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

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Date:   Mon, 26 Dec 2016 18:06:01 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     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>,
        Dave Hansen <dave.hansen@...el.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 Mon, Dec 26, 2016 at 5:54 PM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
> This patch introduces new rlimit resource to manage maximum virtual
> address available to userspace to map.
>
> On x86, 5-level paging enables 56-bit userspace virtual address space.
> Not all user space is ready to handle wide addresses. It's known that
> at least some JIT compilers use high bit in pointers to encode their
> information. It collides with valid pointers with 5-level paging and
> leads to crashes.
>
> The patch aims to address this compatibility issue.
>
> MM would use min(RLIMIT_VADDR, TASK_SIZE) as upper limit of virtual
> address available to map by userspace.
>
> The default hard limit will be RLIM_INFINITY, which basically means that
> TASK_SIZE limits available address space.
>
> The soft limit will also be RLIM_INFINITY everywhere, but the machine
> with 5-level paging enabled. In this case, soft limit would be
> (1UL << 47) - PAGE_SIZE. It’s current x86-64 TASK_SIZE_MAX with 4-level
> paging which known to be safe
>
> New rlimit resource would follow usual semantics with regards to
> inheritance: preserved on fork(2) and exec(2). This has potential to
> break application if limits set too wide or too narrow, but this is not
> uncommon for other resources (consider RLIMIT_DATA or RLIMIT_AS).
>
> As with other resources you can set the limit lower than current usage.
> It would affect only future virtual address space allocations.
>
> Use-cases for new rlimit:
>
>   - Bumping the soft limit to RLIM_INFINITY, allows current process all
>     its children to use addresses above 47-bits.
>
>   - Bumping the soft limit to RLIM_INFINITY after fork(2), but before
>     exec(2) allows the child to use addresses above 47-bits.
>
>   - Lowering the hard limit to 47-bits would prevent current process all
>     its children to use addresses above 47-bits, unless a process has
>     CAP_SYS_RESOURCES.
>
>   - It’s also can be handy to lower hard or soft limit to arbitrary
>     address. User-mode emulation in QEMU may lower the limit to 32-bit
>     to emulate 32-bit machine on 64-bit host.

I tend to think that this should be a personality or an ELF flag, not
an rlimit.  That way setuid works right.