Re: [kernel-hardening] [PATCH 00/18] Introduce struct layout randomization plugin

[Laura Abbott <labbott@redhat.com>]

On 04/06/2017 02:18 PM, Kees Cook wrote:
> This series brings grsecurity's structure layout randomization plugin
> to upstream. The plugin randomizes the layout of selected structures at
> compile time, as a probabilistic defense against attacks that need to
> know the layout of structures within the kernel. While less useful for
> distribution kernels (where the randomization seed must be exposed for
> third party kernel module builds), it still has some value there since
> now all kernel builds would need to be tracked by an attacker. It is
> most useful to "in-house" kernel builds where the randomization seed
> is not available to an attacker.
>     
> One requirement of the plugin is that randomized structures must use
> designated initializers. Many of these have been landing already as
> I've been sending them over the past couple months, but there are
> still some stragglers, which are included here.
> 
> Another area to address are places where randomized structures are
> cast to other structures, since there may be implicit positional
> details that need to be addressed. Luckily, there are only a few
> of these false positives, and they have been worked around either
> by adjusting the source or whitelisting them in the plugin.
> 
> The plugin selects structures in two ways: manually marked with the
> new __randomize_layout annotation, or automatically when a structure
> is found to consist entirely of function pointers (which can be opted
> out of with the new __no_randomize_layout annotation).
> 
> A structure that is especially sensitive and regularly abused in
> exploits is task_struct, but randomizing it requires some special
> handling due to some fields needing to be at the start and end. To
> deal with this, an internal anonymous struct is used to mark the
> portion that will be randomized. I'd love feedback on whether I
> should bite the bullet and perform indenting or violate indenting
> rules to avoid a massive white-space change.
> 
> As mentioned, the bulk of this feature is ported over from grsecurity.
> The implementation is almost entirely identical to the original code
> written by Brad Spengler and the PaX Team and Brad Spengler. The
> changes are addition of improved designated initializer markings,
> a whitelisting mechanism, and a different approach to handling the
> task_struct randomization.
> 
> I've been doing boot tests with instrumentation showing successfully
> changing offsets within the task_struct, which ran overnight without
> problems. So far, the 0day builder hasn't alerted on anything, but
> it's probably still a bit early.
> 
> This series is based on next-20170404.
> 
> Patches are:
> 
> [PATCH 01/18] gcc-plugins: Add the randstruct plugin
> 	The plugin itself, with struct auto-detection disabled.
> 
> [PATCH 02/18] compiler: Add __designated_init annotation
> [PATCH 03/18] randstruct: Set designated_init attribute
> 	Adds marking of structures needing designated initialization.
> 	
> [PATCH 04/18] randstruct: Differentiate bad cast warnings
> 	Minor clarifications to bad cast warning output.
> 
> [PATCH 05/18] af_unix: Use designated initializers
> 	Designated initializer fix for af_unix (taken for -next already)
> 	https://lkml.org/lkml/2017/4/6/846
> 
> [PATCH 06/18] NFS: Avoid cross-structure casting
> 	Avoids a false positive in casting (waiting for feedback)
> 	https://lkml.org/lkml/2017/4/5/530
> 
> [PATCH 07/18] randstruct: Whitelist struct security_hook_heads cast
> [PATCH 08/18] randstruct: Whitelist UNIXCB cast
> 	Whitelist two more false positive cases where source-level
> 	fixes aren't obvious/possible.
> 
> [PATCH 09/18] randstruct: Mark various structs for randomization
> 	Adds the manual annotation for structures to randomize.
> 
> [PATCH 10/18] scsi/bfa: use designated initializers
> [PATCH 11/18] scsi: qedi,qedf: Use designated initializers
> [PATCH 12/18] ovl: Use designated initializers
> 	The remaining designated initializer fixes for automatic
> 	struct randomization.
> 
> [PATCH 13/18] randstruct: opt-out externally exposed function pointer
> 	Opt out of some externally-exposed structs that would be
> 	otherwise automatically randomized.
> 
> [PATCH 14/18] randstruct: Disable randomization of ACPICA structs
> 	Temporary disabling of automatic randomization of ACPICA struct.
> 	
> [PATCH 15/18] randstruct: Enable function pointer struct detection
> 	Enables automatic struct randomization.
> 
> [PATCH 16/18] task_struct: Allow randomized layout
> 	Adds selected portion of task_struct to be randomized.
> 
> [PATCH 17/18] sgi-xp: Use designated initializers
> 	Enable randomization of sgi-xp struct, pending feedback.
> 	https://lkml.org/lkml/2017/3/29/808
> 
> [PATCH 18/18] ACPICA: Use designated initializers
> 	Enable randomization of ACPICA struct, pending feedback.
> 	https://github.com/acpica/acpica/pull/248/
> 
> Testing/feedback appreciated!
> 
> -Kees
> 

I gave this a quick spin on my aarch64 Rawhide machine. I needed to
change the following to work with gcc7

diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c b/scripts/gcc-plugins/randomize_layout_plugin.c
index a2d7e933c33f..8dd5134f161a 100644
--- a/scripts/gcc-plugins/randomize_layout_plugin.c
+++ b/scripts/gcc-plugins/randomize_layout_plugin.c
@@ -529,8 +529,8 @@ static void randomize_layout_finish_decl(void *event_data, void *data)
 
        DECL_SIZE(decl) = 0;
        DECL_SIZE_UNIT(decl) = 0;
-       DECL_ALIGN(decl) = 0;
-       DECL_MODE (decl) = VOIDmode;
+       SET_DECL_ALIGN(decl, 0);
+       SET_DECL_MODE(decl, VOIDmode);
        SET_DECL_RTL(decl, 0);
        update_decl_size(decl);
        layout_decl(decl, 0);

It boots but dies with a bunch of kernel faults somewhere after reaching
userspace. I'll pick up the debugging next week.

Thanks,
Laura