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 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.

Jan