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 <are...@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 -- 2.26.2
