Re: [PATCH] capabilities: Introduce CAP_RESTORE

Adrian Reber
2020-05-25T08:06:10Z

On Fri, May 22, 2020 at 09:40:37AM -0700, Casey Schaufler wrote:
> On 5/21/2020 10:53 PM, Adrian Reber wrote:
> > This enables CRIU to checkpoint and restore a process as non-root.
>
> I know it sounds pedantic, but could you spell out CRIU once?
> While I know that everyone who cares either knows or can guess
> what you're talking about, it may be a mystery to some of the
> newer kernel developers.

Sure. CRIU - Checkpoint/Restore In Userspace.

> > Over the last years CRIU upstream has been asked a couple of time if it
> > is possible to checkpoint and restore a process as non-root. The answer
> > usually was: 'almost'.
> >
> > The main blocker to restore a process was that selecting the PID of the
> > restored process, which is necessary for CRIU, is guarded by CAP_SYS_ADMIN.
>
> What are the other blockers? Are you going to suggest additional new
> capabilities to clear them?

As mentioned somewhere else access to /proc/<pid>/map_files/ would be
helpful. Right now I am testing with a JVM and it works without root
just with the attached patch. Without access to /proc/<pid>/map_files/
not everything CRIU can do will actually work, but we are a lot closer
to what our users have been asking for.

> > In the last two years the questions about checkpoint/restore as non-root
> > have increased and especially in the last few months we have seen
> > multiple people inventing workarounds.
>
> Giving a process CAP_SYS_ADMIN is a non-root solution.

Yes, but like mentioned somewhere else not a solution that actually
works, because CAP_SYS_ADMIN allows too much. Especially for the
checkpoint/restore case, we really need one (setting the PID of a new
process) and to make it complete a second (reading map_files).

Reading the comments in include/uapi/linux/capability.h concerning
CAP_SYS_ADMIN it allows the binary to do at least 35 things. The two
(three) I mentioned above (ns_last_pid (clone3) map_files) are not
mentioned in that list, so CAP_SYS_ADMIN allows probably much more.

To allow checkpoint/restore as non-root nobody will give CRIU
CAP_SYS_ADMIN because it is too wide.

> > The use-cases so far and their workarounds:
> >
> > * Checkpoint/Restore in an HPC environment in combination with
> > a resource manager distributing jobs. Users are always running
> > as non root, but there was the desire to provide a way to
> > checkpoint and restore long running jobs.
> > Workaround: setuid wrapper to start CRIU as root as non-root
> > https://github.com/FredHutch/slurm-examples/blob/master/checkpointer/lib/checkpointer/checkpointer-suid.c
>
> This is a classic and well understood mechanism for dealing with
> this kind of situation. You could have checkpointer-filecap-sys_admin.c
> instead, if you want to reduce use of the super-user.
>
> > * Another use case to checkpoint/restore processes as non-root
> > uses as workaround a non privileged process which cycles through
> > PIDs by calling fork() as fast as possible with a rate of
> > 100,000 pids/s instead of writing to ns_last_pid
> > https://github.com/twosigma/set_ns_last_pid
>
> Oh dear.
>
> > * Fast Java startup using checkpoint/restore.
> > We have been in contact with JVM developers who are integrating
> > CRIU into a JVM to decrease the startup time.
> > Workaround so far: patch out CAP_SYS_ADMIN checks in the kernel
>
> That's not a workaround, it's a policy violation.
> Bad JVM! No biscuit!

This was used as a proof of concept to see if we can checkpoint and
restore a JVM without root. Only the ns_last_pid check was removed to
see if it works and it does.

> > * Container migration as non root. There are people already
> > using CRIU to migrate containers as non-root. The solution
> > there is to run it in a user namespace. So if you are able
> > to carefully setup your environment with the namespaces
> > it is already possible to restore a container/process as non-root.
>
> This is exactly the kind of situation that user namespaces are
> supposed to address.
>
> > Unfortunately it is not always possible to setup an environment
> > in such a way and for easier access to non-root based container
> > migration this patch is also required.
>
> If a user namespace solution is impossible or (more likely) too
> expensive, there's always the checkpointer-filecap-sys_admin option.

But then again we open up all of CAP_SYS_ADMIN, which is not necessary.

> > There are probably a few more things guarded by CAP_SYS_ADMIN required
> > to run checkpoint/restore as non-root,
>
> If you need CAP_SYS_ADMIN anyway you're not gaining anything by
> separating out CAP_RESTORE.

No, as described we can checkpoint and restore a JVM with this patch and
it also solves the problem the set_ns_last_pid fork() loop daemon tries
to solve. It is not enough to support the full functionality of CRIU as
map_files is also important, but we do not need CAP_SYS_ADMIN and
CAP_RESTORE. Only CAP_RESTORE would be necessary.

With a new capability users can enable checkpoint/restore as non-root
without giving CRIU access to any of the other possibilities offered by
CAP_SYS_ADMIN. Setting a PID and map_files have been introduced for CRIU
and used to live behind CONFIG_CHECKPOINT_RESTORE. Having a capability
for checkpoint/restore would make it easier for CRIU users to run it as
non-root and make it very clear what is possible when giving CRIU the
new capability. No other things would be allowed than necessary for
checkpoint/restore. Setting a PID is most important for the restore part
and reading map_files would be helpful during checkpoint. So it actually
should be called CAP_CHECKPOINT_RESTORE as Christian mentioned in
another email.

> > but by applying this patch I can
> > already checkpoint and restore processes as non-root. As there are
> > already multiple workarounds I would prefer to do it correctly in the
> > kernel to avoid that CRIU users are starting to invent more workarounds.
>
> You've presented a couple of really inappropriate implementations
> that would qualify as workarounds. But the other two are completely
> appropriate within the system security policy. They don't "get around"
> the problem, they use existing mechanisms as they are intended.

I agree with the user namespace approach to be appropriate, but not the
CAP_SYS_ADMIN approach as CRIU only needs a tiny subset (2 things) of
what CAP_SYS_ADMIN allows.

> > I have used the following tests to verify that this change works as
> > expected by setting the new capability CAP_RESTORE on the two resulting
> > test binaries:
> >
> > $ cat ns_last_pid.c
> > // http://efiop-notes.blogspot.com/2014/06/how-to-set-pid-using-nslastpid.html
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <string.h>
> > #include <sys/file.h>
> > #include <sys/types.h>
> > #include <unistd.h>
> >
> > int main(int argc, char *argv[])
> > {
> > pid_t pid, new_pid;
> > char buf[32];
> > int fd;
> >
> > if (argc != 2)
> > return 1;
> >
> > printf("Opening ns_last_pid...\n");
> > fd = open("/proc/sys/kernel/ns_last_pid", O_RDWR | O_CREAT, 0644);
> > if (fd < 0) {
> > perror("Cannot open ns_last_pid");
> > return 1;
> > }
> >
> > printf("Locking ns_last_pid...\n");
> > if (flock(fd, LOCK_EX)) {
> > close(fd);
> > printf("Cannot lock ns_last_pid\n");
> > return 1;
> > }
> >
> > pid = atoi(argv[1]);
> > snprintf(buf, sizeof(buf), "%d", pid - 1);
> > printf("Writing pid-1 to ns_last_pid...\n");
> > if (write(fd, buf, strlen(buf)) != strlen(buf)) {
> > printf("Cannot write to buf\n");
> > return 1;
> > }
> >
> > printf("Forking...\n");
> > new_pid = fork();
> > if (new_pid == 0) {
> > printf("I am the child!\n");
> > exit(0);
> > } else if (new_pid == pid)
> > printf("I am the parent. My child got the pid %d!\n", new_pid);
> > else
> > printf("pid (%d) does not match expected pid (%d)\n", new_pid, pid);
> >
> > printf("Cleaning up...\n");
> > if (flock(fd, LOCK_UN))
> > printf("Cannot unlock\n");
> > close(fd);
> > return 0;
> > }
> > $ id -u; /home/libcap/ns_last_pid 300000
> > 1001
> > Opening ns_last_pid...
> > Locking ns_last_pid...
> > Writing pid-1 to ns_last_pid...
> > Forking...
> > I am the parent. My child got the pid 300000!
> > I am the child!
> > Cleaning up...
> >
> > For the clone3() based approach:
> > $ cat clone3_set_tid.c
> > #define _GNU_SOURCE
> > #include <linux/sched.h>
> > #include <stdint.h>
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <string.h>
> > #include <sys/types.h>
> > #include <sys/stat.h>
> > #include <sys/syscall.h>
> > #include <unistd.h>
> >
> > #define ptr_to_u64(ptr) ((__u64)((uintptr_t)(ptr)))
> >
> > int main(int argc, char *argv[])
> > {
> > struct clone_args c_args = { };
> > pid_t pid, new_pid;
> >
> > if (argc != 2)
> > return 1;
> >
> > pid = atoi(argv[1]);
> > c_args.set_tid = ptr_to_u64(&pid);
> > c_args.set_tid_size = 1;
> >
> > printf("Forking...\n");
> > new_pid = syscall(__NR_clone3, &c_args, sizeof(c_args));
> > if (new_pid == 0) {
> > printf("I am the child!\n");
> > exit(0);
> > } else if (new_pid == pid)
> > printf("I am the parent. My child got the pid %d!\n", new_pid);
> > else
> > printf("pid (%d) does not match expected pid (%d)\n", new_pid, pid);
> > printf("Done\n");
> >
> > return 0;
> > }
> > $ id -u; /home/libcap/clone3_set_tid 300000
> > 1001
> > Forking...
> > I am the parent. My child got the pid 300000!
> > Done
> > I am the child!
> >
> > Signed-off-by: Adrian Reber <areber@redhat.com>
> > ---
> > include/linux/capability.h | 5 +++++
> > include/uapi/linux/capability.h | 9 ++++++++-
> > kernel/pid.c | 2 +-
> > kernel/pid_namespace.c | 2 +-
> > security/selinux/include/classmap.h | 5 +++--
> > 5 files changed, 18 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/capability.h b/include/linux/capability.h
> > index b4345b38a6be..1278313cb2bc 100644
> > --- a/include/linux/capability.h
> > +++ b/include/linux/capability.h
> > @@ -261,6 +261,11 @@ static inline bool bpf_capable(void)
> > return capable(CAP_BPF) || capable(CAP_SYS_ADMIN);
> > }
> >
> > +static inline bool restore_ns_capable(struct user_namespace *ns)
> > +{
> > + return ns_capable(ns, CAP_RESTORE) || ns_capable(ns, CAP_SYS_ADMIN);
> > +}
> > +
> > /* audit system wants to get cap info from files as well */
> > extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
> >
> > diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> > index c7372180a0a9..4bcc4e3d41ff 100644
> > --- a/include/uapi/linux/capability.h
> > +++ b/include/uapi/linux/capability.h
> > @@ -406,7 +406,14 @@ struct vfs_ns_cap_data {
> > */
> > #define CAP_BPF 39
> >
> > -#define CAP_LAST_CAP CAP_BPF
> > +
> > +/* Allow checkpoint/restore related operations */
> > +/* Allow PID selection during clone3() */
> > +/* Allow writing to ns_last_pid */
> > +
> > +#define CAP_RESTORE 40
> > +
> > +#define CAP_LAST_CAP CAP_RESTORE
> >
> > #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
> >
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 3122043fe364..bbc26f2bcff6 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -198,7 +198,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
> > if (tid != 1 && !tmp->child_reaper)
> > goto out_free;
> > retval = -EPERM;
> > - if (!ns_capable(tmp->user_ns, CAP_SYS_ADMIN))
> > + if (!restore_ns_capable(tmp->user_ns))
> > goto out_free;
> > set_tid_size--;
> > }
> > diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> > index 0e5ac162c3a8..f58186b31ce6 100644
> > --- a/kernel/pid_namespace.c
> > +++ b/kernel/pid_namespace.c
> > @@ -269,7 +269,7 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
> > struct ctl_table tmp = *table;
> > int ret, next;
> >
> > - if (write && !ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
> > + if (write && !restore_ns_capable(pid_ns->user_ns))
> > return -EPERM;
> >
> > /*
> > diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> > index 98e1513b608a..f8b8f12a6ebd 100644
> > --- a/security/selinux/include/classmap.h
> > +++ b/security/selinux/include/classmap.h
> > @@ -27,9 +27,10 @@
> > "audit_control", "setfcap"
> >
> > #define COMMON_CAP2_PERMS "mac_override", "mac_admin", "syslog", \
> > - "wake_alarm", "block_suspend", "audit_read", "perfmon", "bpf"
> > + "wake_alarm", "block_suspend", "audit_read", "perfmon", "bpf", \
> > + "restore"
> >
> > -#if CAP_LAST_CAP > CAP_BPF
> > +#if CAP_LAST_CAP > CAP_RESTORE
> > #error New capability defined, please update COMMON_CAP2_PERMS.
> > #endif
> >
> >
> > base-commit: e8f3274774b45b5f4e9e3d5cad7ff9f43ae3add5
>