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Date:   Thu, 8 Oct 2020 18:07:28 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Topi Miettinen <toiwoton@gmail.com>
Cc:     linux-hardening@vger.kernel.org, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()
Message-ID: <20201008170728.GK20115@casper.infradead.org>
References: <20201008165408.38228-1-toiwoton@gmail.com>
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On Thu, Oct 08, 2020 at 07:54:08PM +0300, Topi Miettinen wrote:
> +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.
> +
> +    On 32 bit systems this may cause problems due to increased VM
> +    fragmentation if the address space gets crowded.

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.

> +	if ((flags & MREMAP_MAYMOVE) && randomize_va_space >= 3) {
> +		/*
> +		 * Caller is happy with a different address, so let's
> +		 * move even if not necessary!
> +		 */
> +		new_addr = arch_mmap_rnd();
> +
> +		ret = mremap_to(addr, old_len, new_addr, new_len,
> +				&locked, flags, &uf, &uf_unmap_early,
> +				&uf_unmap);
> +		goto out;
> +	}
> +
> +

Overly enthusiastic newline