io_uring/timeout: immediate timeout arg

io_uring/timeout: immediate timeout arg

One the things the user has always keep in mind is that any user
pointers they put into an SQE is not going to be read by the kernel
until submission happens, and the user has to ensure the pointee stays
alive until then. For example, snippet below will lead to UAF of the on
stack variable ts. Instead of passing the timeout value as a pointer
allow to store it immediately in the SQE. The user has to set a new flag
called IORING_TIMEOUT_IMMEDIATE_ARG, in which case sqe->addr for timeout
or sqe->addr2 for timeout update requests will be interpreted as a time
value in nanosecods.

void prep_timeout(struct io_uring_sqe *sqe) {
    struct __kernel_timespec ts = {...};
    prep_timeout(sqe, &ts);
}

void submit() {
    sqe = get_sqe();
    prep_timeout(sqe);
    io_uring_submit();
}

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>