Re: frequent lockups in 3.18rc4

From: Andy Lutomirski
Date: Wed Nov 19 2014 - 19:49:31 EST

On Wed, Nov 19, 2014 at 4:40 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Nov 19, 2014 at 4:30 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> This is why I think that the grsec kernels will crash on very large
>> memory systems. They don't seem to get this right for the kernel
>> stack, and a page fault trying to access the stack is a big no-no.
> For something like a stack, that's trivial, you could just probe it
> before the actual task switch.

I thought so for a while, too, but now I disagree. On PGE hardware,
it seems entirely possible that the new stack would be in the TLB even
if it's not visible via cr3. Then, as soon as the TLB entry expires,
we double-fault.

> So I wouldn't worry about the kernel stack itself (although I think
> vmallocing it isn't likely worth it),

I don't want vmalloc to avoid low-order allocations -- I want it to
have guard pages. The fact that a user-triggerable stack overflow is
basically root right now and doesn't reliably OOPS scares me.

> I'd worry more about some other
> random dynamic percpu allocation. Although they arguably shouldn't
> happen for low-level code that cannot handle the dynamic
> pgd-population. And they generally don't.

This issue ought to be limited to nokprobes code, and I doubt that any
of that code touches dynamic per-cpu things.

> It's really tracing that tends to be a special case not because of any
> particular low-level code issue, but because instrumenting itself
> recursively tends to be a bad idea.
> Linus

Andy Lutomirski
AMA Capital Management, LLC
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