Re: [PATCH] capabilities: Introduce CAP_RESTORE

Casey Schaufler
2020-05-25T18:55:29Z

On 5/25/2020 1:05 AM, Adrian Reber wrote:
> 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.

Thanks. I blew out my acronym processor in the 1990's while
working on trusted Unix system security evaluations.

>>> 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.

Are you talking about read access to map_files owned by other users
or write access to map_files for the current user?

>>> 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,

It's a solution that will execute and do what you're asking of it ...

> because CAP_SYS_ADMIN allows too much.

... but apparently not one that your users find satisfactory.

> 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.

CAP_SYS_ADMIN exists for system behaviors that are not policy enforcement,
but important to the system nonetheless. If you argue that checkpoint/restart
is system policy enforcement rather then an administrative task it would
be easier to sell.

Nobody likes CAP_SYS_ADMIN, but usually a process that does one of the
things it covers will do more (sometimes many more) of the things it
covers. The longstanding problem with breaking up CAP_SYS_ADMIN is that
most breakouts result in programs that still need CAP_SYS_ADMIN anyway.

>>> 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.

Right, I understand that.

>>> 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.

Excellent!

Now, is there any reason other than your program that a process would
use CAP_RESTORE? If a process has this capability what damage could it
do to the system?

>
> 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