linux-kernel - Re: [5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace on i386

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Date:   Sat, 27 Jul 2019 10:49:52 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Paul Bolle <pebolle@...cali.nl>
Subject: Re: [5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace on i386

On Fri, Jul 26, 2019 at 11:01 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> +cc Paul
>
> On Wed, Jul 24, 2019 at 01:56:34AM +0200, Thomas Gleixner wrote:
> > On Tue, 23 Jul 2019, Kees Cook wrote:
> >
> > > On Wed, Jul 24, 2019 at 12:59:03AM +0200, Thomas Gleixner wrote:
> > > > And as we have sys_clock_gettime64() exposed for 32bit anyway you need to
> > > > deal with that in seccomp independently of the VDSO. It does not make sense
> > > > to treat sys_clock_gettime() differently than sys_clock_gettime64(). They
> > > > both expose the same information, but the latter is y2038 safe.
> > >
> > > Okay, so combining Andy's ideas on aliasing and "more seccomp flags",
> > > we could declare that clock_gettime64() is not filterable on 32-bit at
> > > all without the magic SECCOMP_IGNORE_ALIASES flag or something. Then we
> > > would alias clock_gettime64 to clock_gettime _before_ the first evaluation
> > > (unless SECCOMP_IGNORE_ALIASES is set)?
> > >
> > > (When was clock_gettime64() introduced? Is it too long ago to do this
> > > "you can't filter it without a special flag" change?)
> >
> > clock_gettime64() and the other sys_*time64() syscalls which address the
> > y2038 issue were added in 5.1
>
> Paul Bolle pointed out that this regression showed up in v5.3-rc1, not
> v5.2.  In Paul's case, systemd-journal is failing.

I think it's getting quite late to start inventing new seccomp
features to fix this.  I think the right solution for 5.3 is to change
the 32-bit vdso fallback to use the old clock_gettime, i.e.
clock_gettime32.  This is obviously not an acceptable long-term
solution.