randomize_kstack: Improve docs on requirements/rationale

There were some recent questions about where and why to use the
random_kstack routines when applying them to new architectures[1].
Update the header comments to reflect the design choices for the
routines.

[1] https://lore.kernel.org/lkml/1652173338.7bltwybi0c.astroid@bobo.none

Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

include/linux/randomize_kstack.h

@@ -40,10 +40,14 @@ DECLARE_PER_CPU(u32, kstack_offset);
  */
 #define KSTACK_OFFSET_MAX(x)	((x) & 0x3FF)
 
-/*
- * These macros must be used during syscall entry when interrupts and
+/**
+ * add_random_kstack_offset - Increase stack utilization by previously
+ *			      chosen random offset
+ *
+ * This should be used in the syscall entry path when interrupts and
  * preempt are disabled, and after user registers have been stored to
- * the stack.
+ * the stack. For testing the resulting entropy, please see:
+ * tools/testing/selftests/lkdtm/stack-entropy.sh
  */
 #define add_random_kstack_offset() do {					\
 	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
@@ -55,6 +59,23 @@ DECLARE_PER_CPU(u32, kstack_offset);
 	}								\
 } while (0)
 
+/**
+ * choose_random_kstack_offset - Choose the random offset for the next
+ *				 add_random_kstack_offset()
+ *
+ * This should only be used during syscall exit when interrupts and
+ * preempt are disabled. This position in the syscall flow is done to
+ * frustrate attacks from userspace attempting to learn the next offset:
+ * - Maximize the timing uncertainty visible from userspace: if the
+ *   offset is chosen at syscall entry, userspace has much more control
+ *   over the timing between choosing offsets. "How long will we be in
+ *   kernel mode?" tends to be more difficult to predict than "how long
+ *   will we be in user mode?"
+ * - Reduce the lifetime of the new offset sitting in memory during
+ *   kernel mode execution. Exposure of "thread-local" memory content
+ *   (e.g. current, percpu, etc) tends to be easier than arbitrary
+ *   location memory exposure.
+ */
 #define choose_random_kstack_offset(rand) do {				\
 	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
 				&randomize_kstack_offset)) {		\