Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 19 Dec 2014 14:11:37 -0800 | Subject | Re: [PATCH] ASLRv3: randomize_va_space=3 preventing offset2lib attack |
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On Fri, Dec 19, 2014 at 2:04 PM, Hector Marco <hecmargi@upv.es> wrote: > > > El 12/12/14 a las 18:17, Andy Lutomirski escribió: > >> On Dec 12, 2014 8:33 AM, "Hector Marco" <hecmargi@upv.es> wrote: >>> >>> >>> Hello, >>> >>> I agree. I don't think a new randomization mode will be needed, just fix >>> the current randomize_va_space=2. Said other way: fixing the offset2lib >>> will not break any current program and so, no need to add additional >>> configuration options. May be we shall wait for some inputs >>> from the list (may be we are missing something). >>> >>> >>> Regarding to VDSO, definitively, is not randomized enough in 64bits. >>> Brute force attacks would be pretty fast even from the network. >>> I have identified the bug and seems quite easy to fix it. >>> >>> On 32bit systems, this is not a issue because it is mapped in the >>> mmap area. In order to fix the VDSO on 64bit, the following >>> considerations shall >>> be discussed: >>> >>> >>> Performance: >>> It seems (reading the kernel comments) that the random allocation >>> algorithm tries to place the VDSO in the same PTE than the stack. >> >> >> The comment is wrong. It means PTE table. >> >>> But since the permissions of the stack and the VDSO are different >>> it seems that are getting right the opposite. >> >> >> Permissions have page granularity, so this isn't a problem. >> >>> >>> Effectively VDSO shall be correctly randomized because it contains >>> enough useful exploitable stuff. >>> >>> I think that the possible solution is follow the x86_32 approach >>> which consist on map the VDSO in the mmap area. >>> >>> It would be better fix VDSO in a different patch ? I can send a >>> patch which fixes the VDSO on 64 bit. >>> >> >> What are the considerations for 64-bit memory layout? I haven't >> touched it because I don't want to break userspace, but I don't know >> what to be careful about. >> >> --Andy > > > I don't think that mapping the VDSO in the mmap area breaks the > userspace. Actually, this is already happening with the current > implementation. You can see it by running: > > setarch x86_64 -R cat /proc/self/maps >
Hmm. So apparently we even switch which side of the stack the vdso is on depending on the randomization setting.
> > Do this break the userspace in some way ? > > > Regarding the solution to the offset2lib it seems that placing the > executable in a different memory region area could increase the > number of pages for the pages table (because it is more spread). > We should consider this before fixing the current implementation > (randomize_va_space=2). > > I guess that the current implementation places the PIE executable in > the mmap base area jointly with the libraries in an attempt to reduce > the size of the page table. > > Therefore, I can fix the current implementation (maintaining the > randomize_va_space=2) by moving the PIE executable from the mmap base > area to another one for x86*, ARM* and MIPS (as s390 and PowerPC do). > But we shall agree that this increment in the page table is not a > issue. Otherwise, the randomize_va_space=3 shall be considered.
Wrt the vdso itself, though, there is an extra consideration: CRIU. I *think* that the CRIU vdso proxying scheme will work even if the vdso changes sizes and is adjacent to other mappings. Cyrill and/or Pavel, am I right?
I'm not fundamentally opposed to mapping the vdso just like any other shared library. I still think that we should have an extra-strong randomization mode in which all the libraries are randomized wrt each other, though. For many applications, the extra page table cost will be negligible.
--Andy
> > > Hector Marco. > >> >>> >>> >>> Regards, >>> Hector Marco.
-- Andy Lutomirski AMA Capital Management, LLC
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