On 7/21/20 9:27 AM, Andy Lutomirski wrote:
> On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella <sgarzare@xxxxxxxxxx> 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.
How so?
> 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 <axboe@xxxxxxxxx>
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.
--
Jens Axboe
