Re: Why two stacks per process?

[Patrick McManus <mcmanus@xxxxxxxxxxxx>]

On Fri, 2008-04-25 at 00:54 +0800, Peter Teoh wrote:

> this violate the first principle mentioned above.....the current CPU
> may be processing in kernel mode, but another CPU may be processing in
> userspace mode, same process, and as all the userspace memory are
> visible to him, he can view the kernel's data.   


yes! and vice versa - this other thread can also write to the shared
stack - changing this hypothetical shared user/kernel stack while the
kernel is using it on a different thread certainly has very direct
security implications...


T       kernel code                                       userspace code
-- --------------------------------------              -----------------------
1  int access_check;
2   
3  access_check = security_check(foobar);
4                                                         access_check = 1;
5  if (access_check != 0)
6     do_something_scary();


I think its pretty clear that you can't share the userspace stack
effectively. As for having 1 in kernel space for every userspace stack,
instead of some kind of sharing there certainly seem to be advantages to
1:1 but it isn't the only approach.

-Patrick
--
www.ducksong.com





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