From 666ed7facf4524bf6d19b11b20faa2cf93fdf591 Mon Sep 17 00:00:00 2001

From: "meir@redislabs.com" <meir@redislabs.com>

Date: Sun, 13 Jun 2021 14:27:18 +0300

Subject: [PATCH] Fix invalid memory write on lua stack overflow

 {CVE-2021-32626}

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When LUA call our C code, by default, the LUA stack has room for 20

elements. In most cases, this is more than enough but sometimes it's not

and the caller must verify the LUA stack size before he pushes elements.

On 3 places in the code, there was no verification of the LUA stack size.

On specific inputs this missing verification could have lead to invalid

memory write:

1. On 'luaReplyToRedisReply', one might return a nested reply that will

   explode the LUA stack.

2. On 'redisProtocolToLuaType', the Redis reply might be deep enough

   to explode the LUA stack (notice that currently there is no such

   command in Redis that returns such a nested reply, but modules might

   do it)

3. On 'ldbRedis', one might give a command with enough arguments to

   explode the LUA stack (all the arguments will be pushed to the LUA

   stack)

This commit is solving all those 3 issues by calling 'lua_checkstack' and

verify that there is enough room in the LUA stack to push elements. In

case 'lua_checkstack' returns an error (there is not enough room in the

LUA stack and it's not possible to increase the stack), we will do the

following:

1. On 'luaReplyToRedisReply', we will return an error to the user.

2. On 'redisProtocolToLuaType' we will exit with panic (we assume this

   scenario is rare because it can only happen with a module).

3. On 'ldbRedis', we return an error.

---

 src/scripting.c | 41 +++++++++++++++++++++++++++++++++++++++++

 1 file changed, 41 insertions(+)

diff --git a/src/scripting.c b/src/scripting.c

index dea5f516561..afa6adb0c47 100644

--- a/src/scripting.c

+++ b/src/scripting.c

@@ -128,6 +128,16 @@ void sha1hex(char *digest, char *script, size_t len) {

  */

 char *redisProtocolToLuaType(lua_State *lua, char* reply) {

+

+    if (!lua_checkstack(lua, 5)) {

+        /*

+         * Increase the Lua stack if needed, to make sure there is enough room

+         * to push 5 elements to the stack. On failure, exit with panic.

+         * Notice that we need, in the worst case, 5 elements because redisProtocolToLuaType_Aggregate

+         * might push 5 elements to the Lua stack.*/

+        serverPanic("lua stack limit reach when parsing redis.call reply");

+    }

+

     char *p = reply;

     switch(*p) {

@@ -220,6 +230,11 @@ char *redisProtocolToLuaType_Aggregate(lua_State *lua, char *reply, int atype) {

             if (atype == '%') {

                 p = redisProtocolToLuaType(lua,p);

             } else {

+                if (!lua_checkstack(lua, 1)) {

+                    /* Notice that here we need to check the stack again because the recursive

+                     * call to redisProtocolToLuaType might have use the room allocated in the stack */

+                    serverPanic("lua stack limit reach when parsing redis.call reply");

+                }

                 lua_pushboolean(lua,1);

             }

             lua_settable(lua,-3);

@@ -339,6 +354,17 @@ void luaSortArray(lua_State *lua) {

 /* Reply to client 'c' converting the top element in the Lua stack to a

  * Redis reply. As a side effect the element is consumed from the stack.  */

 void luaReplyToRedisReply(client *c, lua_State *lua) {

+

+    if (!lua_checkstack(lua, 4)) {

+        /* Increase the Lua stack if needed to make sure there is enough room

+         * to push 4 elements to the stack. On failure, return error.

+         * Notice that we need, in the worst case, 4 elements because returning a map might

+         * require push 4 elements to the Lua stack.*/

+        addReplyErrorFormat(c, "reached lua stack limit");

+        lua_pop(lua,1); // pop the element from the stack

+        return;

+    }

+

     int t = lua_type(lua,-1);

     switch(t) {

@@ -362,6 +388,7 @@ void luaReplyToRedisReply(client *c, lua_State *lua) {

          * field. */

         /* Handle error reply. */

+        // we took care of the stack size on function start

         lua_pushstring(lua,"err");

         lua_gettable(lua,-2);

         t = lua_type(lua,-1);

@@ -407,6 +434,7 @@ void luaReplyToRedisReply(client *c, lua_State *lua) {

         if (t == LUA_TTABLE) {

             int maplen = 0;

             void *replylen = addReplyDeferredLen(c);

+            /* we took care of the stack size on function start */

             lua_pushnil(lua); /* Use nil to start iteration. */

             while (lua_next(lua,-2)) {

                 /* Stack now: table, key, value */

@@ -429,6 +457,7 @@ void luaReplyToRedisReply(client *c, lua_State *lua) {

         if (t == LUA_TTABLE) {

             int setlen = 0;

             void *replylen = addReplyDeferredLen(c);

+            /* we took care of the stack size on function start */

             lua_pushnil(lua); /* Use nil to start iteration. */

             while (lua_next(lua,-2)) {

                 /* Stack now: table, key, true */

@@ -448,6 +477,7 @@ void luaReplyToRedisReply(client *c, lua_State *lua) {

         void *replylen = addReplyDeferredLen(c);

         int j = 1, mbulklen = 0;

         while(1) {

+            /* we took care of the stack size on function start */

             lua_pushnumber(lua,j++);

             lua_gettable(lua,-2);

             t = lua_type(lua,-1);

@@ -2506,6 +2536,17 @@ void ldbEval(lua_State *lua, sds *argv, int argc) {

 void ldbRedis(lua_State *lua, sds *argv, int argc) {

     int j, saved_rc = server.lua_replicate_commands;

+    if (!lua_checkstack(lua, argc + 1)) {

+        /* Increase the Lua stack if needed to make sure there is enough room

+         * to push 'argc + 1' elements to the stack. On failure, return error.

+         * Notice that we need, in worst case, 'argc + 1' elements because we push all the arguments

+         * given by the user (without the first argument) and we also push the 'redis' global table and

+         * 'redis.call' function so:

+         * (1 (redis table)) + (1 (redis.call function)) + (argc - 1 (all arguments without the first)) = argc + 1*/

+        ldbLogRedisReply("max lua stack reached");

+        return;

+    }

+

     lua_getglobal(lua,"redis");

     lua_pushstring(lua,"call");

     lua_gettable(lua,-2);       /* Stack: redis, redis.call */