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.
