From 332386ae7bc02618d1860f726065448324a6734a Mon Sep 17 00:00:00 2001
Message-Id: <332386ae7bc02618d1860f726065448324a6734a@dist-git>
From: Michal Privoznik <mprivozn@redhat.com>
Date: Mon, 5 Sep 2022 12:37:16 +0200
Subject: [PATCH] kbase: Document QEMU private mount NS limitations
There are two points I've taken for granted:
1) the mount points are set before starting a guest,
2) the / and its submounts are marked as shared, so that mount
events propagate into child namespaces when assumption 1) is
not held.
But what's obvious to me might not be obvious to our users.
Document these known limitations.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2123196
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
(cherry picked from commit d3397885d589c25b8962ae221fd0a71ced5597cb)
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2152083
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
docs/kbase/qemu-passthrough-security.rst | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/docs/kbase/qemu-passthrough-security.rst b/docs/kbase/qemu-passthrough-security.rst
index 4381d9f3a6..106c3cc5b9 100644
--- a/docs/kbase/qemu-passthrough-security.rst
+++ b/docs/kbase/qemu-passthrough-security.rst
@@ -156,3 +156,25 @@ will affect all virtual machines. These settings are all made in
* Cgroups - set ``cgroup_device_acl`` to include the desired device node, or
``cgroup_controllers = [...]`` to exclude the ``devices`` controller.
+
+Private monunt namespace
+----------------------------
+
+As mentioned above, libvirt launches each QEMU process in its own ``mount``
+namespace. It's recommended that all mount points are set up prior starting any
+guest. For cases when that can't be assured, mount points in the namespace are
+marked as slave so that mount events happening in the parent namespace are
+propagated into this child namespace. But this may require an additional step:
+mounts in the parent namespace need to be marked as shared (if the distribution
+doesn't do that by default). This can be achieved by running the following
+command before any guest is started:
+
+::
+
+ # mount --make-rshared /
+
+Another requirement for dynamic mount point propagation is to not place
+``hugetlbfs`` mount points under ``/dev`` because these won't be propagated as
+corresponding directories do not exist in the private namespace. Or just use
+``memfd`` memory backend instead which does not require ``hugetlbfs`` mount
+points.
--
2.39.0