x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

The STACKLEAK feature (initially developed by PaX Team) has the following
benefits:

1. Reduces the information that can be revealed through kernel stack leak
   bugs. The idea of erasing the thread stack at the end of syscalls is
   similar to CONFIG_PAGE_POISONING and memzero_explicit() in kernel
   crypto, which all comply with FDP_RIP.2 (Full Residual Information
   Protection) of the Common Criteria standard.

2. Blocks some uninitialized stack variable attacks (e.g. CVE-2017-17712,
   CVE-2010-2963). That kind of bugs should be killed by improving C
   compilers in future, which might take a long time.

This commit introduces the code filling the used part of the kernel
stack with a poison value before returning to userspace. Full
STACKLEAK feature also contains the gcc plugin which comes in a
separate commit.

The STACKLEAK feature is ported from grsecurity/PaX. More information at:
  https://grsecurity.net/
  https://pax.grsecurity.net/

This code is modified from Brad Spengler/PaX Team's code in the last
public patch of grsecurity/PaX based on our understanding of the code.
Changes or omissions from the original code are ours and don't reflect
the original grsecurity/PaX code.

Performance impact:

Hardware: Intel Core i7-4770, 16 GB RAM

Test #1: building the Linux kernel on a single core
        0.91% slowdown

Test #2: hackbench -s 4096 -l 2000 -g 15 -f 25 -P
        4.2% slowdown

So the STACKLEAK description in Kconfig includes: "The tradeoff is the
performance impact: on a single CPU system kernel compilation sees a 1%
slowdown, other systems and workloads may vary and you are advised to
test this feature on your expected workload before deploying it".

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
index cb0c889e13aa..977b84e69787 100644
--- a/scripts/gcc-plugins/Kconfig
+++ b/scripts/gcc-plugins/Kconfig
@@ -139,4 +139,23 @@ config GCC_PLUGIN_RANDSTRUCT_PERFORMANCE
           in structures.  This reduces the performance hit of RANDSTRUCT
           at the cost of weakened randomization.
 
+config GCC_PLUGIN_STACKLEAK
+        bool "Erase the kernel stack before returning from syscalls"
+        depends on GCC_PLUGINS
+        depends on HAVE_ARCH_STACKLEAK
+        help
+          This option makes the kernel erase the kernel stack before
+          returning from system calls. That reduces the information which
+          kernel stack leak bugs can reveal and blocks some uninitialized
+          stack variable attacks.
+
+          The tradeoff is the performance impact: on a single CPU system kernel
+          compilation sees a 1% slowdown, other systems and workloads may vary
+          and you are advised to test this feature on your expected workload
+          before deploying it.
+
+          This plugin was ported from grsecurity/PaX. More information at:
+           * https://grsecurity.net/
+           * https://pax.grsecurity.net/
+
 endif