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 09:55:43 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.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 7:29 PM, Andy Lutomirski luto@...nel.org wrote:

> On Thu, Jun 28, 2018 at 2:22 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Thu, Jun 28, 2018 at 1:23 PM Andy Lutomirski <luto@...nel.org> wrote:
>>>
>>> This is okay with me for a fix outside the merge window.  Can you do a
>>> followup for the next merge window that fixes it better, though?  In
>>> particular, TASK_SIZE is generally garbage.  I think a better fix
>>> would be something like adding a new arch-overridable helper like:
>>>
>>> static inline unsigned long current_max_user_addr(void) { return TASK_SIZE; }
>>
>> We already have that. It's called "user_addr_max()".
> 
> Nah, that one is more or less equivalent to TASK_SIZE_MAX, except that
> it's different if set_fs() is used.

So which one would be right in this case ? AFAIU we want to ensure we don't
populate regs->ip with a bogus address that would make SYSRET or other return
to userspace instructions explode.

Is that guaranteed by TASK_SIZE or TASK_SIZE_MAX (aliased by user_addr_max()) ?

Thanks,

Mathieu

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