• Hector Marco-Gisbert's avatar
    x86, mm/ASLR: Fix stack randomization on 64-bit systems · 4e7c22d4
    Hector Marco-Gisbert authored
    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
      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
    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.
    Signed-off-by: default avatarHector Marco-Gisbert <hecmargi@upv.es>
    Signed-off-by: default avatarIsmael Ripoll <iripoll@upv.es>
    [ Rebased, fixed 80 char bugs, cleaned up commit message, added test example and CVE ]
    Signed-off-by: default avatarKees Cook <keescook@chromium.org>
    Cc: <stable@vger.kernel.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Fixes: CVE-2015-1593
    Link: http://lkml.kernel.org/r/20150214173350.GA18393@www.outflux.net
    Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
binfmt_elf.c 60.6 KB