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SubjectRe: [PATCH v4] mm: Optional full ASLR for mmap() and mremap()
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On 17.11.2020 18.54, Matthew Wilcox wrote:
> On Mon, Oct 26, 2020 at 06:05:18PM +0200, Topi Miettinen wrote:
>> Writing a new value of 3 to /proc/sys/kernel/randomize_va_space
>> enables 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.
>
> Is this worth it?
>
> https://www.ndss-symposium.org/ndss2017/ndss-2017-programme/aslrcache-practical-cache-attacks-mmu/

Thanks, very interesting. The paper presents an attack (AnC) which can
break ASLR even from JavaScript in browsers. In the process it compares
the memory allocators of Firefox and Chrome. Firefox relies on Linux
mmap() to randomize the memory location, but Chrome internally chooses
the randomized address. The paper doesn't present exact numbers to break
ASLR for Chrome case, but it seems to require more effort. Chrome also
aggressively randomizes the memory on each allocation, which seems to
enable further possibilities for AnC to probe the MMU tables.

Disregarding the difference in aggressiveness of memory allocators, I
think with sysctl.kernel.randomize_va_space=3, the effort for breaking
ASLR with Firefox should be increased closer to Chrome case since mmap()
will use the address space more randomly.

I have used this setting now for a month without any visible performance
issues, so I think the extra bits (for some additional effort to
attackers) are definitely worth the low cost.

Furthermore, the paper does not describe in detail how the attack would
continue after breaking ASLR. Perhaps there are assumptions which are
not valid when the different memory areas are no longer sequential. For
example, if ASLR is initially broken wrt. the JIT buffer but continuing
the attack would require other locations to be determined (like stack,
data segment for main exe or libc etc), further efforts may be needed to
resolve these locations. With randomize_va_space=2, resolving any
address (JIT buffer) can reveal the addresses of many other memory areas
but this is not the case with 3.

-Topi

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