Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

[Andy Lutomirski <luto@kernel.org>]

On Mon, Sep 21, 2020 at 5:24 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
>
>
> On 22/09/2020 02:51, Andy Lutomirski wrote:
> > On Mon, Sep 21, 2020 at 9:15 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
> >>
> >> On 21/09/2020 19:10, Pavel Begunkov wrote:
> >>> On 20/09/2020 01:22, Andy Lutomirski wrote:
> >>>>
> >>>>> On Sep 19, 2020, at 2:16 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> >>>>>
> >>>>> ´╗┐On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski <luto@kernel.org> wrote:
> >>>>>>> On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig <hch@lst.de> wrote:
> >>>>>>> On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote:
> >>>>>>>> Said that, why not provide a variant that would take an explicit
> >>>>>>>> "is it compat" argument and use it there?  And have the normal
> >>>>>>>> one pass in_compat_syscall() to that...
> >>>>>>>
> >>>>>>> That would help to not introduce a regression with this series yes.
> >>>>>>> But it wouldn't fix existing bugs when io_uring is used to access
> >>>>>>> read or write methods that use in_compat_syscall().  One example that
> >>>>>>> I recently ran into is drivers/scsi/sg.c.
> >>>>>
> >>>>> Ah, so reading /dev/input/event* would suffer from the same issue,
> >>>>> and that one would in fact be broken by your patch in the hypothetical
> >>>>> case that someone tried to use io_uring to read /dev/input/event on x32...
> >>>>>
> >>>>> For reference, I checked the socket timestamp handling that has a
> >>>>> number of corner cases with time32/time64 formats in compat mode,
> >>>>> but none of those appear to be affected by the problem.
> >>>>>
> >>>>>> Aside from the potentially nasty use of per-task variables, one thing
> >>>>>> I don't like about PF_FORCE_COMPAT is that it's one-way.  If we're
> >>>>>> going to have a generic mechanism for this, shouldn't we allow a full
> >>>>>> override of the syscall arch instead of just allowing forcing compat
> >>>>>> so that a compat syscall can do a non-compat operation?
> >>>>>
> >>>>> The only reason it's needed here is that the caller is in a kernel
> >>>>> thread rather than a system call. Are there any possible scenarios
> >>>>> where one would actually need the opposite?
> >>>>>
> >>>>
> >>>> I can certainly imagine needing to force x32 mode from a kernel thread.
> >>>>
> >>>> As for the other direction: what exactly are the desired bitness/arch semantics of io_uring?  Is the operation bitness chosen by the io_uring creation or by the io_uring_enter() bitness?
> >>>
> >>> It's rather the second one. Even though AFAIR it wasn't discussed
> >>> specifically, that how it works now (_partially_).
> >>
> >> Double checked -- I'm wrong, that's the former one. Most of it is based
> >> on a flag that was set an creation.
> >>
> >
> > Could we get away with making io_uring_enter() return -EINVAL (or
> > maybe -ENOTTY?) if you try to do it with bitness that doesn't match
> > the io_uring?  And disable SQPOLL in compat mode?
>
> Something like below. If PF_FORCE_COMPAT or any other solution
> doesn't lend by the time, I'll take a look whether other io_uring's
> syscalls need similar checks, etc.
>
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 0458f02d4ca8..aab20785fa9a 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -8671,6 +8671,10 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
>         if (ctx->flags & IORING_SETUP_R_DISABLED)
>                 goto out;
>
> +       ret = -EINVAl;
> +       if (ctx->compat != in_compat_syscall())
> +               goto out;
> +

This seems entirely reasonable to me.  Sharing an io_uring ring
between programs with different ABIs seems a bit nutty.

>         /*
>          * For SQ polling, the thread will do all submissions and completions.
>          * Just return the requested submit count, and wake the thread if
> @@ -9006,6 +9010,10 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
>         if (ret)
>                 goto err;
>
> +       ret = -EINVAL;
> +       if (ctx->compat)
> +               goto err;
> +

I may be looking at a different kernel than you, but aren't you
preventing creating an io_uring regardless of whether SQPOLL is
requested?

>         /* Only gets the ring fd, doesn't install it in the file table */
>         fd = io_uring_get_fd(ctx, &file);
>         if (fd < 0) {
> --
> Pavel Begunkov