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Date: Wed,  6 Dec 2017 02:33:41 +0300
From: Alexander Popov <>
	Kees Cook <>,
	PaX Team <>,
	Brad Spengler <>,
	Ingo Molnar <>,
	Andy Lutomirski <>,
	Tycho Andersen <>,
	Laura Abbott <>,
	Mark Rutland <>,
	Ard Biesheuvel <>,
	Borislav Petkov <>,
	Thomas Gleixner <>,
	"H . Peter Anvin" <>,
	Peter Zijlstra <>,,
Subject: [PATCH RFC v6 0/6] Introduce the STACKLEAK feature and a test for it

This is the 6th version of the patch series introducing STACKLEAK to the
mainline kernel. STACKLEAK is a security feature developed by Grsecurity/PaX
(kudos to them), which:
 - reduces the information that can be revealed through kernel stack leak bugs;
 - blocks some uninitialized stack variable attacks (e.g. CVE-2010-2963);
 - introduces some runtime checks for kernel stack overflow detection.

STACKLEAK instrumentation statistics

These numbers were obtained for the 4th version of the patch series.

Size of vmlinux (x86_64_defconfig):
 file size:
  - STACKLEAK disabled: 35014784 bytes
  - STACKLEAK enabled: 35044952 bytes (+0.086%)
 .text section size (calculated by size utility):
  - STACKLEAK disabled: 10752983
  - STACKLEAK enabled: 11062221 (+2.876%)

The readelf utility shows 45602 functions in vmlinux. The STACKLEAK gcc plugin
inserted 36 check_alloca() calls and 1265 track_stack() calls (42274 calls are
inserted during GIMPLE pass and 41009 calls are deleted during RTL pass).
So 2.853% of functions are instrumented.

STACKLEAK performance impact

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".

Here are the results of a brief performance test on x86_64 (for the 2nd version
of the patch). The numbers are very different because the STACKLEAK performance
penalty depends on the workloads.

Hardware: Intel Core i7-4770, 16 GB RAM

Test #1: building the Linux kernel with Ubuntu config (time make -j9)
Result on 4.11-rc8:
  real	32m14.893s
  user	237m30.886s
  sys	11m12.273s
Result on 4.11-rc8+stackleak:
  real	32m26.881s (+0.62%)
  user	238m38.926s (+0.48%)
  sys	11m36.426s (+3.59%)

Test #2: hackbench -s 4096 -l 2000 -g 15 -f 25 -P
Average result on 4.11-rc8: 8.71s
Average result on 4.11-rc8+stackleak: 9.08s (+4.29%)

Changes in v6

1. Examined syscall entry/exit paths.
    - Added missing erase_kstack() call at ret_from_fork() for x86_32.
    - Added missing erase_kstack() call at syscall_trace_enter().
    - Solved questions previously marked with TODO.

2. Rebased onto v4.15-rc2, which includes Andy Lutomirski's entry changes.
   Andy removed sp0 from thread_struct for x86_64, which was the only issue
   during rebasing.

3. Removed the recursive BUG() in track_stack() that was caused by the code
   instrumentation. Instead, CONFIG_GCC_PLUGIN_STACKLEAK now implies
   an optimal solution.

4. Put stack erasing in syscall_trace_enter() into a separate patch and
   fixed my mistake with secure_computing() (found by Tycho Andersen).

5. After some experiments, kept the asm implementation of erase_kstack(),
   because it gives a full control over the stack for clearing it neatly
   and doesn't offend KASAN.

6. Improved the comments describing STACKLEAK.

Changes in v5 (mostly according to the feedback from Ingo Molnar)

1. Introduced the CONFIG_STACKLEAK_METRICS providing STACKLEAK information
   about tasks via the /proc file system. That information can be useful for
   estimating the STACKLEAK performance impact for different workloads.
   In particular, /proc/<pid>/lowest_stack shows the current lowest_stack
   value and its final value from the previous syscall.

2. Introduced a single check_alloca() implementation working for both
   x86_64 and x86_32.

3. Fixed coding style issues and did some refactoring in the STACKLEAK
   gcc plugin.

4. Put the gcc plugin and the kernel stack erasing into separate (working)

Changes in v4

1. Introduced the CONFIG_STACKLEAK_TRACK_MIN_SIZE parameter instead of
   hard-coded track-lowest-sp.

2. Carefully looked into the assertions in track_stack() and check_alloca().
    - Fixed the incorrect BUG() condition in track_stack(), thanks to Tycho
       Andersen. Also disabled that check if CONFIG_VMAP_STACK is enabled.
    - Fixed the surplus and erroneous code for calculating stack_left in
       check_alloca() on x86_64. That code repeats the work which is already
       done in get_stack_info() and it misses the fact that different
       exception stacks on x86_64 have different size.

3. Introduced two lkdtm tests for the STACKLEAK feature (developed together
   with Tycho Andersen).

4. Added info about STACKLEAK to Documentation/security/self-protection.rst.

5. Improved the comments.

6. Decided not to change "unsigned long sp = (unsigned long)&sp" to
   current_stack_pointer. The original variant is more platform independent
   since current_stack_pointer has different type on x86 and arm.

Changes in v3

1. Added the detailed comments describing the STACKLEAK gcc plugin.
   Read the plugin from the bottom up, like you do for Linux kernel drivers.
   Additional information:
    - gcc internals documentation available from gcc sources;
    - wonderful slides by Diego Novillo
    - nice introduction to gcc plugins at LWN

2. Improved the commit message and Kconfig description according to the
   feedback from Kees Cook. Also added the original notice describing
   the performance impact of the STACKLEAK feature.

3. Removed the arch-specific ix86_cmodel check in stackleak_track_stack_gate().
   It caused skipping the kernel code instrumentation for i386.

4. Fixed a minor mistake in stackleak_tree_instrument_execute().
   First versions of the plugin used ENTRY_BLOCK_PTR_FOR_FN(cfun)->next_bb
   to get the basic block with the function prologue. That was not correct
   since the control flow graph edge from ENTRY_BLOCK_PTR doesn't always
   go to that basic block.

   So later it was changed to single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun)),
   but not completely. next_bb was still used for entry_bb assignment,
   which could cause the wrong value of the prologue_instrumented variable.

   I've reported this issue to Grsecurity and proposed the fix for it, but
   unfortunately didn't get any reply.

5. Introduced the STACKLEAK_POISON macro and renamed the config option
   according to the feedback from Kees Cook.

Ideas for further work

 - Think of erasing stack on the way out of exception handlers (idea by
   Andy Lutomirski).
 - Think of erasing the kernel stack after invoking EFI runtime services
   (idea by Mark Rutland).
 - Try to port STACKLEAK to arm64 (Laura Abbott is working on it).

Alexander Popov (6):
  x86/entry: Add STACKLEAK erasing the kernel stack at the end of
  gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack
  x86/entry: Erase kernel stack in syscall_trace_enter()
  lkdtm: Add a test for STACKLEAK
  fs/proc: Show STACKLEAK metrics in the /proc file system
  doc: self-protection: Add information about STACKLEAK feature

 Documentation/security/self-protection.rst |  23 +-
 arch/Kconfig                               |  53 ++++
 arch/x86/Kconfig                           |   1 +
 arch/x86/entry/common.c                    |  19 +-
 arch/x86/entry/entry_32.S                  |  69 +++++
 arch/x86/entry/entry_64.S                  |  93 ++++++
 arch/x86/entry/entry_64_compat.S           |   8 +
 arch/x86/include/asm/processor.h           |   7 +
 arch/x86/kernel/asm-offsets.c              |  14 +
 arch/x86/kernel/dumpstack.c                |  15 +
 arch/x86/kernel/process_32.c               |   8 +
 arch/x86/kernel/process_64.c               |   8 +
 drivers/misc/Makefile                      |   3 +
 drivers/misc/lkdtm.h                       |   4 +
 drivers/misc/lkdtm_core.c                  |   2 +
 drivers/misc/lkdtm_stackleak.c             | 139 +++++++++
 fs/exec.c                                  |  25 ++
 fs/proc/base.c                             |  14 +
 include/linux/compiler.h                   |   5 +
 scripts/Makefile.gcc-plugins               |   3 +
 scripts/gcc-plugins/stackleak_plugin.c     | 470 +++++++++++++++++++++++++++++
 21 files changed, 971 insertions(+), 12 deletions(-)
 create mode 100644 drivers/misc/lkdtm_stackleak.c
 create mode 100644 scripts/gcc-plugins/stackleak_plugin.c


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