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Date: Wed, 18 Nov 2020 23:42:28 +0100
From: Jann Horn <>
To: Matthew Wilcox <>
Cc: Topi Miettinen <>,, 
	Andrew Morton <>, Linux-MM <>, 
	kernel list <>, Kees Cook <>, 
	Mike Rapoport <>, Mateusz Jurczyk <>, 
	Kernel Hardening <>
Subject: Re: [PATCH v4] mm: Optional full ASLR for mmap() and mremap()

On Tue, Nov 17, 2020 at 5:55 PM Matthew Wilcox <> 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?

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
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

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