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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jun 18, 2020 at 08:30:08AM +0200, Jan Beulich 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.

Well, I guess Tamas avoids this because of the get_gfn call in
hap_update_paging_modes will have already populated the entry, so it's
never going to hit the p2m_is_hole check in __get_gfn_type_access.

> The change is an improvement, so I'd be fine with it going in this
> way, but only as long as the description mentions that there's still
> an open issue here (which may be non-trivial to address). Or perhaps
> combining with your v1 change is the way to go (for now or even
> permanently)?

If vmx_load_pdptrs only requires P2M_ALLOC then this is already
covered by the call to get_gfn performed in hap_update_paging_modes,
so I don't think there's much point in merging with v1, as forcing
hap_update_paging_modes to unshare the entry won't affect
vmx_load_pdptrs anymore.

I'm however worried about other code paths that can call into
vmx_load_pdptrs with mm locks taken, and I agree it would be safer to
assert that all the higher layers make sure the cr3 loaded is
correctly populated for a query without P2M_ALLOC to succeed.

Thanks, Roger.