On Mon, May 25, 2020 at 11:55:20AM -0700, Casey Schaufler wrote:

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

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On Fri, May 22, 2020 at 09:40:37AM -0700, Casey Schaufler wrote:

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On 5/21/2020 10:53 PM, Adrian Reber wrote:

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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.

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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?

If I understand part of CRIU correctly, then we only need read-access
for the current user. I am sure Andrei, Pavel or Cyrill will correct me
if I am wrong concerning map_files.

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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 ...

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 because CAP_SYS_ADMIN allows too much.

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

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 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.

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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.

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* 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.

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 * 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.

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 * 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.

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   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.

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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?

When I introduced clone3() with sett_tid to create process with a
certain PID I first did it without the CAP_SYS_ADMIN check and there
were concerns that this would make it too easy to re-use PIDs. If I
understood it correctly. So if you are only asking about the damage that
could be done to the system if any user can create a process with any
PID via ns_last_pid or clone3(), then the damage could be easy re-use of
PIDs and probably easier re-creation of re-use PID problems.

I cannot comment on what damage could be done by allowing read-access to
map_files. Eric commented in another part of the thread that he thinks
that it might not be necessary at all.

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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.

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 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.

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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 <redacted>
---
 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