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List: oss-security
Subject: [oss-security] Xen Security Advisory 330 v3 (CVE-2020-29485) - oxenstored memory leak in reset_watch
From: Xen.org security team <security () xen ! org>
Date: 2020-12-15 12:20:20
Message-ID: E1kp9JU-00073K-Jw () xenbits ! xenproject ! org
[Download RAW message or body]
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256
Xen Security Advisory CVE-2020-29485 / XSA-330
version 3
oxenstored memory leak in reset_watches
UPDATES IN VERSION 3
====================
Public release.
ISSUE DESCRIPTION
=================
When acting upon a guest XS_RESET_WATCHES request, not all tracking
information is freed.
IMPACT
======
A guest can cause unbounded memory usage in oxenstored. This can lead
to a system-wide DoS.
VULNERABLE SYSTEMS
==================
All version of Xen since 4.6 are vulnerable.
Only systems using the Ocaml Xenstored implementation are vulnerable.
Systems using the C Xenstored implementaion are not vulnerable.
MITIGATION
==========
There are no mitigations.
Changing to use of C xenstored would avoid this vulnerability. However,
given the other vulnerabilities in both versions of xenstored being
reported at this time, changing xenstored implementation is not a
recommended approach to mitigation of individual issues.
CREDITS
=======
This issue was discovered by Edwin Török of Citrix.
RESOLUTION
==========
Applying the appropriate attached patch resolves this issue.
Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball. Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.
xsa330.patch Xen 4.12 - xen-unstable
xsa330-4.11.patch Xen 4.10 - 4.11
$ sha256sum xsa330*
efd95a883f227d63366a745b6007aa0c59cc612573235ba72108c8f89ecef7f3 xsa330.meta
1cda4fd8c91ceb132c5770d90375626521025e078c6ac1b53b68d78815997722 xsa330.patch
87284eaf6df92a78476f49a5587e28e1f5b9ca16ace5ad2e10b4b13abf50e034 xsa330-4.11.patch
$
DEPLOYMENT DURING EMBARGO
=========================
Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
administrators.
But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).
Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
Team.
(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable. This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)
For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
http://www.xenproject.org/security-policy.html
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["xsa330.meta" (application/octet-stream)]
["xsa330.patch" (application/octet-stream)]
From: =?UTF-8?q?Edwin Török?= <edvin.torok@citrix.com>
Subject: tools/ocaml/xenstored: delete watch from trie too when resetting
watches
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
c/s f8c72b526129 "oxenstored: implement XS_RESET_WATCHES" from Xen 4.6
introduced reset watches support in oxenstored by mirroring the change
in cxenstored.
However the OCaml version has some additional data structures to
optimize watch firing, and just resetting the watches in one of the data
structures creates a security bug where a malicious guest kernel can
exceed its watch quota, driving oxenstored into OOM:
* create watches
* reset watches (this still keeps the watches lingering in another data
structure, using memory)
* create some more watches
* loop until oxenstored dies
The guest kernel doesn't necessarily have to be malicious to trigger
this:
* if control/platform-feature-xs_reset_watches is set
* the guest kexecs (e.g. because it crashes)
* on boot more watches are set up
* this will slowly "leak" memory for watches in oxenstored, driving it
towards OOM.
This is XSA-330.
Fixes: f8c72b526129 ("oxenstored: implement XS_RESET_WATCHES")
Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
diff --git a/tools/ocaml/xenstored/connections.ml b/tools/ocaml/xenstored/connections.ml
index 9f9f7ee2f0..6ee3552ec2 100644
--- a/tools/ocaml/xenstored/connections.ml
+++ b/tools/ocaml/xenstored/connections.ml
@@ -134,6 +134,10 @@ let del_watch cons con path token cons.watches <- Trie.set cons.watches \
key watches; watch
+let del_watches cons con + Connection.del_watches con;
+ cons.watches <- Trie.map (del_watches_of_con con) cons.watches
+
(* path is absolute *)
let fire_watches ?oldroot root cons path recurse let key = key_of_path path in
diff --git a/tools/ocaml/xenstored/process.ml b/tools/ocaml/xenstored/process.ml
index 73e04cc18b..437d2dcf9e 100644
--- a/tools/ocaml/xenstored/process.ml
+++ b/tools/ocaml/xenstored/process.ml
@@ -179,8 +179,8 @@ let do_isintroduced con _t domains _cons data if domid = \
Define.domid_self || Domains.exist domains domid then "T\000" else "F\000"
(* only in xen >= 4.2 *)
-let do_reset_watches con _t _domains _cons _data - Connection.del_watches con;
+let do_reset_watches con _t _domains cons _data + Connections.del_watches cons con;
Connection.del_transactions con
(* only in >= xen3.3 \
*)
["xsa330-4.11.patch" (application/octet-stream)]
From: =?UTF-8?q?Edwin Török?= <edvin.torok@citrix.com>
Subject: tools/ocaml/xenstored: delete watch from trie too when resetting
watches
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
c/s f8c72b526129 "oxenstored: implement XS_RESET_WATCHES" from Xen 4.6
introduced reset watches support in oxenstored by mirroring the change
in cxenstored.
However the OCaml version has some additional data structures to
optimize watch firing, and just resetting the watches in one of the data
structures creates a security bug where a malicious guest kernel can
exceed its watch quota, driving oxenstored into OOM:
* create watches
* reset watches (this still keeps the watches lingering in another data
structure, using memory)
* create some more watches
* loop until oxenstored dies
The guest kernel doesn't necessarily have to be malicious to trigger
this:
* if control/platform-feature-xs_reset_watches is set
* the guest kexecs (e.g. because it crashes)
* on boot more watches are set up
* this will slowly "leak" memory for watches in oxenstored, driving it
towards OOM.
This is XSA-330.
Fixes: f8c72b526129 ("oxenstored: implement XS_RESET_WATCHES")
Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
diff --git a/tools/ocaml/xenstored/connections.ml b/tools/ocaml/xenstored/connections.ml
index 020b875dcd..4e69de1d42 100644
--- a/tools/ocaml/xenstored/connections.ml
+++ b/tools/ocaml/xenstored/connections.ml
@@ -134,6 +134,10 @@ let del_watch cons con path token cons.watches <- Trie.set cons.watches \
key watches; watch
+let del_watches cons con + Connection.del_watches con;
+ cons.watches <- Trie.map (del_watches_of_con con) cons.watches
+
(* path is absolute *)
let fire_watches ?oldroot root cons path recurse let key = key_of_path path in
diff --git a/tools/ocaml/xenstored/process.ml b/tools/ocaml/xenstored/process.ml
index 6a998f8764..12ad66fce6 100644
--- a/tools/ocaml/xenstored/process.ml
+++ b/tools/ocaml/xenstored/process.ml
@@ -179,8 +179,8 @@ let do_isintroduced con _t domains _cons data if domid = \
Define.domid_self || Domains.exist domains domid then "T\000" else "F\000"
(* only in xen >= 4.2 *)
-let do_reset_watches con t domains cons data - Connection.del_watches con;
+let do_reset_watches con _t _domains cons _data + Connections.del_watches cons con;
Connection.del_transactions con
(* only in >= xen3.3 \
*)
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