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

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Message-ID: <201907221135.2C2D262D8@keescook>
Date:   Mon, 22 Jul 2019 11:39:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace
 on i386

On Mon, Jul 22, 2019 at 08:31:32PM +0200, Thomas Gleixner wrote:
> On Mon, 22 Jul 2019, Kees Cook wrote:
> > Just so I'm understanding: the vDSO change introduced code to make an
> > actual syscall on i386, which for most seccomp filters would be rejected?
> 
> No. The old x86 specific VDSO implementation had a fallback syscall as
> well, i.e. clock_gettime(). On 32bit clock_gettime() uses the y2038
> endangered timespec.
> 
> So when the VDSO was made generic we changed the internal data structures
> to be 2038 safe right away. As a consequence the fallback syscall is not
> clock_gettime(), it's clock_gettime64(). which seems to surprise seccomp.

Okay, it's didn't add a syscall, it just changed it. Results are the
same: conservative filters suddenly start breaking due to the different
call. (And now I see why Andy's alias suggestion would help...)

I'm not sure which direction to do with this. It seems like an alias
list is a large hammer for this case, and a "seccomp-bypass when calling
from vDSO" solution seems too fragile?

-- 
Kees Cook