Rework stack overflow handling.

Reported by pwnhacker0x18. Fixed by Peter Cawley. #1152

src/lj_debug.c

@@ -63,6 +63,7 @@ static BCPos debug_framepc(lua_State *L, GCfunc *fn, cTValue *nextframe)
     if (cf == NULL || (char *)cframe_pc(cf) == (char *)cframe_L(cf))
       return NO_BCPOS;
     ins = cframe_pc(cf);  /* Only happens during error/hook handling. */
+    if (!ins) return NO_BCPOS;
   } else {
     if (frame_islua(nextframe)) {
       ins = frame_pc(nextframe);

src/lj_err.c

@@ -488,7 +488,14 @@ LJ_NOINLINE void lj_err_mem(lua_State *L)
 {
   if (L->status == LUA_ERRERR+1)  /* Don't touch the stack during lua_open. */
     lj_vm_unwind_c(L->cframe, LUA_ERRMEM);
-  if (curr_funcisL(L)) L->top = curr_topL(L);
+  if (curr_funcisL(L)) {
+    L->top = curr_topL(L);
+    if (LJ_UNLIKELY(L->top > tvref(L->maxstack))) {
+      /* The current Lua frame violates the stack. Replace it with a dummy. */
+      L->top = L->base;
+      setframe_gc(L->base - 1, obj2gco(L));
+    }
+  }
   setstrV(L, L->top++, lj_err_str(L, LJ_ERR_ERRMEM));
   lj_err_throw(L, LUA_ERRMEM);
 }
@@ -551,9 +558,11 @@ LJ_NOINLINE void LJ_FASTCALL lj_err_run(lua_State *L)
 {
   ptrdiff_t ef = finderrfunc(L);
   if (ef) {
-    TValue *errfunc = restorestack(L, ef);
-    TValue *top = L->top;
+    TValue *errfunc, *top;
+    lj_state_checkstack(L, LUA_MINSTACK * 2);  /* Might raise new error. */
     lj_trace_abort(G(L));
+    errfunc = restorestack(L, ef);
+    top = L->top;
     if (!tvisfunc(errfunc) || L->status == LUA_ERRERR) {
       setstrV(L, top-1, lj_err_str(L, LJ_ERR_ERRERR));
       lj_err_throw(L, LUA_ERRERR);
@@ -567,6 +576,13 @@ LJ_NOINLINE void LJ_FASTCALL lj_err_run(lua_State *L)
   lj_err_throw(L, LUA_ERRRUN);
 }
 
+/* Stack overflow error. */
+void LJ_FASTCALL lj_err_stkov(lua_State *L)
+{
+  lj_debug_addloc(L, err2msg(LJ_ERR_STKOV), L->base-1, NULL);
+  lj_err_run(L);
+}
+
 /* Formatted runtime error message. */
 LJ_NORET LJ_NOINLINE static void err_msgv(lua_State *L, ErrMsg em, ...)
 {

src/lj_err.h

@@ -23,6 +23,7 @@ LJ_DATA const char *lj_err_allmsg;
 LJ_FUNC GCstr *lj_err_str(lua_State *L, ErrMsg em);
 LJ_FUNCA_NORET void LJ_FASTCALL lj_err_throw(lua_State *L, int errcode);
 LJ_FUNC_NORET void lj_err_mem(lua_State *L);
+LJ_FUNC_NORET void LJ_FASTCALL lj_err_stkov(lua_State *L);
 LJ_FUNCA_NORET void LJ_FASTCALL lj_err_run(lua_State *L);
 LJ_FUNC_NORET void lj_err_msg(lua_State *L, ErrMsg em);
 LJ_FUNC_NORET void lj_err_lex(lua_State *L, GCstr *src, const char *tok,

src/lj_state.c

@@ -96,27 +96,45 @@ void lj_state_shrinkstack(lua_State *L, MSize used)
 /* Try to grow stack. */
 void LJ_FASTCALL lj_state_growstack(lua_State *L, MSize need)
 {
-  MSize n;
-  if (L->stacksize >= LJ_STACK_MAXEX) {
-    /* 4. Throw 'error in error handling' when we are _over_ the limit. */
-    if (L->stacksize > LJ_STACK_MAXEX)
+  MSize n = L->stacksize + need;
+  if (LJ_LIKELY(n < LJ_STACK_MAX)) {  /* The stack can grow as requested. */
+    if (n < 2 * L->stacksize) {  /* Try to double the size. */
+      n = 2 * L->stacksize;
+      if (n > LJ_STACK_MAX)
+	n = LJ_STACK_MAX;
+    }
+    resizestack(L, n);
+  } else {  /* Request would overflow. Raise a stack overflow error. */
+    if (curr_funcisL(L)) {
+      L->top = curr_topL(L);
+      if (L->top > tvref(L->maxstack)) {
+	/* The current Lua frame violates the stack, so replace it with a
+	** dummy. This can happen when BC_IFUNCF is trying to grow the stack.
+	*/
+	L->top = L->base;
+	setframe_gc(L->base - 1, obj2gco(L));
+      }
+    }
+    if (L->stacksize <= LJ_STACK_MAXEX) {
+      /* An error handler might want to inspect the stack overflow error, but
+      ** will need some stack space to run in. We give it a stack size beyond
+      ** the normal limit in order to do so, then rely on lj_state_relimitstack
+      ** calls during unwinding to bring us back to a convential stack size.
+      ** The + 1 is space for the error message, and 2 * LUA_MINSTACK is for
+      ** the lj_state_checkstack() call in lj_err_run().
+      */
+      resizestack(L, LJ_STACK_MAX + 1 + 2 * LUA_MINSTACK);
+      lj_err_stkov(L);  /* May invoke an error handler. */
+    } else {
+      /* If we're here, then the stack overflow error handler is requesting
+      ** to grow the stack even further. We have no choice but to abort the
+      ** error handler.
+      */
+      GCstr *em = lj_err_str(L, LJ_ERR_STKOV);  /* Might OOM. */
+      setstrV(L, L->top++, em);  /* There is always space to push an error. */
       lj_err_throw(L, LUA_ERRERR);  /* Does not invoke an error handler. */
-    /* 1. We are _at_ the limit after the last growth. */
-    if (L->status < LUA_ERRRUN) {  /* 2. Throw 'stack overflow'. */
-      L->status = LUA_ERRRUN;  /* Prevent ending here again for pushed msg. */
-      lj_err_msg(L, LJ_ERR_STKOV);  /* May invoke an error handler. */
     }
-    /* 3. Add space (over the limit) for pushed message and error handler. */
-  }
-  n = L->stacksize + need;
-  if (n > LJ_STACK_MAX) {
-    n += 2*LUA_MINSTACK;
-  } else if (n < 2*L->stacksize) {
-    n = 2*L->stacksize;
-    if (n >= LJ_STACK_MAX)
-      n = LJ_STACK_MAX;
   }
-  resizestack(L, n);
 }
 
 void LJ_FASTCALL lj_state_growstack1(lua_State *L)