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

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