ -702,7 +702,7 @@ 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 ->
@@ -741,7 +741,7 @@ 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)
        )
 
--
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