On Wed, Jul 01, 2020 at 08:49:04AM +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> > --- I think that now looks reasonable. A few comments. Before we proceed, please split the addition of checkpoint_restore_ns_capable() out into a separate patch. In fact, I think the cleanest way of doing this would be: - 0/n capability: add CAP_CHECKPOINT_RESTORE - 1/n pid: use checkpoint_restore_ns_capable() for set_tid - 2/n pid_namespace: use checkpoint_restore_ns_capable() for ns_last_pid - 3/n: proc: require checkpoint_restore_ns_capable() in init userns for map_files (commit subjects up to you of course) and a nice commit message for each time we relax a permissions on something so we have a clear separate track record for each change in case we need to revert something. Then the rest of the patches in this series. Testing patches probably last. > fs/proc/base.c | 8 ++++---- > include/linux/capability.h | 6 ++++++ > include/uapi/linux/capability.h | 9 ++++++++- > kernel/pid.c | 2 +- > kernel/pid_namespace.c | 2 +- > security/selinux/include/classmap.h | 5 +++-- > 6 files changed, 23 insertions(+), 9 deletions(-) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index d86c0afc8a85..ad806069c778 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -2189,16 +2189,16 @@ struct map_files_info { > }; > > /* > - * Only allow CAP_SYS_ADMIN to follow the links, due to concerns about how the > - * symlinks may be used to bypass permissions on ancestor directories in the > - * path to the file in question. > + * Only allow CAP_SYS_ADMIN and CAP_CHECKPOINT_RESTORE to follow the links, due > + * to concerns about how the symlinks may be used to bypass permissions on > + * ancestor directories in the path to the file in question. > */ > static const char * > proc_map_files_get_link(struct dentry *dentry, > struct inode *inode, > struct delayed_call *done) > { > - if (!capable(CAP_SYS_ADMIN)) > + if (!capable(CAP_SYS_ADMIN) && !capable(CAP_CHECKPOINT_RESTORE)) > return ERR_PTR(-EPERM); I think it's clearer if you just use: checkpoint_restore_ns_capable(&init_user_ns) > +static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns) > > return proc_pid_get_link(dentry, inode, done); > 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/kernel/pid.c b/kernel/pid.c > index 5799ae54b89e..2d0a97b7ed7a 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 (!checkpoint_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..ac135bd600eb 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 && !checkpoint_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..40cebde62856 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 >
