Re: strace of io_uring events?

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Jul 21, 2020 at 08:27:34AM -0700, Andy Lutomirski wrote:
> On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
> >
> > On Thu, Jul 16, 2020 at 08:12:35AM -0700, Kees Cook wrote:
> > > On Thu, Jul 16, 2020 at 03:14:04PM +0200, Stefano Garzarella wrote:
> 
> > > access (IIUC) is possible without actually calling any of the io_uring
> > > syscalls. Is that correct? A process would receive an fd (via SCM_RIGHTS,
> > > pidfd_getfd, or soon seccomp addfd), and then call mmap() on it to gain
> > > access to the SQ and CQ, and off it goes? (The only glitch I see is
> > > waking up the worker thread?)
> >
> > It is true only if the io_uring istance is created with SQPOLL flag (not the
> > default behaviour and it requires CAP_SYS_ADMIN). In this case the
> > kthread is created and you can also set an higher idle time for it, so
> > also the waking up syscall can be avoided.
> 
> I stared at the io_uring code for a while, and I'm wondering if we're
> approaching this the wrong way. It seems to me that most of the
> complications here come from the fact that io_uring SQEs don't clearly
> belong to any particular security principle.  (We have struct creds,
> but we don't really have a task or mm.)  But I'm also not convinced
> that io_uring actually supports cross-mm submission except by accident
> -- as it stands, unless a user is very careful to only submit SQEs
> that don't use user pointers, the results will be unpredictable.
> Perhaps we can get away with this:
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 74bc4a04befa..92266f869174 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -7660,6 +7660,20 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int,
> fd, u32, to_submit,
>      if (!percpu_ref_tryget(&ctx->refs))
>          goto out_fput;
> 
> +    if (unlikely(current->mm != ctx->sqo_mm)) {
> +        /*
> +         * The mm used to process SQEs will be current->mm or
> +         * ctx->sqo_mm depending on which submission path is used.
> +         * It's also unclear who is responsible for an SQE submitted
> +         * out-of-process from a security and auditing perspective.
> +         *
> +         * Until a real usecase emerges and there are clear semantics
> +         * for out-of-process submission, disallow it.
> +         */
> +        ret = -EACCES;
> +        goto out;
> +    }
> +
>      /*
>       * For SQ polling, the thread will do all submissions and completions.
>       * Just return the requested submit count, and wake the thread if
> 
> If we can do that, then we could bind seccomp-like io_uring filters to
> an mm, and we get obvious semantics that ought to cover most of the
> bases.
> 
> Jens, Christoph?
> 
> Stefano, what's your intended usecase for your restriction patchset?
> 

Hi Andy,
my use case concerns virtualization. The idea, that I described in the
proposal of io-uring restrictions [1], is to share io_uring CQ and SQ queues
with a guest VM for block operations.

In the PoC that I realized, there is a block device driver in the guest that
uses io_uring queues coming from the host to submit block requests.

Since the guest is not trusted, we need restrictions to allow only
a subset of syscalls on a subset of file descriptors and memory.


Cheers,
Stefano

[1] https://lore.kernel.org/io-uring/20200609142406.upuwpfmgqjeji4lc@steredhat/