From f03a50938decaa601f02294e4d057fb0048ca9ca Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Wed, 27 Apr 2022 18:31:25 +0100 Subject: [PATCH] lkdtm/stackleak: prevent unexpected stack usage The lkdtm_STACKLEAK_ERASING() test is instrumentable and runs with IRQs unmasked, so it's possible for unrelated code to clobber the task stack and/or manipulate current->lowest_stack while the test is running, resulting in spurious failures. The regular stackleak erasing code is non-instrumentable and runs with IRQs masked, preventing similar issues. Make the body of the test non-instrumentable, and run it with IRQs masked, avoiding such spurious failures. Signed-off-by: Mark Rutland Cc: Alexander Popov Cc: Andrew Morton Cc: Andy Lutomirski Cc: Catalin Marinas Cc: Kees Cook Cc: Will Deacon Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20220427173128.2603085-11-mark.rutland@arm.com --- drivers/misc/lkdtm/stackleak.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) diff --git a/drivers/misc/lkdtm/stackleak.c b/drivers/misc/lkdtm/stackleak.c index 0aafa46ced7d..46c60761a05e 100644 --- a/drivers/misc/lkdtm/stackleak.c +++ b/drivers/misc/lkdtm/stackleak.c @@ -11,7 +11,20 @@ #include "lkdtm.h" #include -void lkdtm_STACKLEAK_ERASING(void) +/* + * Check that stackleak tracks the lowest stack pointer and erases the stack + * below this as expected. + * + * To prevent the lowest stack pointer changing during the test, IRQs are + * masked and instrumentation of this function is disabled. We assume that the + * compiler will create a fixed-size stack frame for this function. + * + * Any non-inlined function may make further use of the stack, altering the + * lowest stack pointer and/or clobbering poison values. To avoid spurious + * failures we must avoid printing until the end of the test or have already + * encountered a failure condition. + */ +static void noinstr check_stackleak_irqoff(void) { const unsigned long task_stack_base = (unsigned long)task_stack_page(current); const unsigned long task_stack_low = stackleak_task_low_bound(current); @@ -81,3 +94,12 @@ void lkdtm_STACKLEAK_ERASING(void) pr_info("OK: the rest of the thread stack is properly erased\n"); } } + +void lkdtm_STACKLEAK_ERASING(void) +{ + unsigned long flags; + + local_irq_save(flags); + check_stackleak_irqoff(); + local_irq_restore(flags); +}