Re: [libvirt] [PATCH] LXC: create a bind mount for sysfs when enable userns but disable netns
On Mon, Jul 14, 2014 at 12:01 PM, Chen Hanxiao
<chenhanxiao(a)cn.fujitsu.com> wrote:
kernel commit 7dc5dbc879bd0779924b5132a48b731a0bc04a1e
forbid us doing a fresh mount for sysfs
when enable userns but disable netns.
This patch will create a bind mount in this senario.
Sorry for exhuming an already merged patch but today I ran into a
nasty issue caused by it.
Signed-off-by: Chen Hanxiao <chenhanxiao(a)cn.fujitsu.com>
---
src/lxc/lxc_container.c | 44 +++++++++++++++++++++++++++++++++-----------
1 file changed, 33 insertions(+), 11 deletions(-)
diff --git a/src/lxc/lxc_container.c b/src/lxc/lxc_container.c
index 4d89677..8a27215 100644
--- a/src/lxc/lxc_container.c
+++ b/src/lxc/lxc_container.c
@@ -815,10 +815,13 @@ static int lxcContainerSetReadOnly(void)
}
-static int lxcContainerMountBasicFS(bool userns_enabled)
+static int lxcContainerMountBasicFS(bool userns_enabled,
+ bool netns_disabled)
{
size_t i;
int rc = -1;
+ char* mnt_src = NULL;
+ int mnt_mflags;
VIR_DEBUG("Mounting basic filesystems");
@@ -826,8 +829,25 @@ static int lxcContainerMountBasicFS(bool userns_enabled)
bool bindOverReadonly;
virLXCBasicMountInfo const *mnt = &lxcBasicMounts[i];
+ /* When enable userns but disable netns, kernel will
+ * forbid us doing a new fresh mount for sysfs.
+ * So we had to do a bind mount for sysfs instead.
+ */
+ if (userns_enabled && netns_disabled &&
+ STREQ(mnt->src, "sysfs")) {
+ if (VIR_STRDUP(mnt_src, "/sys") < 0) {
+ goto cleanup;
+ }
This is clearly broken and looks very untested to me.
It will issue this mount call:
mount("/sys", "/sys", "sysfs",
MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_BIND, NULL)
because the code runs after pivot_root(2).
i.e, /sys will be still empty after that and no sysfs at all there.
As libvirt will later remount /sys readonly creating a container will
fail with the most useless error message:
Error: internal error: guest failed to start: Unable to create
directory /sys/fs/: Read-only file system
or
Error: internal error: guest failed to start: Unable to create
directory /sys/fs/cgroup: Read-only file system
Please note that changing "/sys" to "/.oldroot/sys" will not solve
the
issue as this code runs already in the new
namespace and therefore the old mount tree is locked, thus MS_BIND is
not allowed.
This brings me to the question, why do you handle the netns_disabled
case anyway?
If in the XML file no network is specified just create a new and empty
network namespace.
Bindmounting /sys into the container is a security issue. This is why
mounting sysfs without a netns
was disabled to begin with.
P.S: Sorry for the grumpy mail, I've wasted almost the whole day with
debugging that issue.
--
Thanks,
//richard