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Date: Thu, 26 Nov 2015 17:39:20 +0100
From: Quentin Casasnovas <quentin.casasnovas@...cle.com>
To: kernel-hardening@...ts.openwall.com
Cc: Kees Cook <keescook@...omium.org>,
Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: Re: status: GRKERNSEC_KSTACKOVERFLOW
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.
Quentin
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