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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag
Date:   Sat, 19 Sep 2020 15:22:04 -0700
Message-Id: <D0791499-1190-4C3F-A984-0A313ECA81C7@amacapital.net>
References: <CAK8P3a2Mi+1yttyGk4k7HxRVrMtmFqJewouVhynqUL0PJycmog@mail.gmail.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Christoph Hellwig <hch@lst.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        David Howells <dhowells@redhat.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        Parisc List <linux-parisc@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        sparclinux <sparclinux@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        Linux SCSI List <linux-scsi@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        linux-aio <linux-aio@kvack.org>, io-uring@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Network Development <netdev@vger.kernel.org>,
        keyrings@vger.kernel.org,
        LSM List <linux-security-module@vger.kernel.org>
In-Reply-To: <CAK8P3a2Mi+1yttyGk4k7HxRVrMtmFqJewouVhynqUL0PJycmog@mail.gmail.com>
To:     Arnd Bergmann <arnd@arndb.de>
Precedence: bulk


> On Sep 19, 2020, at 2:16 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>=20
> =EF=BB=BFOn 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...
>>>=20
>>> 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.
>=20
> 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...=

>=20
> 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.
>=20
>> 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?
>=20
> 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?
>=20

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 semant=
ics of io_uring?  Is the operation bitness chosen by the io_uring creation o=
r by the io_uring_enter() bitness?=