Re: [PATCH] mm: mmu_notifier: fix inconsistent memory betweensecondary MMU and host

[Andrea Arcangeli]

On Tue, Aug 21, 2012 at 05:46:39PM +0800, Xiao Guangrong wrote:
> There has a bug in set_pte_at_notify which always set the pte to the
> new page before release the old page in secondary MMU, at this time,
> the process will access on the new page, but the secondary MMU still
> access on the old page, the memory is inconsistent between them
> 
> Below scenario shows the bug more clearly:
> 
> at the beginning: *p = 0, and p is write-protected by KSM or shared with
> parent process
> 
> CPU 0                                       CPU 1
> write 1 to p to trigger COW,
> set_pte_at_notify will be called:
>   *pte = new_page + W; /* The W bit of pte is set */
> 
>                                      *p = 1; /* pte is valid, so no #PF */
> 
>                                      return back to secondary MMU, then
>                                      the secondary MMU read p, but get:
>                                      *p == 0;
> 
>                          /*
>                           * !!!!!!
>                           * the host has already set p to 1, but the secondary
>                           * MMU still get the old value 0
>                           */
> 
>   call mmu_notifier_change_pte to release
>   old page in secondary MMU

The KSM usage of it looks safe because it will only establish readonly
ptes with it.

It seems a problem only for do_wp_page. It wasn't safe to setup
writable ptes with it. I guess we first introduced it for KSM and then
we added it to do_wp_page too by mistake.

The race window is really tiny, it's unlikely it has ever triggered,
however this one seem to be possible so it's slightly more serious
than the other race you recently found (the previous one in the exit
path I think it was impossible to trigger with KVM).

> We can fix it by release old page first, then set the pte to the new
> page.
> 
> Note, the new page will be firstly used in secondary MMU before it is
> mapped into the page table of the process, but this is safe because it
> is protected by the page table lock, there is no race to change the pte
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/mmu_notifier.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index 1d1b1e1..8c7435a 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -317,8 +317,8 @@ static inline void mmu_notifier_mm_destroy(struct mm_struct *mm)
>  	unsigned long ___address = __address;				\
>  	pte_t ___pte = __pte;						\
>  									\
> -	set_pte_at(___mm, ___address, __ptep, ___pte);			\
>  	mmu_notifier_change_pte(___mm, ___address, ___pte);		\
> +	set_pte_at(___mm, ___address, __ptep, ___pte);			\
>  })

If we establish the spte on the new page, what will happen is the same
race in reverse. The fundamental problem is that the first guy that
writes to the "newpage" (guest or host) won't fault again and so it
will fail to serialize against the PT lock.

CPU0  		    	    	CPU1
				oldpage[1] == 0 (both guest & host)
oldpage[0] = 1
trigger do_wp_page
mmu_notifier_change_pte
spte = newpage + writable
				guest does newpage[1] = 1
				vmexit
				host read oldpage[1] == 0
pte = newpage + writable (too late)

I think the fix is to use ptep_clear_flush_notify whenever
set_pte_at_notify will establish a writable pte/spte. If the pte/spte
established by set_pte_at_notify/change_pte is readonly we don't need
to do the ptep_clear_flush_notify instead because when the host will
write to the page that will fault and serialize against the
PT lock (set_pte_at_notify must always run under the PT lock of course).

How about this:

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