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

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Message-Id: <9200ED2A-AE4B-4094-81C9-E92240B4840F@amacapital.net>
Date:   Thu, 28 Jun 2018 18:08:20 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul 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 - ARM Linux <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>,
        Christoph Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        Steven 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>, 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 5:18 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.

You sure?  Because, unless I remember wrong, a 32-bit user program on a 64-bit kernel will actually work at least most of the time even if high bits are set. I’m okay with straight-up promising “will always work” or “will never work”, but “sometimes” is bad.

> 
> 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.
> 
>             Linus