Messages in this thread | | | Date | Mon, 16 Feb 2015 12:49:04 -0800 | From | Andy Lutomirski <> | Subject | Re: [PATCH] ASLR: fix stack randomization on 64-bit systems |
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On 02/14/2015 09:33 AM, Kees Cook wrote: > From: Hector Marco-Gisbert <hecmargi@upv.es> > > The issue is that the stack for processes is not properly randomized on 64 bit > architectures due to an integer overflow. > > The affected function is randomize_stack_top() in file "fs/binfmt_elf.c": > > static unsigned long randomize_stack_top(unsigned long stack_top) > { > unsigned int random_variable = 0; > > if ((current->flags & PF_RANDOMIZE) && > !(current->personality & ADDR_NO_RANDOMIZE)) { > random_variable = get_random_int() & STACK_RND_MASK; > random_variable <<= PAGE_SHIFT; > } > return PAGE_ALIGN(stack_top) + random_variable; > return PAGE_ALIGN(stack_top) - random_variable; > } > > Note that, it declares the "random_variable" variable as "unsigned int". Since > the result of the shifting operation between STACK_RND_MASK (which is > 0x3fffff on x86_64, 22 bits) and PAGE_SHIFT (which is 12 on x86_64): > > random_variable <<= PAGE_SHIFT; > > then the two leftmost bits are dropped when storing the result in the > "random_variable". This variable shall be at least 34 bits long to hold the > (22+12) result. > > These two dropped bits have an impact on the entropy of process stack. > Concretely, the total stack entropy is reduced by four: from 2^28 to 2^30 (One > fourth of expected entropy). > > This patch restores back the entropy by correcting the types involved in the > operations in the functions randomize_stack_top() and stack_maxrandom_size(). > > The successful fix can be tested with: > $ for i in `seq 1 10`; do cat /proc/self/maps | grep stack; done > 7ffeda566000-7ffeda587000 rw-p 00000000 00:00 0 [stack] > 7fff5a332000-7fff5a353000 rw-p 00000000 00:00 0 [stack] > 7ffcdb7a1000-7ffcdb7c2000 rw-p 00000000 00:00 0 [stack] > 7ffd5e2c4000-7ffd5e2e5000 rw-p 00000000 00:00 0 [stack] > ... > > Once corrected, the leading bytes should be between 7ffc and 7fff, rather > than always being 7fff. > > CVE-2015-1593
Awesome. So the vdso randomization *and* the stack randomization implementations were buggy. Anyone want to check the mmap and brk randomization implementations?
--Andy
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