seccomp: Support atomic "addfd + send reply"

Alban Crequy reported a race condition userspace faces when we want to add some fds and make the syscall return them[1] using seccomp notify. The problem is that currently two different ioctl() calls are needed by the process handling the syscalls (agent) for another userspace process (target): SECCOMP_IOCTL_NOTIF_ADDFD to allocate the fd and SECCOMP_IOCTL_NOTIF_SEND to return that value. 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, as suggested by Kees Cook[2]. This is done by simply allowing seccomp_do_user_notification() to add the fd and return it in this case. Therefore, in this case the target wakes up from the wait in seccomp_do_user_notification() either to interrupt the syscall or to add the fd and return it. This "allocate an fd and return" functionality is useful for syscalls that return a file descriptor only, like connect(2). Other syscalls that return a file descriptor but not as return value (or return more than one fd), like socketpair(), pipe(), recvmsg with SCM_RIGHTs, will not work with this flag. This effectively combines SECCOMP_IOCTL_NOTIF_ADDFD and SECCOMP_IOCTL_NOTIF_SEND into an atomic opteration. The notification's return value, nor error can be set by the user. Upon successful invocation of the SECCOMP_IOCTL_NOTIF_ADDFD ioctl with the SECCOMP_ADDFD_FLAG_SEND flag, the notifying process's errno will be 0, and the return value will be the file descriptor number that was installed. [1]: https://lore.kernel.org/lkml/CADZs7q4sw71iNHmV8EOOXhUKJMORPzF7thraxZYddTZsxta-KQ@mail.gmail.com/ [2]: https://lore.kernel.org/lkml/202012011322.26DCBC64F2@keescook/

RCRodrigo Campos committed 5 years ago