x86/mm: Improve AMD Bulldozer ASLR workaround

x86/mm: Improve AMD Bulldozer ASLR workaround

The ASLR implementation needs to special-case AMD F15h processors by
clearing out bits [14:12] of the virtual address in order to avoid I$
cross invalidations and thus performance penalty for certain workloads.
For details, see:

  dfb09f9b7ab0 ("x86, amd: Avoid cache aliasing penalties on AMD family 15h")

This special case reduces the mmapped file's entropy by 3 bits.

The following output is the run on an AMD Opteron 62xx class CPU
processor under x86_64 Linux 4.0.0:

  $ for i in `seq 1 10`; do cat /proc/self/maps | grep "r-xp.*libc" ; done
  b7588000-b7736000 r-xp 00000000 00:01 4924       /lib/i386-linux-gnu/libc.so.6
  b7570000-b771e000 r-xp 00000000 00:01 4924       /lib/i386-linux-gnu/libc.so.6
  b75d0000-b777e000 r-xp 00000000 00:01 4924       /lib/i386-linux-gnu/libc.so.6
  b75b0000-b775e000 r-xp 00000000 00:01 4924       /lib/i386-linux-gnu/libc.so.6
  b7578000-b7726000 r-xp 00000000 00:01 4924       /lib/i386-linux-gnu/libc.so.6
  ...

Bits [12:14] are always 0, i.e. the address always ends in 0x8000 or
0x0000.

32-bit systems, as in the example above, are especially sensitive
to this issue because 32-bit randomness for VA space is 8 bits (see
mmap_rnd()). With the Bulldozer special case, this diminishes to only 32
different slots of mmap virtual addresses.

This patch randomizes per boot the three affected bits rather than
setting them to zero. Since all the shared pages have the same value
at bits [12..14], there is no cache aliasing problems. This value gets
generated during system boot and it is thus not known to a potential
remote attacker. Therefore, the impact from the Bulldozer workaround
gets diminished and ASLR randomness increased.

More details at:

  http://hmarco.org/bugs/AMD-Bulldozer-linux-ASLR-weakness-reducing-mmaped-files-by-eight.html

Original white paper by AMD dealing with the issue:

  http://developer.amd.com/wordpress/media/2012/10/SharedL1InstructionCacheonAMD15hCPU.pdf

Mentored-by: Ismael Ripoll <iripoll@disca.upv.es>
Signed-off-by: Hector Marco-Gisbert <hecmargi@upv.es>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jan-Simon <dl9pf@gmx.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-fsdevel@vger.kernel.org
Link: http://lkml.kernel.org/r/1427456301-3764-1-git-send-email-hecmargi@upv.es
Signed-off-by: Ingo Molnar <mingo@kernel.org>