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4.9 KiB
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128 lines
4.9 KiB
ReStructuredText
===========================================
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Control Flow Integrity Design Documentation
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===========================================
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This page documents the design of the :doc:`ControlFlowIntegrity` schemes
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supported by Clang.
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Forward-Edge CFI for Virtual Calls
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----------------------------------
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This scheme works by allocating, for each static type used to make a virtual
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call, a region of read-only storage in the object file holding a bit vector
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that maps onto to the region of storage used for those virtual tables. Each
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set bit in the bit vector corresponds to the `address point`_ for a virtual
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table compatible with the static type for which the bit vector is being built.
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For example, consider the following three C++ classes:
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.. code-block:: c++
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struct A {
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virtual void f1();
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virtual void f2();
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virtual void f3();
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};
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struct B : A {
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virtual void f1();
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virtual void f2();
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virtual void f3();
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};
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struct C : A {
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virtual void f1();
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virtual void f2();
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virtual void f3();
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};
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The scheme will cause the virtual tables for A, B and C to be laid out
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consecutively:
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.. csv-table:: Virtual Table Layout for A, B, C
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:header: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
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A::offset-to-top, &A::rtti, &A::f1, &A::f2, &A::f3, B::offset-to-top, &B::rtti, &B::f1, &B::f2, &B::f3, C::offset-to-top, &C::rtti, &C::f1, &C::f2, &C::f3
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The bit vector for static types A, B and C will look like this:
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.. csv-table:: Bit Vectors for A, B, C
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:header: Class, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
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A, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0
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B, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0
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C, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0
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To emit a virtual call, the compiler will assemble code that checks that
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the object's virtual table pointer is in-bounds and aligned and that the
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relevant bit is set in the bit vector.
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For example on x86 a typical virtual call may look like this if the bit
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vector is stored in memory:
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.. code-block:: none
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159a: 48 8b 03 mov (%rbx),%rax
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159d: 48 8d 15 6c 33 00 00 lea 0x336c(%rip),%rdx
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15a4: 48 89 c1 mov %rax,%rcx
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15a7: 48 29 d1 sub %rdx,%rcx
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15aa: 48 c1 c1 3d rol $0x3d,%rcx
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15ae: 48 83 f9 51 cmp $0x51,%rcx
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15b2: 77 3b ja 15ef <main+0xcf>
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15b4: 48 89 ca mov %rcx,%rdx
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15b7: 48 c1 ea 05 shr $0x5,%rdx
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15bb: 48 8d 35 b8 07 00 00 lea 0x7b8(%rip),%rsi
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15c2: 8b 14 96 mov (%rsi,%rdx,4),%edx
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15c5: 0f a3 ca bt %ecx,%edx
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15c8: 73 25 jae 15ef <main+0xcf>
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15ca: 48 89 df mov %rbx,%rdi
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15cd: ff 10 callq *(%rax)
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[...]
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15ef: 0f 0b ud2
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Or if the bit vector fits in 32 bits:
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.. code-block:: none
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dc2: 48 8b 03 mov (%rbx),%rax
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dc5: 48 8d 15 14 1e 00 00 lea 0x1e14(%rip),%rdx
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dcc: 48 89 c1 mov %rax,%rcx
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dcf: 48 29 d1 sub %rdx,%rcx
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dd2: 48 c1 c1 3d rol $0x3d,%rcx
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dd6: 48 83 f9 03 cmp $0x3,%rcx
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dda: 77 2f ja e0b <main+0x9b>
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ddc: ba 09 00 00 00 mov $0x9,%edx
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de1: 0f a3 ca bt %ecx,%edx
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de4: 73 25 jae e0b <main+0x9b>
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de6: 48 89 df mov %rbx,%rdi
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de9: ff 10 callq *(%rax)
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[...]
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e0b: 0f 0b ud2
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Or if the bit vector fits in 64 bits:
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.. code-block:: none
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11a6: 48 8b 03 mov (%rbx),%rax
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11a9: 48 8d 15 d0 28 00 00 lea 0x28d0(%rip),%rdx
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11b0: 48 89 c1 mov %rax,%rcx
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11b3: 48 29 d1 sub %rdx,%rcx
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11b6: 48 c1 c1 3d rol $0x3d,%rcx
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11ba: 48 83 f9 2a cmp $0x2a,%rcx
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11be: 77 35 ja 11f5 <main+0xb5>
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11c0: 48 ba 09 00 00 00 00 movabs $0x40000000009,%rdx
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11c7: 04 00 00
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11ca: 48 0f a3 ca bt %rcx,%rdx
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11ce: 73 25 jae 11f5 <main+0xb5>
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11d0: 48 89 df mov %rbx,%rdi
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11d3: ff 10 callq *(%rax)
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[...]
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11f5: 0f 0b ud2
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The compiler relies on co-operation from the linker in order to assemble
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the bit vector for the whole program. It currently does this using LLVM's
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`bit sets`_ mechanism together with link-time optimization.
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.. _address point: https://mentorembedded.github.io/cxx-abi/abi.html#vtable-general
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.. _bit sets: http://llvm.org/docs/BitSets.html
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