On Wed, Jul 01, 2020 at 10:27:08AM +0200, Christian Brauner wrote: > 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. Yes, makes sense. I was thinking about this already, but I was not sure if it I should do it or not. But I had the same idea already. Adrian
