linux-kernel - Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT

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Date:   Thu, 5 Jan 2017 12:14:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR

On Thu, Jan 5, 2017 at 11:39 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 01/05/2017 11:29 AM, Kirill A. Shutemov wrote:
>> On Thu, Jan 05, 2017 at 11:13:57AM -0800, Dave Hansen wrote:
>>> On 12/26/2016 05:54 PM, Kirill A. Shutemov wrote:
>>>> MM would use min(RLIMIT_VADDR, TASK_SIZE) as upper limit of virtual
>>>> address available to map by userspace.
>>>
>>> What happens to existing mappings above the limit when this upper limit
>>> is dropped?
>>
>> Nothing: we only prevent creating new mappings. All existing are not
>> affected.
>>
>> The semantics here the same as with other resource limits.
>>
>>> Similarly, why do we do with an application running with something
>>> incompatible with the larger address space that tries to raise the
>>> limit?  Say, legacy MPX.
>>
>> It has to know what it does. Yes, it can change limit to the point where
>> application is unusable. But you can to the same with other limits.
>
> I'm not sure I'm comfortable with this.  Do other rlimit changes cause
> silent data corruption?  I'm pretty sure doing this to MPX would.
>

What actually goes wrong in this case?  That is, what combination of
MPX setup of subsequent allocations will cause a problem, and is the
problem worse than just a segfault?  IMO it would be really nice to
keep the messy case confined to MPX.

FWIW, this problem is kind of generic.  If you run code in a process,
MPX or otherwise, that assumes something about pointer values and then
create a pointer that violates its assumptions, you will cause
problems.  For example, some VMs use high bits to store metadata.  If
you feed a pointer that's too big to such code, boom.  This is exactly
why high addresses need to be opt-in.