Re: [Xen-devel] [PATCH] viridian: fix assertion failure

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Roger Pau Monne
> Sent: 26 November 2018 15:04
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Jan Beulich <jbeulich@xxxxxxxx>;
> Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>
> Subject: Re: [PATCH] viridian: fix assertion failure
> 
> On Mon, Nov 26, 2018 at 02:54:54PM +0000, Paul Durrant wrote:
> > Whilst attempting to crash an apparently wedged Windows domain using
> > 'xen-hvmcrash' I managed to trigger the following ASSERT:
> >
> > (XEN) Assertion '!vp->ptr' failed at viridian.c:607
> >
> > with stack:
> >
> > (XEN)    [<ffff82d08032c55d>] viridian_map_guest_page+0x1b4/0x1b6
> > (XEN)    [<ffff82d08032b1db>] viridian_synic_load_vcpu_ctxt+0x39/0x3b
> > (XEN)    [<ffff82d08032b90d>]
> viridian.c#viridian_load_vcpu_ctxt+0x93/0xcc
> > (XEN)    [<ffff82d0803096d6>] hvm_load+0x10e/0x19e
> > (XEN)    [<ffff82d080274c6d>] arch_do_domctl+0xb74/0x25b4
> > (XEN)    [<ffff82d0802068ab>] do_domctl+0x16f7/0x19d8
> >
> > This happened because viridian_map_guest_page() was not written to cope
> > with being called multiple times, but this is unfortunately exactly what
> > happens when xen-hvmcrash re-loads the domain context (having clobbered
> > the values of RIP).
> >
> > This patch simply makes viridian_map_guest_page() return immediately if
> it
> > finds the page already mapped (i.e. vp->ptr != NULL).
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > ---
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/hvm/viridian/viridian.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/xen/arch/x86/hvm/viridian/viridian.c
> b/xen/arch/x86/hvm/viridian/viridian.c
> > index 7d73f41de6..b99501eea2 100644
> > --- a/xen/arch/x86/hvm/viridian/viridian.c
> > +++ b/xen/arch/x86/hvm/viridian/viridian.c
> > @@ -604,7 +604,8 @@ void viridian_map_guest_page(struct vcpu *v, struct
> viridian_page *vp)
> >      unsigned long gmfn = vp->msr.fields.pfn;
> >      struct page_info *page = get_page_from_gfn(d, gmfn, NULL,
> P2M_ALLOC);
> >
> > -    ASSERT(!vp->ptr);
> > +    if ( vp->ptr )
> > +        return;
> 
> Isn't this likely to get the page reference counting out of sync? You
> will return without a put_page, and viridian_unmap_guest_page will set
> vp->ptr = NULL and call put_page_and_type without taking into account
> the reference count.
> 

Damn. Missed that the side effect was in the initializer. I'll need to move it 
below the check.

> If viridian_map_guest_page needs to be called multiple times you
> likely need some reference counting AFAICT, so that the last call to
> viridian_unmap_guest_page is the one that releases the page and sets
> vp->ptr = NULL.

I shouldn't need to reference count. When the MSR is set by the guest there is 
always an unmap then a map. It is just the context load case that should only 
map on the first call (to handle resume correctly).

  Paul

> 
> Thanks, Roger.

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