linux-kernel - Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK

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Date:   Thu, 28 Jun 2018 20:54:46 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are <
 TASK_SIZE

----- On Jun 28, 2018, at 8:18 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Thu, Jun 28, 2018 at 4:30 PM Andy Lutomirski <luto@...nel.org> wrote:
>>
>> The idea is that, if someone screws up and sticks a number like
>> 0xbaadf00d00045678 into their rseq abort_ip in a 32-bit x86 program
>> (when they actually mean 0x00045678), we want to something consistent.
> 
> I think the "something consistent" is perfectly fine with just "it won't work".
> 
> Make it do
> 
>        if (rseq_cs->abort_ip != (unsigned long)rseq_cs->abort_ip)
>                return -EINVAL;
> 
> at abort time.
> 
> Done.
> 
> If it's a 32-bit kernel, the above will reject the thing, and if it's
> a 64-bit kernel, it will be a no-op, but the abort won't work in a
> 32-bit caller.
> 
> Problem solved.

This assumes a 64-bit kernel returning to a 32-bit compat task with
garbage it the upper 32 bits of regs->ip behaves correctly (e.g.
kill the offending process rather than crash the kernel) on all
architectures.

Is this something we can rely on ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com