util.xml: Do not allow doctypes, comments or processing instructions
Yes. This is as bad as it sounds. CVE pending.
In Prosody itself, this only affects mod_websocket, which uses util.xml
to parse the <open/> frame, thus allowing unauthenticated remote DoS
using Billion Laughs. However, third-party modules using util.xml may
also be affected by this.
This commit installs handlers which disallow the use of doctype
declarations and processing instructions without any escape hatch. It,
by default, also introduces such a handler for comments, however, there
is a way to enable comments nontheless.
This is because util.xml is used to parse human-facing data, where
comments are generally a desirable feature, and also because comments
are generally harmless.
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <lua.h>
#include <lauxlib.h>
typedef struct {
size_t rpos; /* read position */
size_t wpos; /* write position */
size_t alen; /* allocated size */
size_t blen; /* current content size */
char buffer[];
} ringbuffer;
char readchar(ringbuffer *b) {
b->blen--;
return b->buffer[(b->rpos++) % b->alen];
}
void writechar(ringbuffer *b, char c) {
b->blen++;
b->buffer[(b->wpos++) % b->alen] = c;
}
/* make sure position counters stay within the allocation */
void modpos(ringbuffer *b) {
b->rpos = b->rpos % b->alen;
b->wpos = b->wpos % b->alen;
}
int find(ringbuffer *b, const char *s, size_t l) {
size_t i, j;
int m;
if(b->rpos == b->wpos) { /* empty */
return 0;
}
/* look for a matching first byte */
for(i = 0; i <= b->blen - l; i++) {
if(b->buffer[(b->rpos + i) % b->alen] == *s) {
m = 1;
/* check if the following byte also match */
for(j = 1; j < l; j++)
if(b->buffer[(b->rpos + i + j) % b->alen] != s[j]) {
m = 0;
break;
}
if(m) {
return i + l;
}
}
}
return 0;
}
/*
* Find first position of a substring in buffer
* (buffer, string) -> number
*/
int rb_find(lua_State *L) {
size_t l, m;
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
const char *s = luaL_checklstring(L, 2, &l);
m = find(b, s, l);
if(m > 0) {
lua_pushinteger(L, m);
return 1;
}
return 0;
}
/*
* Move read position forward without returning the data
* (buffer, number) -> boolean
*/
int rb_discard(lua_State *L) {
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
size_t r = luaL_checkinteger(L, 2);
if(r > b->blen) {
lua_pushboolean(L, 0);
return 1;
}
b->blen -= r;
b->rpos += r;
modpos(b);
lua_pushboolean(L, 1);
return 1;
}
/*
* Read bytes from buffer
* (buffer, number, boolean?) -> string
*/
int rb_read(lua_State *L) {
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
size_t r = luaL_checkinteger(L, 2);
int peek = lua_toboolean(L, 3);
if(r > b->blen) {
lua_pushnil(L);
return 1;
}
if((b->rpos + r) > b->alen) {
/* Substring wraps around to the beginning of the buffer */
lua_pushlstring(L, &b->buffer[b->rpos], b->alen - b->rpos);
lua_pushlstring(L, b->buffer, r - (b->alen - b->rpos));
lua_concat(L, 2);
} else {
lua_pushlstring(L, &b->buffer[b->rpos], r);
}
if(!peek) {
b->blen -= r;
b->rpos += r;
modpos(b);
}
return 1;
}
/*
* Read buffer until first occurrence of a substring
* (buffer, string) -> string
*/
int rb_readuntil(lua_State *L) {
size_t l, m;
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
const char *s = luaL_checklstring(L, 2, &l);
m = find(b, s, l);
if(m > 0) {
lua_settop(L, 1);
lua_pushinteger(L, m);
return rb_read(L);
}
return 0;
}
/*
* Write bytes into the buffer
* (buffer, string) -> integer
*/
int rb_write(lua_State *L) {
size_t l, w = 0;
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
const char *s = luaL_checklstring(L, 2, &l);
/* Does `l` bytes fit? */
if((l + b->blen) > b->alen) {
lua_pushnil(L);
return 1;
}
while(l-- > 0) {
writechar(b, *s++);
w++;
}
modpos(b);
lua_pushinteger(L, w);
return 1;
}
int rb_tostring(lua_State *L) {
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
lua_pushfstring(L, "ringbuffer: %p %d/%d", b, b->blen, b->alen);
return 1;
}
int rb_length(lua_State *L) {
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
lua_pushinteger(L, b->blen);
return 1;
}
int rb_size(lua_State *L) {
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
lua_pushinteger(L, b->alen);
return 1;
}
int rb_free(lua_State *L) {
ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");
lua_pushinteger(L, b->alen - b->blen);
return 1;
}
int rb_new(lua_State *L) {
size_t size = luaL_optinteger(L, 1, sysconf(_SC_PAGESIZE));
ringbuffer *b = lua_newuserdata(L, sizeof(ringbuffer) + size);
b->rpos = 0;
b->wpos = 0;
b->alen = size;
b->blen = 0;
luaL_getmetatable(L, "ringbuffer_mt");
lua_setmetatable(L, -2);
return 1;
}
int luaopen_util_ringbuffer(lua_State *L) {
#if (LUA_VERSION_NUM > 501)
luaL_checkversion(L);
#endif
if(luaL_newmetatable(L, "ringbuffer_mt")) {
lua_pushcfunction(L, rb_tostring);
lua_setfield(L, -2, "__tostring");
lua_pushcfunction(L, rb_length);
lua_setfield(L, -2, "__len");
lua_createtable(L, 0, 7); /* __index */
{
lua_pushcfunction(L, rb_find);
lua_setfield(L, -2, "find");
lua_pushcfunction(L, rb_discard);
lua_setfield(L, -2, "discard");
lua_pushcfunction(L, rb_read);
lua_setfield(L, -2, "read");
lua_pushcfunction(L, rb_readuntil);
lua_setfield(L, -2, "readuntil");
lua_pushcfunction(L, rb_write);
lua_setfield(L, -2, "write");
lua_pushcfunction(L, rb_size);
lua_setfield(L, -2, "size");
lua_pushcfunction(L, rb_length);
lua_setfield(L, -2, "length");
lua_pushcfunction(L, rb_free);
lua_setfield(L, -2, "free");
}
lua_setfield(L, -2, "__index");
}
lua_createtable(L, 0, 1);
lua_pushcfunction(L, rb_new);
lua_setfield(L, -2, "new");
return 1;
}