linux-hardening - Re: [PATCH v4] mm: Optional full ASLR for mmap() and mremap()

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Date:   Wed, 18 Nov 2020 23:42:28 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Topi Miettinen <toiwoton@...il.com>,
        linux-hardening@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Mike Rapoport <rppt@...nel.org>,
        Mateusz Jurczyk <mjurczyk@...gle.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v4] mm: Optional full ASLR for mmap() and mremap()

On Tue, Nov 17, 2020 at 5:55 PM Matthew Wilcox <willy@...radead.org> wrote:
> On Mon, Oct 26, 2020 at 06:05:18PM +0200, Topi Miettinen wrote:
> > Writing a new value of 3 to /proc/sys/kernel/randomize_va_space
> > enables full randomization of memory mappings created with mmap(NULL,
> > ...). With 2, the base of the VMA used for such mappings is random,
> > but the mappings are created in predictable places within the VMA and
> > in sequential order. With 3, new VMAs are created to fully randomize
> > the mappings. Also mremap(..., MREMAP_MAYMOVE) will move the mappings
> > even if not necessary.
>
> Is this worth it?
>
> https://www.ndss-symposium.org/ndss2017/ndss-2017-programme/aslrcache-practical-cache-attacks-mmu/

Yeah, against local attacks (including from JavaScript), ASLR isn't
very robust; but it should still help against true remote attacks
(modulo crazyness like NetSpectre).

E.g. Mateusz Jurczyk's remote Samsung phone exploit via MMS messages
(https://googleprojectzero.blogspot.com/2020/08/mms-exploit-part-5-defeating-aslr-getting-rce.html)
would've probably been quite a bit harder to pull off if he hadn't
been able to rely on having all those memory mappings sandwiched
together.