kernel/sys.c: fix stack memory content leak via UNAME26

Calling uname() with the UNAME26 personality set allows a leak of kernel
stack contents.  This fixes it by defensively calculating the length of
copy_to_user() call, making the len argument unsigned, and initializing
the stack buffer to zero (now technically unneeded, but hey, overkill).

CVE-2012-0957

Reported-by: PaX Team <pageexec@freemail.hu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Brad Spengler <spender@grsecurity.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
void kernel_halt(void)
{
	kernel_shutdown_prepare(SYSTEM_HALT);
	syscore_shutdown();
	printk(KERN_EMERG "System halted.\n");
	kmsg_dump(KMSG_DUMP_HALT);
	machine_halt();
}

void kernel_halt(void)
{
	kernel_shutdown_prepare(SYSTEM_HALT);
	syscore_shutdown();
	printk(KERN_EMERG "System halted.\n");
	kmsg_dump(KMSG_DUMP_HALT);
	machine_halt();
}
