Re: [PATCH v2 for-4.14] x86/vmx: use P2M_ALLOC in vmx_load_pdptrs instead of P2M_UNSHARE

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Thu, Jun 18, 2020 at 6:52 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>
> On Thu, Jun 18, 2020 at 02:46:24PM +0200, Jan Beulich wrote:
> > On 18.06.2020 14:39, Tamas K Lengyel wrote:
> > > On Thu, Jun 18, 2020 at 12:31 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> > >>
> > >> On 17.06.2020 18:19, Tamas K Lengyel wrote:
> > >>> While forking VMs running a small RTOS system (Zephyr) a Xen crash has 
> > >>> been
> > >>> observed due to a mm-lock order violation while copying the HVM CPU 
> > >>> context
> > >>> from the parent. This issue has been identified to be due to
> > >>> hap_update_paging_modes first getting a lock on the gfn using get_gfn. 
> > >>> This
> > >>> call also creates a shared entry in the fork's memory map for the cr3 
> > >>> gfn. The
> > >>> function later calls hap_update_cr3 while holding the paging_lock, which
> > >>> results in the lock-order violation in vmx_load_pdptrs when it tries to 
> > >>> unshare
> > >>> the above entry when it grabs the page with the P2M_UNSHARE flag set.
> > >>>
> > >>> Since vmx_load_pdptrs only reads from the page its usage of P2M_UNSHARE 
> > >>> was
> > >>> unnecessary to start with. Using P2M_ALLOC is the appropriate flag to 
> > >>> ensure
> > >>> the p2m is properly populated and to avoid the lock-order violation we
> > >>> observed.
> > >>
> > >> Using P2M_ALLOC is not going to address the original problem though
> > >> afaict: You may hit the mem_sharing_fork_page() path that way, and
> > >> via nominate_page() => __grab_shared_page() => mem_sharing_page_lock()
> > >> you'd run into a lock order violation again.
> > >
> > > Note that the nominate_page you see in that path is for the parent VM.
> > > The paging lock is not taken for the parent VM thus nominate_page
> > > succeeds without any issues any time fork_page is called. There is no
> > > nominate_page called for the client domain as there is nothing to
> > > nominate when plugging a hole.
> >
> > But that's still a lock order issue then, isn't it? Just one that
> > the machinery can't detect / assert upon.
>
> Yes, mm lock ordering doesn't differentiate between domains, and the
> current lock order on the pCPU is based on the last lock taken
> (regardless of the domain it belongs to).

I see, makes sense. In that case the issue is avoided purely due to
get_gfn being called that happens before the paging_lock is taken.
That would have to be the way-to-go on other paths leading to
vmx_load_pdptrs as well but since all other paths leading there do it
without the paging lock being taken there aren't any more adjustments
necessary right now that I can see.

Tamas