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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.06.2020 11:40, Roger Pau Monné wrote:
> 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.

Oh, I simply mis-remembered what v1 did; for some reason I thought
it added a call rather than modifying the query type from alloc to
unshare.

Jan