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UserspaceEmulator: Catch use-after-frees by tracking malloc/free :^)

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This patch introduces a "MallocTracer" to the UserspaceEmulator.
If this object is present on the Emulator, it can be notified whenever
the emulated program does a malloc() or free().

The notifications come in via a magic instruction sequence that we
embed in the LibC malloc() and free() functions. The sequence is:

    "salc x2, push reg32 x2, pop reg32 x3"

The data about the malloc/free operation is in the three pushes.
We make sure the sequence is harmless when running natively.

Memory accesses on MmapRegion are then audited to see if they fall
inside a known-to-be-freed malloc chunk. If so, we complain loud
and red in the debugger output. :^)

This is very, very cool! :^)

It's also a whole lot slower than before, since now we're auditing
memory accesses against a new set of metadata. This will need to be
optimized (and running in this mode should be opt-in, perhaps even
a separate program, etc.)
This commit is contained in:
Andreas Kling 2020-07-15 21:46:50 +02:00
parent d7c87e84f3
commit c314292319
9 changed files with 274 additions and 5 deletions
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38
DevTools/UserspaceEmulator/SoftCPU.cpp
View file

@ -65,6 +65,19 @@ void SoftCPU::dump() const
printf("o=%u s=%u z=%u a=%u p=%u c=%u\n", of(), sf(), zf(), af(), pf(), cf());
}
void SoftCPU::did_receive_secret_data()
{
if (m_secret_data[0] == 1) {
if (auto* tracer = m_emulator.malloc_tracer())
tracer->target_did_malloc({}, m_secret_data[2], m_secret_data[1]);
} else if (m_secret_data[0] == 2) {
if (auto* tracer = m_emulator.malloc_tracer())
tracer->target_did_free({}, m_secret_data[1]);
} else {
ASSERT_NOT_REACHED();
}
}
void SoftCPU::update_code_cache()
{
auto* region = m_emulator.mmu().find_region({ cs(), eip() });
@ -604,7 +617,6 @@ ALWAYS_INLINE static T op_shrd(SoftCPU& cpu, T data, T extra_bits, u8 steps)
return result;
}
template<typename T>
ALWAYS_INLINE static T op_shld(SoftCPU& cpu, T data, T extra_bits, u8 steps)
{
@ -633,7 +645,6 @@ ALWAYS_INLINE static T op_shld(SoftCPU& cpu, T data, T extra_bits, u8 steps)
return result;
}
template<bool update_dest, typename Op>
ALWAYS_INLINE void SoftCPU::generic_AL_imm8(Op op, const X86::Instruction& insn)
{
@ -1477,6 +1488,18 @@ void SoftCPU::PUSH_reg16(const X86::Instruction&) { TODO(); }
void SoftCPU::PUSH_reg32(const X86::Instruction& insn)
{
push32(gpr32(insn.reg32()));
if (m_secret_handshake_state == 2) {
m_secret_data[0] = gpr32(insn.reg32());
++m_secret_handshake_state;
} else if (m_secret_handshake_state == 3) {
m_secret_data[1] = gpr32(insn.reg32());
++m_secret_handshake_state;
} else if (m_secret_handshake_state == 4) {
m_secret_data[2] = gpr32(insn.reg32());
m_secret_handshake_state = 0;
did_receive_secret_data();
}
}
void SoftCPU::RCL_RM16_1(const X86::Instruction&) { TODO(); }
@ -1534,7 +1557,16 @@ void SoftCPU::ROR_RM8_1(const X86::Instruction&) { TODO(); }
void SoftCPU::ROR_RM8_CL(const X86::Instruction&) { TODO(); }
void SoftCPU::ROR_RM8_imm8(const X86::Instruction&) { TODO(); }
void SoftCPU::SAHF(const X86::Instruction&) { TODO(); }
void SoftCPU::SALC(const X86::Instruction&) { TODO(); }
void SoftCPU::SALC(const X86::Instruction&)
{
set_al(cf() ? 0x01 : 0x00);
if (m_secret_handshake_state < 2)
++m_secret_handshake_state;
else
m_secret_handshake_state = 0;
}
template<typename T>
static T op_sar(SoftCPU& cpu, T data, u8 steps)