linux-kernel - Re: [Xen-devel] [PATCH] [v2] xen: hypercall: fix out-of-bounds memcpy

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Date:   Fri, 9 Feb 2018 15:21:55 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     David Laight <David.Laight@...lab.com>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [Xen-devel] [PATCH] [v2] xen: hypercall: fix out-of-bounds memcpy

On Fri, Feb 9, 2018 at 3:13 PM, David Laight <David.Laight@...lab.com> wrote:
> From: Arnd Bergmann
>> Sent: 09 February 2018 12:58
> ...
>> However, aside from this driver, I wonder if we should be worried about
>> Spectre type 1 attacks on similar code, when gcc-8 turns a switch/case
>> statement into an array lookup behind our back, e.g. in an ioctl handler.
>> Has anybody got this on their radar?
>
> The canonical code for a switch statement is to jump indirect on an array
> of code pointers.
> ioctl handlers probably use a series of compares because the values are
> sparse.

The majority of ioctl handlers is sparse enough that a table lookup wouldn't
work, but there are still subsystems that never fully adopted the _IOC()
macros, e.g. tty or socket ioctls are just consecutive numbers.

> Also remember that gcc-8 will convert dense switch statements that just
> load a value into a data array lookup.

Right, that's the case I'm interested in here. I don't know how many of
those exist in the kernel, as this would again be a small subset of the
switch()/case statements that use consecutive numbers.

> I guess both those jump tables are potential attack vectors.
> Not quite sure how they might be used to leak info though.

When I tested the xen fallback code with gcc-7.3, I noticed a retpoline
getting generated for pointer array, so that should be safe.

      Arnd