lkdtm/stackleak: check stack boundaries

The stackleak code relies upon the current SP and lowest recorded SP
falling within expected task stack boundaries.

Check this at the start of the test.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220427173128.2603085-12-mark.rutland@arm.com

drivers/misc/lkdtm/stackleak.c

@@ -35,6 +35,25 @@ static void noinstr check_stackleak_irqoff(void)
 	unsigned long poison_high, poison_low;
 	bool test_failed = false;
 
+	/*
+	 * Check that the current and lowest recorded stack pointer values fall
+	 * within the expected task stack boundaries. These tests should never
+	 * fail unless the boundaries are incorrect or we're clobbering the
+	 * STACK_END_MAGIC, and in either casee something is seriously wrong.
+	 */
+	if (current_sp < task_stack_low || current_sp >= task_stack_high) {
+		pr_err("FAIL: current_stack_pointer (0x%lx) outside of task stack bounds [0x%lx..0x%lx]\n",
+		       current_sp, task_stack_low, task_stack_high - 1);
+		test_failed = true;
+		goto out;
+	}
+	if (lowest_sp < task_stack_low || lowest_sp >= task_stack_high) {
+		pr_err("FAIL: current->lowest_stack (0x%lx) outside of task stack bounds [0x%lx..0x%lx]\n",
+		       lowest_sp, task_stack_low, task_stack_high - 1);
+		test_failed = true;
+		goto out;
+	}
+
 	/*
 	 * Depending on what has run prior to this test, the lowest recorded
 	 * stack pointer could be above or below the current stack pointer.
@@ -87,6 +106,7 @@ static void noinstr check_stackleak_irqoff(void)
 		poison_high - task_stack_low,
 		task_stack_low - task_stack_base);
 
+out:
 	if (test_failed) {
 		pr_err("FAIL: the thread stack is NOT properly erased!\n");
 		pr_expected_config(CONFIG_GCC_PLUGIN_STACKLEAK);