Re: [PATCH v5] mm: Optional full ASLR for mmap(), mremap(), vdso and stack

Florian Weimer
2020-12-03T09:49:33Z

* Topi Miettinen:

> +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 and the location of stack and vdso are also
> + randomized.
> +
> + On 32 bit systems this may cause problems due to increased VM
> + fragmentation if the address space gets crowded.

Isn't this a bit of an understatement? I think you'll have to restrict
this randomization to a subregion of the entire address space, otherwise
the reduction in maximum mapping size due to fragmentation will be a
problem on 64-bit architectures as well (which generally do not support
the full 64 bits for user-space addresses).

> + 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. In the worst case,
> + additional page table entries of up to 4 pages are created for
> + each mapping, so with small mappings there's considerable penalty.

The number 4 is architecture-specific, right?

Thanks,
Florian
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