linux-kernel - Re: [PATCH 2/3] xen: correct race in alloc_p2m

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Date:	Fri, 9 Jan 2015 15:09:30 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xensource.com>, <konrad.wilk@...cle.com>,
	<boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 2/3] xen: correct race in alloc_p2m_pmd()

On 08/01/15 17:01, Juergen Gross wrote:
> When allocating a new pmd for the linear mapped p2m list a check is
> done for not introducing another pmd when this just happened on
> another cpu. In this case the old pte pointer was returned which
> points to the p2m_missing or p2m_identity page. The correct value
> would be the pointer to the found new page.

Looking at the check I don't see how it works at all.

alloc_p2m() looks up the address of the PTE page

	ptep = lookup_address(addr, &level);
	pte_pg = (pte_t *)((unsigned long)ptep & ~(PAGE_SIZE - 1));

But the check under the lock that is still true does:

	ptechk = lookup_address(vaddr, &level);
	if (ptechk == pte_pg) {

So I don't see how this works unless (by chance) we happen to get the
first entry in the PTE page.

It also doesn't handle racing with p2m_missing_pte to p2m_identity_pte
(or vice-versa) change.

Something like:

	ptechk = lookup_address(vaddr, &level);
	ptechk = (pte_t *)((unsigned long)ptechk & ~(PAGE_SIZE - 1));
	if (ptechk == p2m_missing_pte || ptechk == p2m_identity) {
              set_pmd(..)

Perhaps?

David

> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -440,10 +440,9 @@ EXPORT_SYMBOL_GPL(get_phys_to_machine);
>   * a new pmd is to replace p2m_missing_pte or p2m_identity_pte by a individual
>   * pmd. In case of PAE/x86-32 there are multiple pmds to allocate!
>   */
> -static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
> +static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *pte_pg)
>  {
>  	pte_t *ptechk;
> -	pte_t *pteret = ptep;
>  	pte_t *pte_newpg[PMDS_PER_MID_PAGE];
>  	pmd_t *pmdp;
>  	unsigned int level;
> @@ -477,8 +476,6 @@ static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
>  		if (ptechk == pte_pg) {
>  			set_pmd(pmdp,
>  				__pmd(__pa(pte_newpg[i]) | _KERNPG_TABLE));
> -			if (vaddr == (addr & ~(PMD_SIZE - 1)))
> -				pteret = pte_offset_kernel(pmdp, addr);
>  			pte_newpg[i] = NULL;
>  		}
>  
> @@ -492,7 +489,7 @@ static pte_t *alloc_p2m_pmd(unsigned long addr, pte_t *ptep, pte_t *pte_pg)
>  		vaddr += PMD_SIZE;
>  	}
>  
> -	return pteret;
> +	return lookup_address(addr, &level);
>  }
>  
>  /*
> @@ -521,7 +518,7 @@ static bool alloc_p2m(unsigned long pfn)
>  
>  	if (pte_pg == p2m_missing_pte || pte_pg == p2m_identity_pte) {
>  		/* PMD level is missing, allocate a new one */
> -		ptep = alloc_p2m_pmd(addr, ptep, pte_pg);
> +		ptep = alloc_p2m_pmd(addr, pte_pg);
>  		if (!ptep)
>  			return false;
>  	}
> 

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