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System call interception

LXD supports intercepting some specific system calls from unprivileged containers and if they're considered to be safe, will executed with elevated privileges on the host.

Doing so comes with a performance impact for the syscall in question and will cause some work for LXD to evaluate the request and if allowed, process it with elevated privileges.

Enabling of specific system call interception options is done on a per-container basis through container configuration options.

Available system calls

mknod / mknodat

The mknod and mknodat system calls can be used to create a variety of special files.

Most commonly inside containers, they may be called to create block or character devices. Creating such devices isn't allowed in unprivileged containers as this is a very easy way to escalate privileges by allowing direct write access to resources like disks or memory.

But there are files which are safe to create. For those, intercepting this syscall may unblock some specific workloads and allow them to run inside an unprivileged containers.

The devices which are currently allowed are:

  • overlayfs whiteout (char 0:0)
  • /dev/console (char 5:1)
  • /dev/full (char 1:7)
  • /dev/null (char 1:3)
  • /dev/random (char 1:8)
  • /dev/tty (char 5:0)
  • /dev/urandom (char 1:9)
  • /dev/zero (char 1:5)

All file types other than character devices are currently sent to the kernel as usual, so enabling this feature doesn't change their behavior at all.

This can be enabled by setting security.syscalls.intercept.mknod to true.

bpf

The bpf system call is used to manage eBPF programs in the kernel. Those can be attached to a variety of kernel subsystems.

In general, loading of eBPF programs that are not trusted can be problematic as it can facilitate timing based attacks.

LXD's eBPF support is currently restricted to programs managing devices cgroup entries. To enable it, you need to set both security.syscalls.intercept.bpf and security.syscalls.intercept.bpf.devices to true.

mount

The mount system call allows for mounting both physical and virtual file systems. By default, unprivileged containers are restricted by the kernel to just a handful of virtual and network file systems.

To allow mounting physical file systems, system call interception can be used. LXD offers a variety of options to handle this.

security.syscalls.intercept.mount is used to control the entire feature and needs to be turned on for any of the other options to work.

security.syscalls.intercept.mount.allowed allows specifying a list of file systems which can be directly mounted in the container. This is the most dangerous option as it allows the user to feed data that is not trusted at the kernel. This can easily be used to crash the host system or to attack it. It should only ever be used in trusted environments.

security.syscalls.intercept.mount.shift can be set on top of that so the resulting mount is shifted to the UID/GID map used by the container. This is needed to avoid everything showing up as nobody/nogroup inside of unprivileged containers.

The much safer alternative to those is security.syscalls.intercept.mount.fuse which can be set to pairs of file-system name and FUSE handler. When this is set, an attempt at mounting one of the configured file systems will be transparently redirected to instead calling the FUSE equivalent of that file system.

As this is all running as the caller, it avoids the entire issue around the kernel attack surface and so is generally considered to be safe, though you should keep in mind that any kind of system call interception makes for an easy way to overload the host system.

sched_setscheduler

The sched_setscheduler system call is used to manage process priority.

Granting this may allow a user to significantly increase the priority of their processes, potentially taking a lot of system resources.

It also allows access to schedulers like SCHED_FIFO which are generally considered to be flawed and can significantly impact overall system stability. This is why under normal conditions, only the real root user (or global CAP_SYS_NICE) would allow its use.

setxattr

The setxattr system call is used to set extended attributes on files.

The attributes which are handled by this currently are:

  • trusted.overlay.opaque (overlayfs directory whiteout)

Note that because the mediation must happen on a number of character strings, there is no easy way at present to only intercept the few attributes we care about. As we only allow the attributes above, this may result in breakage for other attributes that would have been previously allowed by the kernel.

This can be enabled by setting security.syscalls.intercept.setxattr to true.

sysinfo

The sysinfo system call is used by some distributions instead of /proc/ entries to report on resource usage.

In order to provide resource usage information specific to the container, rather than the whole system, this syscall interception mode uses cgroup-based resource usage information to fill in the system call response.