Re: [PATCH v5] mm: Optional full ASLR for mmap(), mremap(), vdso and stack
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From: Florian Weimer <hidden>
Date: 2020-12-03 09:49:33
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linux-hardening, linux-mm, lkml
* Topi Miettinen:
quoted hunk
+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).
quoted hunk
+ 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 -- Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn, Commercial register: Amtsgericht Muenchen, HRB 153243, Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill