Re: [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack

[Joerg Roedel]

On Fri, Jul 13, 2018 at 04:31:02PM -0700, Andy Lutomirski wrote:
> What you're really doing is keeping it available for an extra flag.
> Please update the comment as such.  But see below.

Thanks, will do.

> > +.macro PARANOID_EXIT_TO_KERNEL_MODE
> > +
> > +       /*
> > +        * Test if we entered the kernel with the entry-stack. Most
> > +        * likely we did not, because this code only runs on the
> > +        * return-to-kernel path.
> > +        */
> > +       testl   $CS_FROM_ENTRY_STACK, PT_CS(%esp)
> > +       jz      .Lend_\@
> > +
> > +       /* Unlikely slow-path */
> > +
> > +       /* Clear marker from stack-frame */
> > +       andl    $(~CS_FROM_ENTRY_STACK), PT_CS(%esp)
> > +
> > +       /* Copy the remaining task-stack contents to entry-stack */
> > +       movl    %esp, %esi
> > +       movl    PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %edi
> 
> I'm confused.  Why do we need any special handling here at all?  How
> could we end up with the contents of the stack frame we interrupted in
> a corrupt state?
> 
> I guess I don't understand why this patch is needed.

The patch is needed because we can get exceptions in kernel-mode while
we are already on user-cr3 and entry-stack. In this case we need to
return with user-cr3 and entry-stack to the kernel too, otherwise we
would go to user-space with kernel-cr3.

So based on that, I did the above because the entry-stack is a per-cpu
data structure and I am not sure that we always return from the exception
on the same CPU where we got it. Therefore the path is called
PARANOID_... :)


Regards,

	Joerg