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
