From 6298903e35217ab69c279056f925fb72900ce0b7 Mon Sep 17 00:00:00 2001
From: Roberto Ierusalimschy <roberto@inf.puc-rio.br>
Date: Mon, 6 Jul 2020 12:11:54 -0300
Subject: [PATCH] Keep minimum size when shrinking a stack

When shrinking a stack (during GC), do not make it smaller than the
initial stack size.
---
 ldo.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)
==== end of original header ====

CVE: CVE-2020-15888

Upstream-Status: backport [https://github.com/lua/lua.git]

Signed-off-by: Joe Slater <joe.slater@windriver.com>

====
diff --git a/ldo.c b/ldo.c
index c563b1d9..a89ac010 100644
--- a/src/ldo.c
+++ b/src/ldo.c
@@ -220,7 +220,7 @@ static int stackinuse (lua_State *L) {
 
 void luaD_shrinkstack (lua_State *L) {
   int inuse = stackinuse(L);
-  int goodsize = inuse + (inuse / 8) + 2*EXTRA_STACK;
+  int goodsize = inuse + BASIC_STACK_SIZE;
   if (goodsize > LUAI_MAXSTACK)
     goodsize = LUAI_MAXSTACK;  /* respect stack limit */
   if (L->stacksize > LUAI_MAXSTACK)  /* had been handling stack overflow? */
@@ -229,8 +229,7 @@ void luaD_shrinkstack (lua_State *L) {
     luaE_shrinkCI(L);  /* shrink list */
   /* if thread is currently not handling a stack overflow and its
      good size is smaller than current size, shrink its stack */
-  if (inuse <= (LUAI_MAXSTACK - EXTRA_STACK) &&
-      goodsize < L->stacksize)
+  if (inuse <= (LUAI_MAXSTACK - EXTRA_STACK) && goodsize < L->stacksize)
     luaD_reallocstack(L, goodsize);
   else  /* don't change stack */
     condmovestack(L,{},{});  /* (change only for debugging) */
-- 
2.17.1