Re: [Xen-devel] [PATCH] xen/x86: clear per cpu stub page information in cpu_smpboot_free()

[Jürgen Groß <jgross@xxxxxxxx>]

On 08.01.20 13:23, Wei Liu wrote:
> On Wed, Jan 08, 2020 at 01:18:46PM +0100, Jürgen Groß wrote:
> > On 08.01.20 13:16, Wei Liu wrote:
> > > On Wed, Jan 08, 2020 at 12:01:48PM +0100, Juergen Gross wrote:
> > > > cpu_smpboot_free() removes the stubs for the cpu going offline, but it
> > > > isn't clearing the related percpu variables. This will result in
> > > > crashes when a stub page is released due to all related cpus gone
> > > > offline and one of those cpus going online later.
> > > >
> > > > Fix that by clearing stubs.addr and stubs.mfn in order to allocate a
> > > > new stub page when needed.
> > > >
> > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > > ---
> > > >    xen/arch/x86/smpboot.c | 2 ++
> > > >    1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
> > > > index 7e29704080..46c0729214 100644
> > > > --- a/xen/arch/x86/smpboot.c
> > > > +++ b/xen/arch/x86/smpboot.c
> > > > @@ -945,6 +945,8 @@ static void cpu_smpboot_free(unsigned int cpu, bool remove)
> > > >                                 (per_cpu(stubs.addr, cpu) | ~PAGE_MASK) + 1);
> > > >            if ( i == STUBS_PER_PAGE )
> > > >                free_domheap_page(mfn_to_page(mfn));
> > > > +        per_cpu(stubs.addr, cpu) = 0;
> > > > +        per_cpu(stubs.mfn, cpu) = 0;
> > > Shouldn't the mfn be set to INVALID_MFN instead?
> > This would require modifying alloc_stub_page():
> >
> >      if ( *mfn )
> >          pg = mfn_to_page(_mfn(*mfn));
> >      else
> OK. I think the chance of allocating mfn 0 from the allocator is
> exceedingly low, so I certainly have no objection to reset it to 0.
The chance should be exactly zero. Otherwise we'd have a big problem
due to L1TF...


Juergen

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