On 14.05.2018 08:15, Mark Rutland wrote:

On Sun, May 13, 2018 at 11:40:07AM +0300, Alexander Popov wrote:

quoted

It seems that previously I was very "lucky" to accidentally have those MIN_STACK_LEFT,
call trace depth and oops=panic together to experience a hang on stack overflow
during BUG().


When I run my test in a loop _without_ VMAP_STACK, I manage to corrupt the neighbour
processes with BUG() handling overstepping the stack boundary. It's a pity, but
I have an idea.

I think that in the absence of VMAP_STACK, there will always be cases where we
*could* corrupt a neighbouring stack, but I agree that trying to minimize that
possibility would be good.

Ok!

quoted

In kernel/sched/core.c we already have:

#ifdef CONFIG_SCHED_STACK_END_CHECK
  	if (task_stack_end_corrupted(prev))
		panic("corrupted stack end detected inside scheduler\n");
#endif

So what would you think if I do the following in check_alloca():

	if (size >= stack_left) {
#if !defined(CONFIG_VMAP_STACK) && defined(CONFIG_SCHED_STACK_END_CHECK)
		panic("alloca over the kernel stack boundary\n");
#else
		BUG();
#endif

Given this is already out-of-line, how about we always use panic(), regardless
of VMAP_STACK and SCHED_STACK_END_CHECK? i.e. just

	if (unlikely(size >= stack_left))
		panic("alloca over the kernel stack boundary");

If we have VMAP_STACK selected, and overflow during the panic, it's the same as
if we overflowed during the BUG(). It's likely that panic() will use less stack
space than BUG(), and the compiler can put the call in a slow path that
shouldn't affect most calls, so in all cases it's likely preferable.

I'm sure that maintainers and Linus will strongly dislike my patch if I always
use panic() here. panic() kills the whole kernel and we shouldn't use it when we
can safely continue to work.

Let me describe my logic. So let's have size >= stack_left on a thread stack.

1. If CONFIG_VMAP_STACK is enabled, we can safely use BUG(). Even if BUG()
handling overflows the thread stack into the guard page, handle_stack_overflow()
is called and the neighbour memory is not corrupted. The kernel can proceed to live.

2. If CONFIG_VMAP_STACK is disabled, BUG() handling can corrupt the neighbour
kernel memory and cause the undefined behaviour of the whole kernel. I see it on
my lkdtm test. That is a cogent reason for panic().

2.a. If CONFIG_SCHED_STACK_END_CHECK is enabled, the kernel already does panic()
when STACK_END_MAGIC is corrupted. So we will _not_ break the safety policy if
we do panic() in a similar situation in check_alloca().

2.b. If CONFIG_SCHED_STACK_END_CHECK is disabled, the user has some real reasons
not to do panic() when the kernel stack is corrupted. So we should not do it in
check_alloca() as well, just use BUG() and hope for the best.

That logic can be expressed this way:

	if (size >= stack_left) {
#if !defined(CONFIG_VMAP_STACK) && defined(CONFIG_SCHED_STACK_END_CHECK)
		panic("alloca over the kernel stack boundary\n");
#else
		BUG();
#endif

I think I should add a proper comment to describe it.

Thank you.

Best regards,
Alexander