@@ -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)) { \
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> --- include/linux/randomize_kstack.h | 27 ++++++++++++++++++++++++--- 1 file changed, 24 insertions(+), 3 deletions(-)