Richard Weinberger [off-list ref] writes:

Am 28.05.2015 um 21:57 schrieb Eric W. Biederman:

quoted

quoted

FWIW, it breaks also libvirt-lxc:
Error: internal error: guest failed to start: Failed to re-mount /proc/sys on /proc/sys flags=1021: Operation not permitted

Interesting.  I had not anticipated a failure there?  And it is failing
in remount?  Oh that is interesting.

That implies that there is some flag of the original mount of /proc that
the remount of /proc/sys is clearing, and that previously 

The flags specified are current rdonly,remount,bind so I expect there
are some other flags on proc that libvirt-lxc is clearing by accident
and we did not fail before because the kernel was not enforcing things.

Please see:
http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/lxc/lxc_container.c;h=9a9ae5c2aaf0f90ff472f24fda43c077b44998c7;hb=HEAD#l933
lxcContainerMountBasicFS()

and:
http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/lxc/lxc_container.c;h=9a9ae5c2aaf0f90ff472f24fda43c077b44998c7;hb=HEAD#l850
lxcBasicMounts

quoted

What are the mount flags in a working libvirt-lxc?

See:
test1:~ # cat /proc/self/mountinfo
149 147 0:56 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
150 149 0:56 /sys /proc/sys ro,nodev,relatime - proc proc rw

If you need more info, please let me know. :-)

Oh interesting I had not realized libvirt-lxc had grown an unprivileged
mode using user namespaces.

This does appear to be a classic remount bug, where you are not
preserving the permissions.  It appears the fact that the code
failed to enforce locked permissions on the fresh mount of proc
was hiding this bug until now.

I expect what you actually want is the code below:

diff --git a/src/lxc/lxc_container.c b/src/lxc/lxc_container.c
index 9a9ae5c2aaf0..f008a7484bfe 100644
--- a/src/lxc/lxc_container.c
+++ b/src/lxc/lxc_container.c

@@ -850,7 +850,7 @@ typedef struct {
 
 static const virLXCBasicMountInfo lxcBasicMounts[] = {
     { "proc", "/proc", "proc", MS_NOSUID|MS_NOEXEC|MS_NODEV, false, false, false },
-    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_RDONLY, false, false, false },
+    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, false, false, false },
     { "/.oldroot/proc/sys/net/ipv4", "/proc/sys/net/ipv4", NULL, MS_BIND, false, false, true },
     { "/.oldroot/proc/sys/net/ipv6", "/proc/sys/net/ipv6", NULL, MS_BIND, false, false, true },
     { "sysfs", "/sys", "sysfs", MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, false, false, false },

Or possibly just:

diff --git a/src/lxc/lxc_container.c b/src/lxc/lxc_container.c
index 9a9ae5c2aaf0..a60ccbd12bfc 100644
--- a/src/lxc/lxc_container.c
+++ b/src/lxc/lxc_container.c

@@ -850,7 +850,7 @@ typedef struct {
 
 static const virLXCBasicMountInfo lxcBasicMounts[] = {
     { "proc", "/proc", "proc", MS_NOSUID|MS_NOEXEC|MS_NODEV, false, false, false },
-    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_RDONLY, false, false, false },
+    { "/proc/sys", "/proc/sys", NULL, MS_BIND|MS_RDONLY, true, false, false },
     { "/.oldroot/proc/sys/net/ipv4", "/proc/sys/net/ipv4", NULL, MS_BIND, false, false, true },
     { "/.oldroot/proc/sys/net/ipv6", "/proc/sys/net/ipv6", NULL, MS_BIND, false, false, true },
     { "sysfs", "/sys", "sysfs", MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, false, false, false },

As the there is little point in making /proc/sys read-only in a
user-namespace, as the permission checks are uid based and no-one should
have the global uid 0 in your container.  Making mounting /proc/sys
read-only rather pointless.

Eric