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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Sat, 19 Sep 2020 23:16:10 +0200
Message-ID: <CAK8P3a2Mi+1yttyGk4k7HxRVrMtmFqJewouVhynqUL0PJycmog@mail.gmail.com>
Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
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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?

       Arnd