On Thu, Jun 18, 2020 at 3:42 AM Roger Pau Monné <roger....@citrix.com> 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. > > 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.
Using P2M_ALLOC is always safe if 1) the entry is already populated like in this case but also in 2) in case the gfn is a hole and gets forked. In mem_sharing the paging lock order is only applicable when an already present entry is getting converted to a shared type or a shared typed is getting unshared. It does not apply when a hole is being plugged. So this is safe in other paths as well where the entry is not yet present. Tamas
