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@xxxxxxxxxx> >>> --- >>> 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
