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Message-ID: <0bb962c4-92fb-156b-cee3-df48aff0552d@redhat.com>
Date: Tue, 10 Oct 2017 18:19:00 -0700
From: Laura Abbott <labbott@...hat.com>
To: Alexander Popov <alex.popov@...ux.com>,
kernel-hardening@...ts.openwall.com, keescook@...omium.org,
pageexec@...email.hu, spender@...ecurity.net, tycho@...ker.com,
Mark Rutland <mark.rutland@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Andy Lutomirski <luto@...capital.net>, x86@...nel.org
Subject: Re: [PATCH RFC v4 0/3] Introduce the STACKLEAK feature and a test for
it
On 10/04/2017 03:55 PM, Alexander Popov wrote:
> 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
> https://www.airs.com/dnovillo/200711-GCC-Internals/
> - nice introduction to gcc plugins at LWN
> https://lwn.net/Articles/457543/
>
> 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
> syscalls
> 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
>
I tried this series with CVE-2017-14954 . That particular bug
is not helped here because the poisoning has already been
overwritten by other leaf functions. Given the call stack this
may be a particularly bad case but I'm wondering how common
this might be if we're only erasing at the end of a system
call. One previous copy_to_user which has to go through the
fault path can get fairly deep.
Thanks,
Laura
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