Re: [PATCH v4] mm: Optional full ASLR for mmap() and mremap()

[Mike Rapoport]

(added one of the AnC paper authors)

On Tue, Nov 17, 2020 at 10:21:30PM +0200, Topi Miettinen wrote:
> 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

-- 
Sincerely yours,
Mike.