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Subject: Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-hardening@vger.kernel.org, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20201008165408.38228-1-toiwoton@gmail.com>
 <20201008170728.GK20115@casper.infradead.org>
From:   Topi Miettinen <toiwoton@gmail.com>
Message-ID: <1d08abd1-4343-59e7-108a-68af5f7778c0@gmail.com>
Date:   Thu, 8 Oct 2020 21:11:22 +0300
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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