x86/xen: avoid writing to freed memory after race in p2m handling

In case a race was detected during allocation of a new p2m tree
element in alloc_p2m() the new allocated mid_mfn page is freed without
updating the pointer to the found value in the tree. This will result
in overwriting the just freed page with the mfn of the p2m leaf.
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/p2m.c

@@ -566,6 +566,7 @@ static bool alloc_p2m(unsigned long pfn)
 		/* Separately check the mid mfn level */
 		unsigned long missing_mfn;
 		unsigned long mid_mfn_mfn;
+		unsigned long old_mfn;
 
 		mid_mfn = alloc_p2m_page();
 		if (!mid_mfn)
@@ -575,10 +576,13 @@ static bool alloc_p2m(unsigned long pfn)
 
 		missing_mfn = virt_to_mfn(p2m_mid_missing_mfn);
 		mid_mfn_mfn = virt_to_mfn(mid_mfn);
-		if (cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn) != missing_mfn)
+		old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn);
+		if (old_mfn != missing_mfn) {
 			free_p2m_page(mid_mfn);
-		else
+			mid_mfn = mfn_to_virt(old_mfn);
+		} else {
 			p2m_top_mfn_p[topidx] = mid_mfn;
+		}
 	}
 
 	if (p2m_top[topidx][mididx] == p2m_identity ||