x86/vmx: use P2M_ALLOC in vmx_load_pdptrs instead of P2M_UNSHARE

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.

Note that the lock order violation is avoided because before the paging_lock is
taken a lookup is performed with P2M_ALLOC that forks the page, thus the second
lookup in vmx_load_pdptrs succeeds without having to perform the fork. We keep
P2M_ALLOC in vmx_load_pdptrs because there are code-paths leading up to it
which don't take the paging_lock and that have no previous lookup. Currently no
other code-path exists leading there with the paging_lock taken, thus no
further adjustments are necessary.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@intel.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
master commit: b67e859d08
master date: 2020-06-19 15:24:55 +0200

xen/arch/x86/hvm/vmx/vmx.c

@@ -1320,7 +1320,7 @@ static void vmx_load_pdptrs(struct vcpu *v)
     if ( (cr3 & 0x1fUL) && !hvm_pcid_enabled(v) )
         goto crash;
 
-    page = get_page_from_gfn(v->domain, cr3 >> PAGE_SHIFT, &p2mt, P2M_UNSHARE);
+    page = get_page_from_gfn(v->domain, cr3 >> PAGE_SHIFT, &p2mt, P2M_ALLOC);
     if ( !page )
     {
         /* Ideally you don't want to crash but rather go into a wait