[PATCH for-4.14] x86/hap: use get_gfn_type in hap_update_paging_modes

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>]

While forking VMs running a small RTOS systems (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 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.

This issue has not affected VMs running other OS's as a call to vmx_load_pdptrs
is benign if PAE is not enabled or if EFER_LMA is set and returns before
trying to unshare and map the page.

Using get_gfn_type to get a lock on the gfn avoids this problem as we can
populate the fork's gfn with a copied page instead of a shared entry if its
needed, thus avoiding the lock order violation while holding paging_lock.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
---
The bug seems to have been present since commit 4cb6c4f4941, only discovered
now due to the heavy use of mem_sharing with VM forks. As this is a simple
bug-fix it would be nice to include it in the 4.14 release.
---
 xen/arch/x86/mm/hap/hap.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index 7f84d0c6ea..9ae4c3ae6e 100644
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -748,12 +748,19 @@ static void hap_update_paging_modes(struct vcpu *v)
     struct domain *d = v->domain;
     unsigned long cr3_gfn = v->arch.hvm.guest_cr[3] >> PAGE_SHIFT;
     p2m_type_t t;
+    p2m_query_t q = P2M_ALLOC;
 
-    /* We hold onto the cr3 as it may be modified later, and
-     * we need to respect lock ordering. No need for 
-     * checks here as they are performed by vmx_load_pdptrs
-     * (the potential user of the cr3) */
-    (void)get_gfn(d, cr3_gfn, &t);
+    /*
+     * We hold onto the cr3 as it may be modified later, and
+     * we need to respect lock ordering. Unshare here if we have to as to avoid
+     * a lock-order violation later while we are holding the paging_lock.
+     * Further checks are performed by vmx_load_pdptrs (the potential user of
+     * the cr3).
+     */
+    if ( hvm_pae_enabled(v) && !hvm_long_mode_active(v) )
+        q |= P2M_UNSHARE;
+
+    (void)get_gfn_type(d, cr3_gfn, &t, q);
     paging_lock(d);
 
     v->arch.paging.mode = hap_paging_get_mode(v);
-- 
2.25.1