x86, kaslr: Mix entropy sources together as needed

Depending on availability, mix the RDRAND and RDTSC entropy together with
XOR. Only when neither is available should the i8254 be used. Update
the Kconfig documentation to reflect this. Additionally, since bits
used for entropy is masked elsewhere, drop the needless masking in
the get_random_long(). Similarly, use the entire TSC, not just the low
32 bits.

Finally, to improve the starting entropy, do a simple hashing of a
build-time versions string and the boot-time boot_params structure for
some additional level of unpredictability.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/20131111222839.GA28616@www.outflux.net
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 51f439953d23..596cd9edeb9c 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1735,13 +1735,17 @@ config RANDOMIZE_BASE
            deters exploit attempts relying on knowledge of the location
            of kernel internals.
 
-           Entropy is generated using the RDRAND instruction if it
-           is supported.  If not, then RDTSC is used, if supported. If
-           neither RDRAND nor RDTSC are supported, then no randomness
-           is introduced.
+           Entropy is generated using the RDRAND instruction if it is
+           supported. If RDTSC is supported, it is used as well. If
+           neither RDRAND nor RDTSC are supported, then randomness is
+           read from the i8254 timer.
 
            The kernel will be offset by up to RANDOMIZE_BASE_MAX_OFFSET,
-           and aligned according to PHYSICAL_ALIGN.
+           and aligned according to PHYSICAL_ALIGN. Since the kernel is
+           built using 2GiB addressing, and PHYSICAL_ALGIN must be at a
+           minimum of 2MiB, only 10 bits of entropy is theoretically
+           possible. At best, due to page table layouts, 64-bit can use
+           9 bits of entropy and 32-bit uses 8 bits.
 
 config RANDOMIZE_BASE_MAX_OFFSET
         hex "Maximum ASLR offset allowed"