linux-hardening - Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()

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Date:   Thu, 8 Oct 2020 21:11:22 +0300
From:   Topi Miettinen <toiwoton@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-hardening@...r.kernel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()

On 8.10.2020 20.07, Matthew Wilcox wrote:
> 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.

Right, I'll update the description.

>> +	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
> 

Will remove.

-Topi