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Date: Thu, 4 Apr 2019 09:56:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Khalid Aziz <khalid.aziz@...cle.com>
Cc: juergh@...il.com, tycho@...ho.ws, jsteckli@...zon.de,
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	Khalid Aziz <khalid@...ehiking.org>,
	kernel-hardening@...ts.openwall.com,
	"Vasileios P . Kemerlis" <vpk@...columbia.edu>,
	Juerg Haefliger <juerg.haefliger@...onical.com>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC PATCH v9 11/13] xpfo, mm: optimize spinlock usage in
 xpfo_kunmap

On Wed, Apr 03, 2019 at 11:34:12AM -0600, Khalid Aziz wrote:
> From: Julian Stecklina <jsteckli@...zon.de>
> 
> Only the xpfo_kunmap call that needs to actually unmap the page
> needs to be serialized. We need to be careful to handle the case,
> where after the atomic decrement of the mapcount, a xpfo_kmap
> increased the mapcount again. In this case, we can safely skip
> modifying the page table.
> 
> Model-checked with up to 4 concurrent callers with Spin.
> 
> Signed-off-by: Julian Stecklina <jsteckli@...zon.de>
> Signed-off-by: Khalid Aziz <khalid.aziz@...cle.com>
> Cc: Khalid Aziz <khalid@...ehiking.org>
> Cc: x86@...nel.org
> Cc: kernel-hardening@...ts.openwall.com
> Cc: Vasileios P. Kemerlis <vpk@...columbia.edu>
> Cc: Juerg Haefliger <juerg.haefliger@...onical.com>
> Cc: Tycho Andersen <tycho@...ho.ws>
> Cc: Marco Benatto <marco.antonio.780@...il.com>
> Cc: David Woodhouse <dwmw2@...radead.org>
> ---
>  include/linux/xpfo.h | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/xpfo.h b/include/linux/xpfo.h
> index 2318c7eb5fb7..37e7f52fa6ce 100644
> --- a/include/linux/xpfo.h
> +++ b/include/linux/xpfo.h
> @@ -61,6 +61,7 @@ static inline void xpfo_kmap(void *kaddr, struct page *page)
>  static inline void xpfo_kunmap(void *kaddr, struct page *page)
>  {
>  	unsigned long flags;
> +	bool flush_tlb = false;
>  
>  	if (!static_branch_unlikely(&xpfo_inited))
>  		return;
> @@ -72,18 +73,23 @@ static inline void xpfo_kunmap(void *kaddr, struct page *page)
>  	 * The page is to be allocated back to user space, so unmap it from
>  	 * the kernel, flush the TLB and tag it as a user page.
>  	 */
> -	spin_lock_irqsave(&page->xpfo_lock, flags);
> -
>  	if (atomic_dec_return(&page->xpfo_mapcount) == 0) {
> -#ifdef CONFIG_XPFO_DEBUG
> -		WARN_ON(PageXpfoUnmapped(page));
> -#endif
> -		SetPageXpfoUnmapped(page);
> -		set_kpte(kaddr, page, __pgprot(0));
> -		xpfo_flush_kernel_tlb(page, 0);
> +		spin_lock_irqsave(&page->xpfo_lock, flags);
> +
> +		/*
> +		 * In the case, where we raced with kmap after the
> +		 * atomic_dec_return, we must not nuke the mapping.
> +		 */
> +		if (atomic_read(&page->xpfo_mapcount) == 0) {
> +			SetPageXpfoUnmapped(page);
> +			set_kpte(kaddr, page, __pgprot(0));
> +			flush_tlb = true;
> +		}
> +		spin_unlock_irqrestore(&page->xpfo_lock, flags);
>  	}
>  
> -	spin_unlock_irqrestore(&page->xpfo_lock, flags);
> +	if (flush_tlb)
> +		xpfo_flush_kernel_tlb(page, 0);
>  }

This doesn't help with the TLB invalidation issue, AFAICT this is still
completely buggered. kunmap_atomic() can be called from IRQ context.

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