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SubjectRe: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()
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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

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