seccomp: Support atomic "addfd and send"

[rata]

Here it is a c&p of the commit msg. Github doesn't show the "1" and friends correctly, but that is common use on the linux commit msg IIRC.

Alban Crequy reported a race condition userspace faces when we want to
add an fd and make the same syscall return that value1.

Currently two different ioctl() calls are needed by the process handling
the syscalls (agent) for another userspace process (target). Therefore,
it is possible for the agent to do the first ioctl to add a file
descriptor but the target is interrupted (EINTR) before the agent does
the second ioctl() call.

This patch adds a flag to the ADDFD ioctl() so it adds the fd and
returns that value atomically to the target program. This was suggested
by Kees Cook2. This is done by simply changing
seccomp_do_user_notification() to add the fd and return that. In this
way, the target is waked up from the wait in
seccomp_do_user_notification() either to interrupt the syscall or to add
the fd and return it.

The struct seccomp_notif_resp, used when doing SECCOMP_IOCTL_NOTIF_SEND
ioctl() to send a response to the target, has three more fields that we
don't allow to set when doing the addfd ioctl(). The reasons for each
are:

• val: This will be set to the new allocated fd
• error: If this is non-zero, the value is ignored. Therefore,
  it is pointless in this case as we want to return the value.
• flags: The only flag is to let userspace continue to execute the
  syscall. This seems pointless, as we want the syscall to return the
  allocated fd.

Signed-off-by: Rodrigo Campos rodrigo@sdfg.com.ar

How to use

See the selftest for an example

Testing done

Selftests and a similar local test to read from the fd and check it is the expected data.

Missing stuff:

• Test it with Alban golang examples that hit the race condition
• Rebase to send for 5.11/5.12 (this is currently based on linux 5.10-rc5, this is of course not good)
• Remove printk. Left them in case you test it, they were useful for me
• Add selftests
• Should I use my personal or kinvolk email?

[rata]

Github is crazy detecting so many changes, will re-open to see if that fixes it