On Wed, Jul 15, 2020 at 04:49:49PM +0200, Adrian Reber wrote: > This patch introduces CAP_CHECKPOINT_RESTORE, a new capability facilitating > checkpoint/restore for non-root users. > > Over the last years, The CRIU (Checkpoint/Restore In Userspace) team has been > asked numerous times if it is possible to checkpoint/restore a process as > non-root. The answer usually was: 'almost'. > > The main blocker to restore a process as non-root was to control the PID of the > restored process. This feature available via the clone3 system call, or via > /proc/sys/kernel/ns_last_pid is unfortunately guarded by CAP_SYS_ADMIN. > > In the past two years, requests for non-root checkpoint/restore have increased > due to the following use cases: > * Checkpoint/Restore in an HPC environment in combination with a resource > manager distributing jobs where users are always running as non-root. > There is a desire to provide a way to checkpoint and restore long running > jobs. > * Container migration as non-root > * We have been in contact with JVM developers who are integrating > CRIU into a Java VM to decrease the startup time. These checkpoint/restore > applications are not meant to be running with CAP_SYS_ADMIN. > > We have seen the following workarounds: > * Use a setuid wrapper around CRIU: > See https://github.com/FredHutch/slurm-examples/blob/master/checkpointer/lib/checkpointer/checkpointer-suid.c > * Use a setuid helper that writes to ns_last_pid. > Unfortunately, this helper delegation technique is impossible to use with > clone3, and is thus prone to races. > See https://github.com/twosigma/set_ns_last_pid > * Cycle through PIDs with fork() until the desired PID is reached: > This has been demonstrated to work with cycling rates of 100,000 PIDs/s > See https://github.com/twosigma/set_ns_last_pid > * Patch out the CAP_SYS_ADMIN check from the kernel > * Run the desired application in a new user and PID namespace to provide > a local CAP_SYS_ADMIN for controlling PIDs. This technique has limited use in > typical container environments (e.g., Kubernetes) as /proc is > typically protected with read-only layers (e.g., /proc/sys) for hardening > purposes. Read-only layers prevent additional /proc mounts (due to proc's > SB_I_USERNS_VISIBLE property), making the use of new PID namespaces limited as > certain applications need access to /proc matching their PID namespace. > > The introduced capability allows to: > * Control PIDs when the current user is CAP_CHECKPOINT_RESTORE capable > for the corresponding PID namespace via ns_last_pid/clone3. > * Open files in /proc/pid/map_files when the current user is > CAP_CHECKPOINT_RESTORE capable in the root namespace, useful for recovering > files that are unreachable via the file system such as deleted files, or memfd > files. > > See corresponding selftest for an example with clone3(). > > Signed-off-by: Adrian Reber <areber@xxxxxxxxxx> > Signed-off-by: Nicolas Viennot <Nicolas.Viennot@xxxxxxxxxxxx> > --- Thanks! This looks good now. Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx> > include/linux/capability.h | 6 ++++++ > include/uapi/linux/capability.h | 9 ++++++++- > security/selinux/include/classmap.h | 5 +++-- > 3 files changed, 17 insertions(+), 3 deletions(-) > > diff --git a/include/linux/capability.h b/include/linux/capability.h > index b4345b38a6be..1e7fe311cabe 100644 > --- a/include/linux/capability.h > +++ b/include/linux/capability.h > @@ -261,6 +261,12 @@ static inline bool bpf_capable(void) > return capable(CAP_BPF) || capable(CAP_SYS_ADMIN); > } > > +static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns) > +{ > + return ns_capable(ns, CAP_CHECKPOINT_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 48ff0757ae5e..395dd0df8d08 100644 > --- a/include/uapi/linux/capability.h > +++ b/include/uapi/linux/capability.h > @@ -408,7 +408,14 @@ 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_CHECKPOINT_RESTORE 40 > + > +#define CAP_LAST_CAP CAP_CHECKPOINT_RESTORE > > #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP) > > diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h > index e54d62d529f1..ba2e01a6955c 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", \ > + "checkpoint_restore" > > -#if CAP_LAST_CAP > CAP_BPF > +#if CAP_LAST_CAP > CAP_CHECKPOINT_RESTORE > #error New capability defined, please update COMMON_CAP2_PERMS. > #endif > > -- > 2.26.2 >
