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Date: Thu,  5 Oct 2017 01:55:00 +0300
From: Alexander Popov <>
	Laura Abbott <>,
	Mark Rutland <>,
	Ard Biesheuvel <>,
	Andy Lutomirski <>,,
Subject: [PATCH RFC v4 0/3] Introduce the STACKLEAK feature and a test for it

This is the 4th version of the patch 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 by some kernel stack leak bugs
    (e.g. CVE-2016-4569 and CVE-2016-4578);
 - blocks some uninitialized stack variable attacks (e.g. CVE-2010-2963);
 - introduces some runtime checks for kernel stack overflow detection.

Further work:
 - think of erasing the kernel stack after invoking EFI runtime services;
 - try to port STACKLEAK to arm64 (Laura Abbott is working on it).

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).

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 bottom up, like you do for Linux kernel drivers.
   Additional information:
    - gcc internals documentation, which is 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 the
   feedback from Kees Cook. Also added the original notice describing
   the performance impact of the STACKLEAK feature.

3. Removed arch-specific ix86_cmodel check from 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 the ENTRY_BLOCK_PTR doesn't always
   go to that basic block.

   So later it was changed it 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 the feedback from Kees Cook.

Alexander Popov (3):
  gcc-plugins: Add STACKLEAK erasing the kernel stack at the end of
  lkdtm: Add a test for STACKLEAK
  doc: self-protection: Add information about STACKLEAK feature

 Documentation/security/self-protection.rst |  23 +-
 arch/Kconfig                               |  39 +++
 arch/x86/Kconfig                           |   1 +
 arch/x86/entry/common.c                    |  17 +-
 arch/x86/entry/entry_32.S                  |  69 +++++
 arch/x86/entry/entry_64.S                  |  95 +++++++
 arch/x86/entry/entry_64_compat.S           |   8 +
 arch/x86/include/asm/processor.h           |   4 +
 arch/x86/kernel/asm-offsets.c              |   9 +
 arch/x86/kernel/dumpstack_32.c             |  12 +
 arch/x86/kernel/dumpstack_64.c             |  15 ++
 arch/x86/kernel/process_32.c               |   5 +
 arch/x86/kernel/process_64.c               |   5 +
 drivers/misc/Makefile                      |   3 +
 drivers/misc/lkdtm.h                       |   4 +
 drivers/misc/lkdtm_core.c                  |   2 +
 drivers/misc/lkdtm_stackleak.c             | 139 ++++++++++
 fs/exec.c                                  |  30 +++
 include/linux/compiler.h                   |   4 +
 scripts/Makefile.gcc-plugins               |   3 +
 scripts/gcc-plugins/stackleak_plugin.c     | 397 +++++++++++++++++++++++++++++
 21 files changed, 872 insertions(+), 12 deletions(-)
 create mode 100644 drivers/misc/lkdtm_stackleak.c
 create mode 100644 scripts/gcc-plugins/stackleak_plugin.c


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