x86/HVM: serialize pinned cache attribute list manipulation

While the RCU variants of list insertion and removal allow lockless list
traversal (with RCU just read-locked), insertions and removals still
need serializing amongst themselves. To keep things simple, use the
domain lock for this purpose.

This is CVE-2022-42334 / part of XSA-428.

Fixes: 642123c512 ("x86/hvm: provide XEN_DMOP_pin_memory_cacheattr")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit 829ec245cf66560e3b50d140ccb3168e7fb7c945)

xen/arch/x86/hvm/mtrr.c

@@ -594,7 +594,7 @@ static void free_pinned_cacheattr_entry(struct rcu_head *rcu)
 int hvm_set_mem_pinned_cacheattr(struct domain *d, uint64_t gfn_start,
                                  uint64_t gfn_end, uint32_t type)
 {
-    struct hvm_mem_pinned_cacheattr_range *range;
+    struct hvm_mem_pinned_cacheattr_range *range, *newr;
     unsigned int nr = 0;
     int rc = 1;
 
@@ -608,14 +608,15 @@ int hvm_set_mem_pinned_cacheattr(struct domain *d, uint64_t gfn_start,
     {
     case XEN_DOMCTL_DELETE_MEM_CACHEATTR:
         /* Remove the requested range. */
-        rcu_read_lock(&pinned_cacheattr_rcu_lock);
-        list_for_each_entry_rcu ( range,
-                                  &d->arch.hvm.pinned_cacheattr_ranges,
-                                  list )
+        domain_lock(d);
+        list_for_each_entry ( range,
+                              &d->arch.hvm.pinned_cacheattr_ranges,
+                              list )
             if ( range->start == gfn_start && range->end == gfn_end )
             {
-                rcu_read_unlock(&pinned_cacheattr_rcu_lock);
                 list_del_rcu(&range->list);
+                domain_unlock(d);
+
                 type = range->type;
                 call_rcu(&range->rcu, free_pinned_cacheattr_entry);
                 p2m_memory_type_changed(d);
@@ -636,7 +637,7 @@ int hvm_set_mem_pinned_cacheattr(struct domain *d, uint64_t gfn_start,
                 }
                 return 0;
             }
-        rcu_read_unlock(&pinned_cacheattr_rcu_lock);
+        domain_unlock(d);
         return -ENOENT;
 
     case PAT_TYPE_UC_MINUS:
@@ -651,7 +652,10 @@ int hvm_set_mem_pinned_cacheattr(struct domain *d, uint64_t gfn_start,
         return -EINVAL;
     }
 
-    rcu_read_lock(&pinned_cacheattr_rcu_lock);
+    newr = xzalloc(struct hvm_mem_pinned_cacheattr_range);
+
+    domain_lock(d);
+
     list_for_each_entry_rcu ( range,
                               &d->arch.hvm.pinned_cacheattr_ranges,
                               list )
@@ -669,27 +673,34 @@ int hvm_set_mem_pinned_cacheattr(struct domain *d, uint64_t gfn_start,
         }
         ++nr;
     }
-    rcu_read_unlock(&pinned_cacheattr_rcu_lock);
+
     if ( rc <= 0 )
-        return rc;
+        /* nothing */;
+    else if ( nr >= 64 /* The limit is arbitrary. */ )
+        rc = -ENOSPC;
+    else if ( !newr )
+        rc = -ENOMEM;
+    else
+    {
+        newr->start = gfn_start;
+        newr->end = gfn_end;
+        newr->type = type;
 
-    if ( nr >= 64 /* The limit is arbitrary. */ )
-        return -ENOSPC;
+        list_add_rcu(&newr->list, &d->arch.hvm.pinned_cacheattr_ranges);
 
-    range = xzalloc(struct hvm_mem_pinned_cacheattr_range);
-    if ( range == NULL )
-        return -ENOMEM;
+        newr = NULL;
+        rc = 0;
+    }
 
-    range->start = gfn_start;
-    range->end = gfn_end;
-    range->type = type;
+    domain_unlock(d);
+
+    xfree(newr);
 
-    list_add_rcu(&range->list, &d->arch.hvm.pinned_cacheattr_ranges);
     p2m_memory_type_changed(d);
     if ( type != PAT_TYPE_WRBACK )
         flush_all(FLUSH_CACHE);
 
-    return 0;
+    return rc;
 }
 
 static int hvm_save_mtrr_msr(struct vcpu *v, hvm_domain_context_t *h)