[PATCH 2/2] arm64: Clear the stack

[Alexander Popov]

On 14.05.2018 13:06, Mark Rutland wrote:
> On Mon, May 14, 2018 at 12:35:25PM +0300, Alexander Popov wrote:
>> On 14.05.2018 08:15, Mark Rutland wrote:
>>> On Sun, May 13, 2018 at 11:40:07AM +0300, Alexander Popov wrote:
>>>> 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.
> 
> On arm64 with CONFIG_VMAP_STACK, a stack overflow will result in a
> panic(). My understanding was that the same is true on x86.

No, x86 CONFIG_VMAP_STACK only kills the offending process. I see it on my deep
recursion test, the kernel continues to live. handle_stack_overflow() in
arch/x86/kernel/traps.c calls die().

>> 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().
> 
> In this case, panic() can also corrupt the neighbour stack, and could
> also fail.
> 
> When CONFIG_VMAP_STACK is not selected, a stack overflow simply cannot
> be handled reliably -- while panic() may be more likely to succeed, it
> is not gauranteed to.
>
>> 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().
> 
> Sure, I'm certainly happy with panic() here.

Ok!

>> 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. 
> 
> I believe that CONFIG_SCHED_STACK_END_CHECK is seen as a debug feature,
> and hence people don't select it. 

I see CONFIG_SCHED_STACK_END_CHECK enabled by default in Ubuntu config...

> I strongly doubt that people have
> reasons to disable it other than not wanting the overhead associated
> with debug features.

I think it's not a question of performance here. There are cases when a system
must live as long as possible (even partially corrupted) and must not die
entirely. Oops is ok for those systems, but panic (full DoS) is not.

> I think it is reasonable to panic() here even with CONFIG_VMAP_STACK
> selected.

It's too tough for CONFIG_VMAP_STACK on x86 - the system can proceed to live.
Anyway, the check_alloca() code will not be shared between x86 and arm64, I've
described the reasons in this thread. So I can have BUG() for CONFIG_VMAP_STACK
on x86 and Laura can consistently use panic() on arm64.

>> So we should not do it in check_alloca() as well, just use BUG() and
>> hope for the best.
> 
> Regardless of whether we BUG() or panic(), we're hoping for the best.
> 
> Consistently using panic() here will keep things simpler, so any failure
> reported will be easier to reason about, and easier to debug.

Let me keep BUG() for !CONFIG_SCHED_STACK_END_CHECK. I beware of using panic()
by default, let distro/user decide this. I remember very well how I was shouted
at, when this one was merged:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce6fa91b93630396ca220c33dd38ffc62686d499


Mark, I'm really grateful to you for such a nice code review!
Alexander