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

[Topi Miettinen]

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