Randomness added to table length computation

A bad actor could fill only a few entries in a table (power of twos in
decreasing order, see tests) and produce a small table with a huge
length. If your program builds a table with external data and iterates
over its length, this behavior could be an issue.

lapi.c

@@ -440,7 +440,7 @@ LUA_API lua_Unsigned lua_rawlen (lua_State *L, int idx) {
     case LUA_VSHRSTR: return cast(lua_Unsigned, tsvalue(o)->shrlen);
     case LUA_VLNGSTR: return cast(lua_Unsigned, tsvalue(o)->u.lnglen);
     case LUA_VUSERDATA: return cast(lua_Unsigned, uvalue(o)->len);
-    case LUA_VTABLE: return luaH_getn(hvalue(o));
+    case LUA_VTABLE: return luaH_getn(L, hvalue(o));
     default: return 0;
   }
 }

lobject.c

@@ -31,7 +31,8 @@
 
 
 /*
-** Computes ceil(log2(x))
+** Computes ceil(log2(x)), which is the smallest integer n such that
+** x <= (1 << n).
 */
 lu_byte luaO_ceillog2 (unsigned int x) {
   static const lu_byte log_2[256] = {  /* log_2[i - 1] = ceil(log2(i)) */

ltable.c

@@ -1220,24 +1220,36 @@ void luaH_setint (lua_State *L, Table *t, lua_Integer key, TValue *value) {
 
 /*
 ** Try to find a boundary in the hash part of table 't'. From the
-** caller, we know that 'j' is zero or present and that 'j + 1' is
+** caller, we know that 'asize + 1' is present. We want to find a larger
-** present. We want to find a larger key that is absent from the
+** key that is absent from the table, so that we can do a binary search
-** table, so that we can do a binary search between the two keys to
+** between the two keys to find a boundary. We keep doubling 'j' until
-** find a boundary. We keep doubling 'j' until we get an absent index.
+** we get an absent index.  If the doubling would overflow, we try
-** If the doubling would overflow, we try LUA_MAXINTEGER. If it is
+** LUA_MAXINTEGER. If it is absent, we are ready for the binary search.
-** absent, we are ready for the binary search. ('j', being max integer,
+** ('j', being max integer, is larger or equal to 'i', but it cannot be
-** is larger or equal to 'i', but it cannot be equal because it is
+** equal because it is absent while 'i' is present.) Otherwise, 'j' is a
-** absent while 'i' is present; so 'j > i'.) Otherwise, 'j' is a
+** boundary. ('j + 1' cannot be a present integer key because it is not
-** boundary. ('j + 1' cannot be a present integer key because it is
+** a valid integer in Lua.)
-** not a valid integer in Lua.)
+** About 'rnd': If we used a fixed algorithm, a bad actor could fill
+** a table with only the keys that would be probed, in such a way that
+** a small table could result in a huge length. To avoid that, we use
+** the state's seed as a source of randomness. For the first probe,
+** we "randomly double" 'i' by adding to it a random number roughly its
+** width.
 */
-static lua_Unsigned hash_search (Table *t, lua_Unsigned j) {
+static lua_Unsigned hash_search (lua_State *L, Table *t, unsigned asize) {
-  lua_Unsigned i;
+  lua_Unsigned i = asize + 1;  /* caller ensures t[i] is present */
-  if (j == 0) j++;  /* the caller ensures 'j + 1' is present */
+  unsigned rnd = G(L)->seed;
-  do {
+  int n = (asize > 0) ? luaO_ceillog2(asize) : 0;  /* width of 'asize' */
+  unsigned mask = (1u << n) - 1;  /* 11...111 with the width of 'asize' */
+  unsigned incr = (rnd & mask) + 1;  /* first increment (at least 1) */
+  lua_Unsigned j = (incr <= l_castS2U(LUA_MAXINTEGER) - i) ? i + incr : i + 1;
+  rnd >>= n;  /* used 'n' bits from 'rnd' */
+  while (!hashkeyisempty(t, j)) {  /* repeat until an absent t[j] */
     i = j;  /* 'i' is a present index */
-    if (j <= l_castS2U(LUA_MAXINTEGER) / 2)
+    if (j <= l_castS2U(LUA_MAXINTEGER)/2 - 1) {
-      j *= 2;
+      j = j*2 + (rnd & 1);  /* try again with 2j or 2j+1 */
+      rnd >>= 1;
+    }
     else {
       j = LUA_MAXINTEGER;
       if (hashkeyisempty(t, j))  /* t[j] not present? */
@@ -1245,7 +1257,7 @@ static lua_Unsigned hash_search (Table *t, lua_Unsigned j) {
       else  /* weird case */
         return j;  /* well, max integer is a boundary... */
     }
-  } while (!hashkeyisempty(t, j));  /* repeat until an absent t[j] */
+  }
   /* i < j  &&  t[i] present  &&  t[j] absent */
   while (j - i > 1u) {  /* do a binary search between them */
     lua_Unsigned m = (i + j) / 2;
@@ -1286,7 +1298,7 @@ static lua_Unsigned newhint (Table *t, unsigned hint) {
 ** If there is no array part, or its last element is non empty, the
 ** border may be in the hash part.
 */
-lua_Unsigned luaH_getn (Table *t) {
+lua_Unsigned luaH_getn (lua_State *L, Table *t) {
   unsigned asize = t->asize;
   if (asize > 0) {  /* is there an array part? */
     const unsigned maxvicinity = 4;
@@ -1327,7 +1339,7 @@ lua_Unsigned luaH_getn (Table *t) {
   if (isdummy(t) || hashkeyisempty(t, asize + 1))
     return asize;  /* 'asize + 1' is empty */
   else  /* 'asize + 1' is also non empty */
-    return hash_search(t, asize);
+    return hash_search(L, t, asize);
 }
 
 

ltable.h

@@ -173,7 +173,7 @@ LUAI_FUNC void luaH_resizearray (lua_State *L, Table *t, unsigned nasize);
 LUAI_FUNC lu_mem luaH_size (Table *t);
 LUAI_FUNC void luaH_free (lua_State *L, Table *t);
 LUAI_FUNC int luaH_next (lua_State *L, Table *t, StkId key);
-LUAI_FUNC lua_Unsigned luaH_getn (Table *t);
+LUAI_FUNC lua_Unsigned luaH_getn (lua_State *L, Table *t);
 
 
 #if defined(LUA_DEBUG)

lvm.c

@@ -722,7 +722,7 @@ void luaV_objlen (lua_State *L, StkId ra, const TValue *rb) {
       Table *h = hvalue(rb);
       tm = fasttm(L, h->metatable, TM_LEN);
       if (tm) break;  /* metamethod? break switch to call it */
-      setivalue(s2v(ra), l_castU2S(luaH_getn(h)));  /* else primitive len */
+      setivalue(s2v(ra), l_castU2S(luaH_getn(L, h)));  /* else primitive len */
       return;
     }
     case LUA_VSHRSTR: {

testes/nextvar.lua

@@ -345,6 +345,18 @@ do
   end
 end
 
+
+do  print("testing attack on table length")
+  local t = {}
+  local lim = math.floor(math.log(math.maxinteger, 2)) - 1
+  for i = lim, 0, -1 do
+    t[2^i] = true
+  end
+  assert(t[1 << lim])
+  -- next loop should not take forever
+  for i = 1, #t do end
+end
+
 local nofind = {}