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.

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.

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.