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

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Date:   Fri, 29 Jun 2018 10:05:44 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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 29, 2018, at 10:02 AM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Thu, Jun 28, 2018 at 6:08 PM Andy Lutomirski <luto@...capital.net> wrote:
>> > On Jun 28, 2018, at 5:18 PM, Linus Torvalds <torvalds@...ux-foundation.org>
>> > wrote:
>> >
>> >
>> > 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.
> 
> Sure.
> 
> If you run a 32-bit binary on a 64-bit kernel,. you will have access
> to the 0xc0000000 - 0xffffffff area that you wouldn't have had access
> to if it ran on a 32-bit kernel.
> 
> But exactly *because* you have access to that area, those addresses
> are actually valid addresses for the 32-bit case, so they shouldn't be
> considered bad. They can't happen on a native 32-bit kerne, but a
> 32-bit program doesn't even care. If it has user memory mapped in that
> area, it should work.
> 
> And if it *doesn't* have user memory mapped in that area, then it will
> fail when the trying to execute the (non-existent) abort sequence.
> 
> After all, depending on configuration, a native 32-bit kernel might
> limit user space even more (ie some vendors had a 2G:2G split instead
> of the traditional 3G:1G split.
> 
> Was that the case you were thinking of, or was it something else?

What I'm worried about is setting regs->ip of a compat 32-bit task to
addresses in the range 0x100000000-0xFFFFFFFFFFFFFFFF.

Thanks,

Mathieu

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