Re: [Xen-devel] [PATCH 2/3] xen: correct race in alloc_p2m_pmd()

[Juergen Gross <jgross@xxxxxxxx>]

On 01/09/2015 04:09 PM, David Vrabel wrote:
> 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.
The check under lock tests vaddr which is pmd aligned.

> It also doesn't handle racing with p2m_missing_pte to p2m_identity_pte
> (or vice-versa) change.
The change is always and only for missing/identity to individual pmd.
There is no transition between all missing and identity pmds.

> 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?
Would work, but is more complicated than needed. :-)


Juergen

> 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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