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Message-ID: <87im9j2pbs.fsf@oldenburg2.str.redhat.com>
Date: Thu, 03 Dec 2020 10:47:51 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Topi Miettinen <toiwoton@...il.com>
Cc: linux-hardening@...r.kernel.org, akpm@...ux-foundation.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Jann Horn <jannh@...gle.com>,
Kees Cook <keescook@...omium.org>,
Matthew Wilcox <willy@...radead.org>,
Mike Rapoport <rppt@...nel.org>,
Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5] mm: Optional full ASLR for mmap(), mremap(), vdso
and stack
* Topi Miettinen:
> +3 Additionally enable 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 and the location of stack and vdso are also
> + randomized.
> +
> + On 32 bit systems this may cause problems due to increased VM
> + fragmentation if the address space gets crowded.
Isn't this a bit of an understatement? I think you'll have to restrict
this randomization to a subregion of the entire address space, otherwise
the reduction in maximum mapping size due to fragmentation will be a
problem on 64-bit architectures as well (which generally do not support
the full 64 bits for user-space addresses).
> + On all systems, it will reduce performance and increase memory
> + usage due to less efficient use of page tables and inability to
> + merge adjacent VMAs with compatible attributes. In the worst case,
> + additional page table entries of up to 4 pages are created for
> + each mapping, so with small mappings there's considerable penalty.
The number 4 is architecture-specific, right?
Thanks,
Florian
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