Re: [PATCH v11 01/12] add support for Clang's Shadow Call Stack (SCS)

[Will Deacon]

On Mon, Apr 27, 2020 at 01:45:46PM -0700, Sami Tolvanen wrote:
> On Fri, Apr 24, 2020 at 12:21:14PM +0100, Will Deacon wrote:
> > Also, since you mentioned the lack of redzoning, isn't it a bit dodgy
> > allocating blindly out of the kmem_cache? It means we don't have a redzone
> > or a guard page, so if you can trigger something like a recursion bug then
> > could you scribble past the SCS before the main stack overflows? Would this
> > clobber somebody else's SCS?
> 
> I agree that allocating from a kmem_cache isn't ideal for safety. It's a
> compromise to reduce memory overhead.

Do you think it would be a problem if we always allocated a page for the
SCS?

> > The vmap version that I asked Sami to drop
> > is at least better in this regard, although the guard page is at the wrong
> > end of the stack and we just hope that the allocation below us didn't pass
> > VM_NO_GUARD. Looks like the same story for vmap stack :/
> 
> SCS grows up and the guard page is after the allocation, so how is it at
> the wrong end? Am I missing something here?

Sorry, I'd got the SCS upside-down in my head (hey, that second 'S' stands
for 'Stack'!). But I think I'm right about vmap stack, which feels a
little fragile even though it seems to work out today with the very limited
uses of VM_NO_GUARD.

> > If we split the pointer in two (base, offset) then we could leave the
> > base live in the thread_info, not require alignment of the stacks (which
> > may allow for unconditional redzoning?) and then just update the offset
> > value on context switch, which could be trivially checked as part of the
> > existing stack overflow checking on kernel entry.
> 
> I sent out v13 with split pointers, but I'm not sure it's convenient to
> add an overflow check to kernel_ventry where the VMAP_STACK check is
> done. I suppose I could add a check to kernel_entry after we load x18
> from tsk. Thoughts?

I'll take a look at v13, since at this stage I'm keen to get something
queued up so that we can use it as a base for further improvements without
you having to repost the whole stack every time.

Cheers,

Will