[libvirt] [PATCH 4/3] Control LXC capabilities
Serge E. Hallyn
serue at us.ibm.com
Tue Jun 23 13:52:54 UTC 2009
Quoting Daniel P. Berrange (berrange at redhat.com):
> This patch updates the LXC driver to make use of libcap-ng for managing
> process capabilities. Previously Ryota Ozaki had provided code to remove
> the CAP_BOOT capabilities inside the container, preventing host reboots.
> In addition to that one, I believe we should be removing ability to
> load kernel modules, change the system clock and changing audit/MAC.
> So this patch also clears the following:
>
> CAP_SYS_MODULE, /* No kernel module loading */
> CAP_SYS_TIME, /* No changing the clock */
> CAP_AUDIT_CONTROL, /* No messing with auditing */
> CAP_AUDIT_WRITE, /* No messing with auditing */
> CAP_MAC_ADMIN, /* No messing with LSM */
> CAP_MAC_OVERRIDE, /* No messing with LSM */
Thanks, Daniel, this does look good. I wonder whether there is a more
appropriate list to email caps-related patches (including libcap-ng
itself) to. Not only does the code itself warrant discussion (for
instance, should capng_lock() also set CAP_NOSUID_FIXUP?), but such
discussions, in one place, about converting several programs to dropping
capabilities would help others to do the same with this code.
I can't think of anything other than the LSM list, so I'm cc:ing it
here.
> We use libcap-ng's capng_updatev/apply functions to remove these from
> the permitted, inheritable, effective and bounding sets. Then we use
> capng_lock to set NOROOT and NOROOT_LOCKED in the process securebits
> to prevent them ever being re-acquired.
>
> The other thing I realized is that the 'libvirt_lxc' controller process
> does not need to keep any capabilities at all once it has spawned the
> container process, since all its doing is forwarding I/O between 2 open
> file descripts. So I also clear all capabilities from that. We should
> probably make it chuid/gid to a non-root user in future too.
>
> lxc_container.c | 66 +++++++++++++++++++++++++++++++++++++------------------
> lxc_controller.c | 28 +++++++++++++++++++++++
> 2 files changed, 73 insertions(+), 21 deletions(-)
>
>
> Regards,
> Daniel
>
> diff -r 7e766489c4a2 src/lxc_container.c
> --- a/src/lxc_container.c Tue Jun 23 11:13:45 2009 +0100
> +++ b/src/lxc_container.c Tue Jun 23 11:54:10 2009 +0100
> @@ -41,8 +41,9 @@
> /* For MS_MOVE */
> #include <linux/fs.h>
>
> -#include <sys/prctl.h>
> -#include <linux/capability.h>
> +#if HAVE_CAPNG
> +#include <cap-ng.h>
> +#endif
>
> #include "virterror_internal.h"
> #include "logging.h"
> @@ -642,27 +643,50 @@ static int lxcContainerSetupMounts(virDo
> return lxcContainerSetupExtraMounts(vmDef);
> }
>
> -static int lxcContainerDropCapabilities(virDomainDefPtr vmDef ATTRIBUTE_UNUSED)
> +
> +/*
> + * This is running as the 'init' process insid the container.
> + * It removes some capabilities that could be dangerous to
> + * host system, since they are not currently "containerized"
> + */
> +static int lxcContainerDropCapabilities(void)
> {
> -#ifdef PR_CAPBSET_DROP
> - int i;
> - const struct {
> - int id;
> - const char *name;
> - } caps[] = {
> -#define ID_STRING(name) name, #name
> - { ID_STRING(CAP_SYS_BOOT) },
> - };
> +#if HAVE_CAPNG
> + int ret;
>
> - for (i = 0 ; i < ARRAY_CARDINALITY(caps) ; i++) {
> - if (prctl(PR_CAPBSET_DROP, caps[i].id, 0, 0, 0)) {
> - lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
> - _("failed to drop %s"), caps[i].name);
> - return -1;
> - }
> + capng_get_caps_process();
> +
> + if ((ret = capng_updatev(CAPNG_DROP,
> + CAPNG_EFFECTIVE | CAPNG_PERMITTED |
> + CAPNG_INHERITABLE | CAPNG_BOUNDING_SET,
> + CAP_SYS_BOOT, /* No use of reboot */
> + CAP_SYS_MODULE, /* No kernel module loading */
> + CAP_SYS_TIME, /* No changing the clock */
> + CAP_AUDIT_CONTROL, /* No messing with auditing */
> + CAP_AUDIT_WRITE, /* No messing with auditing */
> + CAP_MAC_ADMIN, /* No messing with LSM */
> + CAP_MAC_OVERRIDE, /* No messing with LSM */
> + -1 /* sentinal */)) < 0) {
> + lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
> + _("failed to remove capabilities %d"), ret);
> + return -1;
> }
> -#else /* ! PR_CAPBSET_DROP */
> - VIR_WARN0(_("failed to drop capabilities PR_CAPBSET_DROP undefined"));
> +
> + if ((ret = capng_apply(CAPNG_SELECT_BOTH)) < 0) {
> + lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
> + _("failed to apply capabilities: %d"), ret);
> + return -1;
> + }
The only problem offhand with this idiom is that you need CAP_SETPCAP to
set securebits and drop caps from bounding set, but I think a lot of
applications could stand to drop CAP_SETPCAP otherwise. So I don't know
if it would help to do the capng_lock() before capng_apply().
(To be clear, not bc you need to do so right here, but because others
may well look at your code as example code.)
> + /* Need to prevent them regaining any caps on exec */
> + if ((ret = capng_lock()) < 0) {
> + lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
> + _("failed to lock capabilities: %d"), ret);
> + return -1;
> + }
> +
> +#else
> + VIR_WARN0(_("libcap-ng support not compiled in, unable to clear capabilities"));
> #endif
> return 0;
> }
> @@ -735,7 +759,7 @@ static int lxcContainerChild( void *data
> return -1;
>
> /* drop a set of root capabilities */
> - if (lxcContainerDropCapabilities(vmDef) < 0)
> + if (lxcContainerDropCapabilities() < 0)
> return -1;
>
> /* this function will only return if an error occured */
> diff -r 7e766489c4a2 src/lxc_controller.c
> --- a/src/lxc_controller.c Tue Jun 23 11:13:45 2009 +0100
> +++ b/src/lxc_controller.c Tue Jun 23 11:54:10 2009 +0100
> @@ -35,6 +35,10 @@
> #include <getopt.h>
> #include <sys/mount.h>
>
> +#if HAVE_CAPNG
> +#include <cap-ng.h>
> +#endif
> +
> #include "virterror_internal.h"
> #include "logging.h"
> #include "util.h"
> @@ -210,6 +214,25 @@ cleanup:
> return rc;
> }
>
> +
> +static int lxcControllerClearCapabilities(void)
> +{
> +#if HAVE_CAPNG
> + int ret;
> +
> + capng_clear(CAPNG_SELECT_BOTH);
> +
> + if ((ret = capng_apply(CAPNG_SELECT_BOTH)) < 0) {
> + lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
> + _("failed to apply capabilities: %d"), ret);
> + return -1;
> + }
> +#else
> + VIR_WARN0(_("libcap-ng support not compiled in, unable to clear capabilities"));
> +#endif
> + return 0;
> +}
> +
> typedef struct _lxcTtyForwardFd_t {
> int fd;
> int active;
> @@ -562,6 +585,11 @@ lxcControllerRun(virDomainDefPtr def,
> if (lxcContainerSendContinue(control[0]) < 0)
> goto cleanup;
>
> + /* Now the container is running, there's no need for us to keep
> + any elevated capabilities */
> + if (lxcControllerClearCapabilities() < 0)
> + goto cleanup;
> +
> rc = lxcControllerMain(monitor, client, appPty, containerPty);
>
> cleanup:
>
>
> --
> |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :|
> |: http://libvirt.org -o- http://virt-manager.org -o- http://ovirt.org :|
> |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :|
> |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|
>
> --
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