linux-kernel - Re: [PATCH 1/9] kernel: add a PF_FORCE

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Date:   Sat, 19 Sep 2020 23:16:10 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        David Howells <dhowells@...hat.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Linux SCSI List <linux-scsi@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-aio <linux-aio@...ck.org>, io-uring@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Network Development <netdev@...r.kernel.org>,
        keyrings@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski <luto@...nel.org> wrote:
> On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig <hch@....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