From: Roberto Ierusalimschy <roberto@inf.puc-rio.br>
Date: Wed, 27 Mar 2019 14:30:12 -0300
Subject: Fixed bug in 'lua_upvaluejoin'

Bug-fix: joining an upvalue with itself could cause a use-after-free
crash.

Origin: https://github.com/lua/lua/commit/89aee84cbc9224f638f3b7951b306d2ee8ecb71e
Bug: http://lua-users.org/lists/lua-l/2019-01/msg00039.html
Bug-Debian: https://security-tracker.debian.org/tracker/CVE-2019-6706
Bug-Debian: https://bugs.debian.org/920321
---
 src/lapi.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/src/lapi.c b/src/lapi.c
index c9455a5..86eac00 100644
--- a/src/lapi.c
+++ b/src/lapi.c
@@ -1253,13 +1253,12 @@ LUA_API const char *lua_setupvalue (lua_State *L, int funcindex, int n) {
 }
 
 
-static UpVal **getupvalref (lua_State *L, int fidx, int n, LClosure **pf) {
+static UpVal **getupvalref (lua_State *L, int fidx, int n) {
   LClosure *f;
   StkId fi = index2addr(L, fidx);
   api_check(L, ttisLclosure(fi), "Lua function expected");
   f = clLvalue(fi);
   api_check(L, (1 <= n && n <= f->p->sizeupvalues), "invalid upvalue index");
-  if (pf) *pf = f;
   return &f->upvals[n - 1];  /* get its upvalue pointer */
 }
 
@@ -1268,7 +1267,7 @@ LUA_API void *lua_upvalueid (lua_State *L, int fidx, int n) {
   StkId fi = index2addr(L, fidx);
   switch (ttype(fi)) {
     case LUA_TLCL: {  /* lua closure */
-      return *getupvalref(L, fidx, n, NULL);
+      return *getupvalref(L, fidx, n);
     }
     case LUA_TCCL: {  /* C closure */
       CClosure *f = clCvalue(fi);
@@ -1285,9 +1284,10 @@ LUA_API void *lua_upvalueid (lua_State *L, int fidx, int n) {
 
 LUA_API void lua_upvaluejoin (lua_State *L, int fidx1, int n1,
                                             int fidx2, int n2) {
-  LClosure *f1;
-  UpVal **up1 = getupvalref(L, fidx1, n1, &f1);
-  UpVal **up2 = getupvalref(L, fidx2, n2, NULL);
+  UpVal **up1 = getupvalref(L, fidx1, n1);
+  UpVal **up2 = getupvalref(L, fidx2, n2);
+  if (*up1 == *up2)
+    return;
   luaC_upvdeccount(L, *up1);
   *up1 = *up2;
   (*up1)->refcount++;