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SubjectRe: [PATCH 1/4] task_struct: Allow randomized layout
On Mon, Jun 19, 2017 at 01:56:38PM -0700, Kees Cook wrote:
> This marks most of the layout of task_struct as randomizable, but leaves
> thread_info and scheduler state untouched at the start, and thread_struct
> untouched at the end.
>
> Other parts of the kernel use unnamed structures, but the 0-day builder
> using gcc-4.4 blows up on static initializers. Officially, it's documented
> as only working on gcc 4.6 and later, which further confuses me:
> https://gcc.gnu.org/wiki/C11Status
> The structure layout randomization already requires gcc 4.7, but instead
> of depending on the plugin being enabled, just check the gcc versions
> for wider build testing. At Linus's suggestion, the marking is hidden
> in a macro to reduce how ugly it looks. Additionally, indenting is left
> unchanged since it would make things harder to read.
>
> Randomization of task_struct is modified from Brad Spengler/PaX Team's
> code in the last public patch of grsecurity/PaX based on my understanding
> of the code. Changes or omissions from the original code are mine and
> don't reflect the original grsecurity/PaX code.
>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Thanks for the Cc :/

> +#define randomized_struct_fields_start struct {
> +#define randomized_struct_fields_end } __randomize_layout;

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f833254fce00..e2ad3531e7fe 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -490,6 +490,13 @@ struct task_struct {
> #endif
> /* -1 unrunnable, 0 runnable, >0 stopped: */
> volatile long state;
> +
> + /*
> + * This begins the randomizable portion of task_struct. Only
> + * scheduling-critical items should be added above here.
> + */
> + randomized_struct_fields_start
> +
> void *stack;
> atomic_t usage;
> /* Per task flags (PF_*), defined further below: */


That now looks like:

struct task_struct {
struct thread_info thread_info; /* 0 16 */
volatile long int state; /* 16 8 */

/* XXX 40 bytes hole, try to pack */

/* --- cacheline 1 boundary (64 bytes) --- */
struct {
void * stack; /* 64 8 */
atomic_t usage; /* 72 4 */
unsigned int flags; /* 76 4 */
unsigned int ptrace; /* 80 4 */
struct llist_node wake_entry; /* 88 8 */


Can we please undo this crap?

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