lua-users home
lua-l archive

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index]

Hi there,
I found a heap-user-after-free bug in lua-5.3.5.

The function `lua_upvaluejoin` in file lapi.c at line 1287 suffers from a use after free bug when supplied the same function for parameter f1 and f2 and the same upvalue index, additionally the bug happens only when the upvalue is closed, this happens because the `luaC_upvdeccount` function found in file lgc.c at line 678 will decrement the refcount and then free the upvalue if the refcount is zero and if the upvalue is closed.
See the comments below for more explanation.
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);
  luaC_upvdeccount(L, *up1);      //Will delete up1
  *up1 = *up2;     //up1 is up2 because it's the same upvalue and now it's freed.
  (*up1)->refcount++;   //up1 is freed, yet it's used here.
  if (upisopen(*up1)) (*up1)-> = 1;
  luaC_upvalbarrier(L, *up1);

To trigger the bug simply use a lua program like this (this one will crash):
f=load(function() end)
Another program that will not crash (unless you use -fsanitize=address):
function w() 
local x = {}
f = function() print(x) end


Fady Othman
Information Security Consultant

Security Consultant # ZINAD IT
G006D-THUB, Dubai Silicon Oasis, Dubai, U.A.E