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

[Jan Beulich <jbeulich@suse.com>]

On 18.06.2020 15:00, Tamas K Lengyel wrote:
> On Thu, Jun 18, 2020 at 6:52 AM Roger Pau Monné <roger.pau@citrix.com> 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@suse.com> 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.

If this is indeed the case, then I guess all that's needed is a further
extended / refined commit message in v3.

Jan