Re: [PATCH] capabilities: Introduce CAP_RESTORE

Christian Brauner
2020-05-22T13:41:56Z

On Fri, May 22, 2020 at 07:53:50AM +0200, Adrian Reber wrote:
> This enables CRIU to checkpoint and restore a process as non-root.
>
> 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.
>
> 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.
>
> 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
> * 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
> * 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
> * 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.
> 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.
>
> There are probably a few more things guarded by CAP_SYS_ADMIN required
> to run checkpoint/restore as non-root, 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.
>
> 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);
> +}
> +

When did everyone start putting their favorite little helpers in here?
I see both bpf_capable() and perform_capable() are going to be in
linux/capability.h if linux-next is to be believed. Seems they could
very well go in their own bpf headers.
Anyway, if everyone is fine with putting specific helpers in
capability.h then ok.

> /* 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

Thinking about what Adrian said this should probably really be:

CAP_CHECKPOINT_RESTORE

I initially thought that might be too long but our config option is the
same name CONFIG_CHECKPOINT_RESTORE so seems ok and other capabilities
have similar long names (CAP_DAC_READ_SEARCH, CAP_NET_BIND_SERVICE come
to mind)

> +
> +#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
> --
> 2.26.2
>