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Date: Sat, 28 Nov 2015 10:19:45 +0100
From: Quentin Casasnovas <quentin.casasnovas@...cle.com>
To: kernel-hardening@...ts.openwall.com
Cc: Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: Re: status: GRKERNSEC_KSTACKOVERFLOW
On Fri, Nov 27, 2015 at 12:09:31PM -0800, Kees Cook wrote:
> On Thu, Nov 26, 2015 at 8:39 AM, Quentin Casasnovas
> <quentin.casasnovas@...cle.com> wrote:
> > On Thu, Nov 26, 2015 at 12:45:42AM +0100, Quentin Casasnovas wrote:
> >> On Tue, Nov 24, 2015 at 11:10:09AM -0800, Kees Cook wrote:
> >>
> >> [snip/]
> >>
> >> It should also be noted that I did not find that the struct thread_info
> >> (which is stuffed at the end of the stack) was protected in any way either.
> >> So even if a write/read _below_ the stack could still be trapped if nothing
> >> is currently mapped there, it looks like deep stack usage could still
> >> overflow it and go unoticed. Here again, I didn't spend a lot of time on
> >> this and it might just be that I'm missing something.
> >>
> >> In the very unlikely event where I didn't miss anything and the struct
> >> thread_info can still be overflown and there isn't any guard page, maybe we
> >> can improve on the current KSTACK_OVERFLOW feature by putting the struct
> >> thread_info on a different page than the kernel stack, and not vmap() it
> >> like the rest of the stack pages, but instead map a PROT_NONE page there.
> >> That would mean the struct thread_info can still be accessed by using its
> >> lowmem mapping (i.e. legit usage from the kernel) but not by deep kernel
> >> stack usage. Maybe the cost of adding an extra page per kernel stack is
> >> too high though.
> >
> > As expected I missed some other changes:
> >
> > /* Load thread_info address into "reg" */
> > #define GET_THREAD_INFO(reg) \
> > - _ASM_MOV PER_CPU_VAR(cpu_current_top_of_stack),reg ; \
> > - _ASM_SUB $(THREAD_SIZE),reg ;
> > + _ASM_MOV PER_CPU_VAR(current_tinfo),reg ;
> >
> > and
> >
> > +DECLARE_PER_CPU(struct thread_info *, current_tinfo);
> > +
> > static inline struct thread_info *current_thread_info(void)
> > {
> > - return (struct thread_info *)(current_top_of_stack() - THREAD_SIZE);
> > + return this_cpu_read_stable(current_tinfo);
> > }
> >
> > So no more thread_info on the stack in the default configuration, which
> > isn't correlated with the KSTACKOVERFLOW config option.
>
> Good find! This seems like it should be its own patch, distinct from
> KSTACKOVERFLOW?
>
We should probably make KSTACKOVERFLOW depend on moving the thread_info off
the stack, since otherwise the thread_info could still be hijacked and that
would counterfeit the KSTACKOVERFLOW purpose.
Quentin
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