linux-kernel - Re: strace of io

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Date:   Tue, 21 Jul 2020 08:27:34 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Stefano Garzarella <sgarzare@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>
Cc:     Kees Cook <keescook@...omium.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        strace-devel@...ts.strace.io, io-uring@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Stefan Hajnoczi <stefanha@...hat.com>
Subject: Re: strace of io_uring events?

On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella <sgarzare@...hat.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?