[libc++] Fix use-after-free when building with _LIBCPP_DEBUG=1

The issue is the following code:

    __cn1->__add(*__ip);
    (*__ip)->__c_ = __cn1;

`__ip` points into the array of iterators for container `__cn2`.  This code adds
the iterator to the array of iterators for `__cn1`, and updates the iterator to
point to the new container.

This code works fine, except when `__cn1` and `__cn2` are the same container.
`__cn1->__add()` might need to grow the array of iterators, and when it does,
`__ip` becomes invalid, so the second line becomes a use-after-free error.

Simply swapping the order of the above two lines is not sufficient, because of
the memmove() below.  The easiest and most performant solution is just to skip
touching any iterators if the containers are the same.

Differential Revision: https://reviews.llvm.org/D58926

llvm-svn: 355550

2 files changed, 53 insertions, 41 deletions

diff --git a/libcxx/include/list b/libcxx/include/list
index 2afe2a205ca..7b59a0eb0fc 100644
--- a/libcxx/include/list
+++ b/libcxx/include/list
@@ -2013,22 +2013,24 @@ list<_Tp, _Alloc>::splice(const_iterator __p, list& __c)
         base::__sz() += __c.__sz();
         __c.__sz() = 0;
 #if _LIBCPP_DEBUG_LEVEL >= 2
-        __libcpp_db* __db = __get_db();
-        __c_node* __cn1 = __db->__find_c_and_lock(this);
-        __c_node* __cn2 = __db->__find_c(&__c);
-        for (__i_node** __ip = __cn2->end_; __ip != __cn2->beg_;)
-        {
-            --__ip;
-            iterator* __i = static_cast<iterator*>((*__ip)->__i_);
-            if (__i->__ptr_ != __c.__end_as_link())
+        if (&__c != this) {
+            __libcpp_db* __db = __get_db();
+            __c_node* __cn1 = __db->__find_c_and_lock(this);
+            __c_node* __cn2 = __db->__find_c(&__c);
+            for (__i_node** __ip = __cn2->end_; __ip != __cn2->beg_;)
             {
-                __cn1->__add(*__ip);
-                (*__ip)->__c_ = __cn1;
-                if (--__cn2->end_ != __ip)
-                    memmove(__ip, __ip+1, (__cn2->end_ - __ip)*sizeof(__i_node*));
+                --__ip;
+                iterator* __i = static_cast<iterator*>((*__ip)->__i_);
+                if (__i->__ptr_ != __c.__end_as_link())
+                {
+                    __cn1->__add(*__ip);
+                    (*__ip)->__c_ = __cn1;
+                    if (--__cn2->end_ != __ip)
+                        memmove(__ip, __ip+1, (__cn2->end_ - __ip)*sizeof(__i_node*));
+                }
             }
+            __db->unlock();
         }
-        __db->unlock();
 #endif
     }
 }
@@ -2056,22 +2058,24 @@ list<_Tp, _Alloc>::splice(const_iterator __p, list& __c, const_iterator __i)
         --__c.__sz();
         ++base::__sz();
 #if _LIBCPP_DEBUG_LEVEL >= 2
-        __libcpp_db* __db = __get_db();
-        __c_node* __cn1 = __db->__find_c_and_lock(this);
-        __c_node* __cn2 = __db->__find_c(&__c);
-        for (__i_node** __ip = __cn2->end_; __ip != __cn2->beg_;)
-        {
-            --__ip;
-            iterator* __j = static_cast<iterator*>((*__ip)->__i_);
-            if (__j->__ptr_ == __f)
+        if (&__c != this) {
+            __libcpp_db* __db = __get_db();
+            __c_node* __cn1 = __db->__find_c_and_lock(this);
+            __c_node* __cn2 = __db->__find_c(&__c);
+            for (__i_node** __ip = __cn2->end_; __ip != __cn2->beg_;)
             {
-                __cn1->__add(*__ip);
-                (*__ip)->__c_ = __cn1;
-                if (--__cn2->end_ != __ip)
-                    memmove(__ip, __ip+1, (__cn2->end_ - __ip)*sizeof(__i_node*));
+                --__ip;
+                iterator* __j = static_cast<iterator*>((*__ip)->__i_);
+                if (__j->__ptr_ == __f)
+                {
+                    __cn1->__add(*__ip);
+                    (*__ip)->__c_ = __cn1;
+                    if (--__cn2->end_ != __ip)
+                        memmove(__ip, __ip+1, (__cn2->end_ - __ip)*sizeof(__i_node*));
+                }
             }
+            __db->unlock();
         }
-        __db->unlock();
 #endif
     }
 }
@@ -2110,26 +2114,28 @@ list<_Tp, _Alloc>::splice(const_iterator __p, list& __c, const_iterator __f, con
         base::__unlink_nodes(__first, __last);
         __link_nodes(__p.__ptr_, __first, __last);
 #if _LIBCPP_DEBUG_LEVEL >= 2
-        __libcpp_db* __db = __get_db();
-        __c_node* __cn1 = __db->__find_c_and_lock(this);
-        __c_node* __cn2 = __db->__find_c(&__c);
-        for (__i_node** __ip = __cn2->end_; __ip != __cn2->beg_;)
-        {
-            --__ip;
-            iterator* __j = static_cast<iterator*>((*__ip)->__i_);
-            for (__link_pointer __k = __f.__ptr_;
-                                          __k != __l.__ptr_; __k = __k->__next_)
+        if (&__c != this) {
+            __libcpp_db* __db = __get_db();
+            __c_node* __cn1 = __db->__find_c_and_lock(this);
+            __c_node* __cn2 = __db->__find_c(&__c);
+            for (__i_node** __ip = __cn2->end_; __ip != __cn2->beg_;)
             {
-                if (__j->__ptr_ == __k)
+                --__ip;
+                iterator* __j = static_cast<iterator*>((*__ip)->__i_);
+                for (__link_pointer __k = __f.__ptr_;
+                                              __k != __l.__ptr_; __k = __k->__next_)
                 {
-                    __cn1->__add(*__ip);
-                    (*__ip)->__c_ = __cn1;
-                    if (--__cn2->end_ != __ip)
-                        memmove(__ip, __ip+1, (__cn2->end_ - __ip)*sizeof(__i_node*));
+                    if (__j->__ptr_ == __k)
+                    {
+                        __cn1->__add(*__ip);
+                        (*__ip)->__c_ = __cn1;
+                        if (--__cn2->end_ != __ip)
+                            memmove(__ip, __ip+1, (__cn2->end_ - __ip)*sizeof(__i_node*));
+                    }
                 }
             }
+            __db->unlock();
         }
-        __db->unlock();
 #endif
     }
 }
diff --git a/libcxx/test/libcxx/debug/containers/db_sequence_container_iterators.pass.cpp b/libcxx/test/libcxx/debug/containers/db_sequence_container_iterators.pass.cpp
index 6ead98ebcb7..2de4c023f1e 100644
--- a/libcxx/test/libcxx/debug/containers/db_sequence_container_iterators.pass.cpp
+++ b/libcxx/test/libcxx/debug/containers/db_sequence_container_iterators.pass.cpp
@@ -65,6 +65,7 @@ public:
       }
       if constexpr (CT == CT_List) {
         SpliceFirstElem();
+        SpliceSameContainer();
       }
     } catch (...) {
       assert(false && "uncaught debug exception");
@@ -110,6 +111,11 @@ private:
     }
   }
 
+  static void SpliceSameContainer() {
+    CHECKPOINT("splice(<same-container>)");
+    Container C = {1, 1};
+    C.splice(C.end(), C, C.begin());
+  }
 
   static void SpliceFirstElemAfter() {
     // See llvm.org/PR35564