CVE-2020-15945 (published 2020-07-24T21:15Z)

Lua through 5.4.0 has a segmentation fault in changedline in ldebug.c (e.g., when called by luaG_traceexec) because it incorrectly expects that an oldpc value is always updated upon a return of the flow of control to a function.

Severity: LOW
CVSS v2 base score: 2.1 (AV:L/AC:L/Au:N/C:N/I:N/A:P)
CVSS v3 base score: 5.5, MEDIUM (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: NVD-CWE-Other
cwe_name: Other
description: NVD is only using a subset of CWE for mapping instead of the entire CWE, and the weakness type is not covered by that subset.
extended_description: Insufficient Information

Fix information:
repo_url: https://github.com/lua/lua
commit_details: repo_url: https://github.com/lua/lua
author: Roberto Ierusalimschy
author_date: 2020-07-17 11:01:05-03:00
msg: Fixed bug: invalid 'oldpc' when returning to a function

The field 'L->oldpc' is not always updated when control returns to a
function; an invalid value can seg. fault when computing 'changedline'.
(One example is an error in a finalizer; control can return to
'luaV_execute' without executing 'luaD_poscall'.) Instead of trying to
fix all possible corner cases, it seems safer to be resilient to invalid
values for 'oldpc'. Valid but wrong values at most cause an extra call
to a line hook.
repository: repo_url: https://github.com/lua/lua
repo_name: lua/lua
description: A copy of the Lua development repository, as seen by the Lua team. Mirrored irregularly. Please DO NOT send pull requests or any other stuff. All communication should be through the Lua mailing list https://www.lua.org/lua-l.html
repo_language: C
owner: lua
file_changes: filename: ldebug.c
change_type: ModificationType.MODIFY
diff: @@ -33,10 +33,8 @@
 
 #define noLuaClosure(f)		((f) == NULL || (f)->c.tt == LUA_VCCL)
 
-
-/* Active Lua function (given call info) */
-#define ci_func(ci)		(clLvalue(s2v((ci)->func)))
-
+/* inverse of 'pcRel' */
+#define invpcRel(pc, p)		((p)->code + (pc) + 1)
 
 static const char *funcnamefromcode (lua_State *L, CallInfo *ci,
                                     const char **name);
@@ -127,20 +125,18 @@ static void settraps (CallInfo *ci) {
 /*
 ** This function can be called during a signal, under "reasonable"
 ** assumptions.
-** Fields 'oldpc', 'basehookcount', and 'hookcount' (set by
-** 'resethookcount') are for debug only, and it is no problem if they
-** get arbitrary values (causes at most one wrong hook call). 'hookmask'
-** is an atomic value. We assume that pointers are atomic too (e.g., gcc
-** ensures that for all platforms where it runs). Moreover, 'hook' is
-** always checked before being called (see 'luaD_hook').
+** Fields 'basehookcount' and 'hookcount' (set by 'resethookcount')
+** are for debug only, and it is no problem if they get arbitrary
+** values (causes at most one wrong hook call). 'hookmask' is an atomic
+** value. We assume that pointers are atomic too (e.g., gcc ensures that
+** for all platforms where it runs). Moreover, 'hook' is always checked
+** before being called (see 'luaD_hook').
 */
 LUA_API void lua_sethook (lua_State *L, lua_Hook func, int mask, int count) {
   if (func == NULL || mask == 0) {  /* turn off hooks? */
     mask = 0;
     func = NULL;
   }
-  if (isLua(L->ci))
-    L->oldpc = L->ci->u.l.savedpc;
   L->hook = func;
   L->basehookcount = count;
   resethookcount(L);
@@ -795,10 +791,24 @@ static int changedline (const Proto *p, int oldpc, int newpc) {
 }
 
 
+/*
+** Traces the execution of a Lua function. Called before the execution
+** of each opcode, when debug is on. 'L->oldpc' stores the last
+** instruction traced, to detect line changes. When entering a new
+** function, 'npci' will be zero and will test as a new line without
+** the need for 'oldpc'; so, 'oldpc' does not need to be initialized
+** before. Some exceptional conditions may return to a function without
+** updating 'oldpc'. In that case, 'oldpc' may be invalid; if so, it is
+** reset to zero.  (A wrong but valid 'oldpc' at most causes an extra
+** call to a line hook.)
+*/
 int luaG_traceexec (lua_State *L, const Instruction *pc) {
   CallInfo *ci = L->ci;
   lu_byte mask = L->hookmask;
+  const Proto *p = ci_func(ci)->p;
   int counthook;
+  /* 'L->oldpc' may be invalid; reset it in this case */
+  int oldpc = (L->oldpc < p->sizecode) ? L->oldpc : 0;
   if (!(mask & (LUA_MASKLINE | LUA_MASKCOUNT))) {  /* no hooks? */
     ci->u.l.trap = 0;  /* don't need to stop again */
     return 0;  /* turn off 'trap' */
@@ -819,15 +829,14 @@ int luaG_traceexec (lua_State *L, const Instruction *pc) {
   if (counthook)
     luaD_hook(L, LUA_HOOKCOUNT, -1, 0, 0);  /* call count hook */
   if (mask & LUA_MASKLINE) {
-    const Proto *p = ci_func(ci)->p;
     int npci = pcRel(pc, p);
     if (npci == 0 ||  /* call linehook when enter a new function, */
-        pc <= L->oldpc ||  /* when jump back (loop), or when */
-        changedline(p, pcRel(L->oldpc, p), npci)) {  /* enter new line */
+        pc <= invpcRel(oldpc, p) ||  /* when jump back (loop), or when */
+        changedline(p, oldpc, npci)) {  /* enter new line */
       int newline = luaG_getfuncline(p, npci);
       luaD_hook(L, LUA_HOOKLINE, newline, 0, 0);  /* call line hook */
     }
-    L->oldpc = pc;  /* 'pc' of last call to line hook */
+    L->oldpc = npci;  /* 'pc' of last call to line hook */
   }
   if (L->status == LUA_YIELD) {  /* did hook yield? */
     if (counthook)

programming_language: C
filename: ldebug.h
change_type: ModificationType.MODIFY
diff: @@ -13,6 +13,11 @@
 
 #define pcRel(pc, p)	(cast_int((pc) - (p)->code) - 1)
 
+
+/* Active Lua function (given call info) */
+#define ci_func(ci)		(clLvalue(s2v((ci)->func)))
+
+
 #define resethookcount(L)	(L->hookcount = L->basehookcount)
 
 /*

programming_language: C
filename: ldo.c
change_type: ModificationType.MODIFY
diff: @@ -327,7 +327,7 @@ static StkId rethook (lua_State *L, CallInfo *ci, StkId firstres, int nres) {
   ptrdiff_t oldtop = savestack(L, L->top);  /* hook may change top */
   int delta = 0;
   if (isLuacode(ci)) {
-    Proto *p = clLvalue(s2v(ci->func))->p;
+    Proto *p = ci_func(ci)->p;
     if (p->is_vararg)
       delta = ci->u.l.nextraargs + p->numparams + 1;
     if (L->top < ci->top)
@@ -340,8 +340,8 @@ static StkId rethook (lua_State *L, CallInfo *ci, StkId firstres, int nres) {
     luaD_hook(L, LUA_HOOKRET, -1, ftransfer, nres);  /* call it */
     ci->func -= delta;
   }
-  if (isLua(ci->previous))
-    L->oldpc = ci->previous->u.l.savedpc;  /* update 'oldpc' */
+  if (isLua(ci = ci->previous))
+    L->oldpc = pcRel(ci->u.l.savedpc, ci_func(ci)->p);  /* update 'oldpc' */
   return restorestack(L, oldtop);
 }
 

programming_language: C
filename: lstate.c
change_type: ModificationType.MODIFY
diff: @@ -301,6 +301,7 @@ static void preinit_thread (lua_State *L, global_State *g) {
   L->openupval = NULL;
   L->status = LUA_OK;
   L->errfunc = 0;
+  L->oldpc = 0;
 }
 
 

programming_language: C
filename: lstate.h
change_type: ModificationType.MODIFY
diff: @@ -286,7 +286,6 @@ struct lua_State {
   StkId top;  /* first free slot in the stack */
   global_State *l_G;
   CallInfo *ci;  /* call info for current function */
-  const Instruction *oldpc;  /* last pc traced */
   StkId stack_last;  /* last free slot in the stack */
   StkId stack;  /* stack base */
   UpVal *openupval;  /* list of open upvalues in this stack */
@@ -297,6 +296,7 @@ struct lua_State {
   volatile lua_Hook hook;
   ptrdiff_t errfunc;  /* current error handling function (stack index) */
   l_uint32 nCcalls;  /* number of allowed nested C calls - 'nci' */
+  int oldpc;  /* last pc traced */
   int stacksize;
   int basehookcount;
   int hookcount;

programming_language: C
filename: lvm.c
change_type: ModificationType.MODIFY
diff: @@ -1794,7 +1794,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
         ProtectNT(luaT_adjustvarargs(L, GETARG_A(i), ci, cl->p));
         if (trap) {
           luaD_hookcall(L, ci);
-          L->oldpc = pc + 1;  /* next opcode will be seen as a "new" line */
+          L->oldpc = 1;  /* next opcode will be seen as a "new" line */
         }
         updatebase(ci);  /* function has new base after adjustment */
         vmbreak;

programming_language: C