Re: strace of io_uring events?

On 7/21/20 11:23 AM, Andy Lutomirski wrote:

On Tue, Jul 21, 2020 at 8:31 AM Jens Axboe [off-list ref] wrote:

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On 7/21/20 9:27 AM, Andy Lutomirski wrote:

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On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella [off-list ref] wrote:

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On Thu, Jul 16, 2020 at 08:12:35AM -0700, Kees Cook wrote:

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On Thu, Jul 16, 2020 at 03:14:04PM +0200, Stefano Garzarella wrote:

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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.

How so?

Unless I've missed something, either current->mm or sqo_mm will be
used depending on which thread ends up doing the IO.  (And there might
be similar issues with threads.)  Having the user memory references
end up somewhere that is an implementation detail seems suboptimal.

current->mm is always used from the entering task - obviously if done
synchronously, but also if it needs to go async. The only exception is a
setup with SQPOLL, in which case ctx->sqo_mm is the task that set up the
ring. SQPOLL requires root privileges to setup, and there's no task
entering the io_uring at all necessarily. It'll just submit sqes with
the credentials that are registered with the ring.

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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

That'll break postgres that already uses this, also see:

commit 73e08e711d9c1d79fae01daed4b0e1fee5f8a275
Author: Jens Axboe [off-list ref]
Date:   Sun Jan 26 09:53:12 2020 -0700

    Revert "io_uring: only allow submit from owning task"

So no, we can't do that.

Yikes, I missed that.

Andres, how final is your Postgres branch?  I'm wondering if we could
get away with requiring a special flag when creating an io_uring to
indicate that you intend to submit IO from outside the creating mm.

Even if we can't make this change, we could plausibly get away with
tying seccomp-style filtering to sqo_mm.  IOW we'd look up a
hypothetical sqo_mm->io_uring_filters to filter SQEs even when
submitted from a different mm.

This is just one known use case, there may very well be others. Outside
of SQPOLL, which is special, I don't see a reason to restrict this.
Given that you may have a fuller understanding of it after the above
explanation, please clearly state what problem you're seeing that
warrants a change.

-- 
Jens Axboe