linux-kernel - Re: [PATCH v3 04/11] arm64, mm: Move generic mmap layout functions to mm

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Date:   Fri, 19 Apr 2019 09:20:36 +0200
From:   Alex Ghiti <alex@...ti.fr>
To:     Kees Cook <keescook@...omium.org>
Cc:     Albert Ou <aou@...s.berkeley.edu>,
        Catalin Marinas <catalin.marinas@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linux-MM <linux-mm@...ck.org>,
        Paul Burton <paul.burton@...s.com>,
        linux-riscv@...ts.infradead.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        James Hogan <jhogan@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mips@...r.kernel.org, Christoph Hellwig <hch@....de>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Luis Chamberlain <mcgrof@...nel.org>
Subject: Re: [PATCH v3 04/11] arm64, mm: Move generic mmap layout functions to
 mm

On 4/18/19 10:19 AM, Kees Cook wrote:
> On Thu, Apr 18, 2019 at 12:55 AM Alex Ghiti <alex@...ti.fr> wrote:
>> Regarding the help text, I agree that it does not seem to be frequent to
>> place
>> comment above config like that, I'll let Christoph and you decide what's
>> best. And I'll
>> add the possibility for the arch to define its own STACK_RND_MASK.
> Yeah, I think it's very helpful to spell out the requirements for new
> architectures with these kinds of features in the help text (see
> SECCOMP_FILTER for example).
>
>>> I think CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT should select
>>> CONFIG_ARCH_HAS_ELF_RANDOMIZE. It would mean moving
>>
>> I don't think we should link those 2 features together: an architecture
>> may want
>> topdown mmap and don't care about randomization right ?
> Given that the mmap randomization and stack randomization are already
> coming along for the ride, it seems weird to make brk randomization an
> optional feature (especially since all the of the architectures you're
> converting include it). I'd also like these kinds of security features
> to be available by default. So, I think one patch to adjust the MIPS
> brk randomization entropy and then you can just include it in this
> move.


Ok that makes sense, and that would bring support for randomization to
riscv at the same time, so I'll look into it, thanks.


>> Actually, I had to add those ifdefs for mmap_rnd_compat_bits, not
>> is_compat_task.
> Oh! In that case, use CONFIG_HAVE_ARCH_MMAP_RND_BITS. :) Actually,
> what would be maybe cleaner would be to add mmap_rnd_bits_min/max
> consts set to 0 for the non-CONFIG_HAVE_ARCH_MMAP_RND_BITS case at the
> top of mm/mmap.c.


Ok I'll do that.


>
> I really like this clean-up! I think we can move x86 to it too without
> too much pain. :)
>

Yeah I think too, I will do that too.


Thanks again,


Alex