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diff --git a/docs/ControlFlowIntegrityDesign.rst b/docs/ControlFlowIntegrityDesign.rst new file mode 100644 index 0000000..89aa038 --- /dev/null +++ b/docs/ControlFlowIntegrityDesign.rst @@ -0,0 +1,275 @@ +=========================================== +Control Flow Integrity Design Documentation +=========================================== + +This page documents the design of the :doc:`ControlFlowIntegrity` schemes +supported by Clang. + +Forward-Edge CFI for Virtual Calls +================================== + +This scheme works by allocating, for each static type used to make a virtual +call, a region of read-only storage in the object file holding a bit vector +that maps onto to the region of storage used for those virtual tables. Each +set bit in the bit vector corresponds to the `address point`_ for a virtual +table compatible with the static type for which the bit vector is being built. + +For example, consider the following three C++ classes: + +.. code-block:: c++ + + struct A { + virtual void f1(); + virtual void f2(); + virtual void f3(); + }; + + struct B : A { + virtual void f1(); + virtual void f2(); + virtual void f3(); + }; + + struct C : A { + virtual void f1(); + virtual void f2(); + virtual void f3(); + }; + +The scheme will cause the virtual tables for A, B and C to be laid out +consecutively: + +.. csv-table:: Virtual Table Layout for A, B, C + :header: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 + + 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 + +The bit vector for static types A, B and C will look like this: + +.. csv-table:: Bit Vectors for A, B, C + :header: Class, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 + + A, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0 + B, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0 + C, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0 + +Bit vectors are represented in the object file as byte arrays. By loading +from indexed offsets into the byte array and applying a mask, a program can +test bits from the bit set with a relatively short instruction sequence. Bit +vectors may overlap so long as they use different bits. For the full details, +see the `ByteArrayBuilder`_ class. + +In this case, assuming A is laid out at offset 0 in bit 0, B at offset 0 in +bit 1 and C at offset 0 in bit 2, the byte array would look like this: + +.. code-block:: c++ + + char bits[] = { 0, 0, 1, 0, 0, 0, 3, 0, 0, 0, 0, 5, 0, 0 }; + +To emit a virtual call, the compiler will assemble code that checks that +the object's virtual table pointer is in-bounds and aligned and that the +relevant bit is set in the bit vector. + +For example on x86 a typical virtual call may look like this: + +.. code-block:: none + + ca7fbb: 48 8b 0f mov (%rdi),%rcx + ca7fbe: 48 8d 15 c3 42 fb 07 lea 0x7fb42c3(%rip),%rdx + ca7fc5: 48 89 c8 mov %rcx,%rax + ca7fc8: 48 29 d0 sub %rdx,%rax + ca7fcb: 48 c1 c0 3d rol $0x3d,%rax + ca7fcf: 48 3d 7f 01 00 00 cmp $0x17f,%rax + ca7fd5: 0f 87 36 05 00 00 ja ca8511 + ca7fdb: 48 8d 15 c0 0b f7 06 lea 0x6f70bc0(%rip),%rdx + ca7fe2: f6 04 10 10 testb $0x10,(%rax,%rdx,1) + ca7fe6: 0f 84 25 05 00 00 je ca8511 + ca7fec: ff 91 98 00 00 00 callq *0x98(%rcx) + [...] + ca8511: 0f 0b ud2 + +The compiler relies on co-operation from the linker in order to assemble +the bit vectors for the whole program. It currently does this using LLVM's +`bit sets`_ mechanism together with link-time optimization. + +.. _address point: https://mentorembedded.github.io/cxx-abi/abi.html#vtable-general +.. _bit sets: http://llvm.org/docs/BitSets.html +.. _ByteArrayBuilder: http://llvm.org/docs/doxygen/html/structllvm_1_1ByteArrayBuilder.html + +Optimizations +------------- + +The scheme as described above is the fully general variant of the scheme. +Most of the time we are able to apply one or more of the following +optimizations to improve binary size or performance. + +In fact, if you try the above example with the current version of the +compiler, you will probably find that it will not use the described virtual +table layout or machine instructions. Some of the optimizations we are about +to introduce cause the compiler to use a different layout or a different +sequence of machine instructions. + +Stripping Leading/Trailing Zeros in Bit Vectors +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If a bit vector contains leading or trailing zeros, we can strip them from +the vector. The compiler will emit code to check if the pointer is in range +of the region covered by ones, and perform the bit vector check using a +truncated version of the bit vector. For example, the bit vectors for our +example class hierarchy will be emitted like this: + +.. csv-table:: Bit Vectors for A, B, C + :header: Class, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 + + A, , , 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, , + B, , , , , , , , 1, , , , , , , + C, , , , , , , , , , , , , 1, , + +Short Inline Bit Vectors +~~~~~~~~~~~~~~~~~~~~~~~~ + +If the vector is sufficiently short, we can represent it as an inline constant +on x86. This saves us a few instructions when reading the correct element +of the bit vector. + +If the bit vector fits in 32 bits, the code looks like this: + +.. code-block:: none + + dc2: 48 8b 03 mov (%rbx),%rax + dc5: 48 8d 15 14 1e 00 00 lea 0x1e14(%rip),%rdx + dcc: 48 89 c1 mov %rax,%rcx + dcf: 48 29 d1 sub %rdx,%rcx + dd2: 48 c1 c1 3d rol $0x3d,%rcx + dd6: 48 83 f9 03 cmp $0x3,%rcx + dda: 77 2f ja e0b <main+0x9b> + ddc: ba 09 00 00 00 mov $0x9,%edx + de1: 0f a3 ca bt %ecx,%edx + de4: 73 25 jae e0b <main+0x9b> + de6: 48 89 df mov %rbx,%rdi + de9: ff 10 callq *(%rax) + [...] + e0b: 0f 0b ud2 + +Or if the bit vector fits in 64 bits: + +.. code-block:: none + + 11a6: 48 8b 03 mov (%rbx),%rax + 11a9: 48 8d 15 d0 28 00 00 lea 0x28d0(%rip),%rdx + 11b0: 48 89 c1 mov %rax,%rcx + 11b3: 48 29 d1 sub %rdx,%rcx + 11b6: 48 c1 c1 3d rol $0x3d,%rcx + 11ba: 48 83 f9 2a cmp $0x2a,%rcx + 11be: 77 35 ja 11f5 <main+0xb5> + 11c0: 48 ba 09 00 00 00 00 movabs $0x40000000009,%rdx + 11c7: 04 00 00 + 11ca: 48 0f a3 ca bt %rcx,%rdx + 11ce: 73 25 jae 11f5 <main+0xb5> + 11d0: 48 89 df mov %rbx,%rdi + 11d3: ff 10 callq *(%rax) + [...] + 11f5: 0f 0b ud2 + +If the bit vector consists of a single bit, there is only one possible +virtual table, and the check can consist of a single equality comparison: + +.. code-block:: none + + 9a2: 48 8b 03 mov (%rbx),%rax + 9a5: 48 8d 0d a4 13 00 00 lea 0x13a4(%rip),%rcx + 9ac: 48 39 c8 cmp %rcx,%rax + 9af: 75 25 jne 9d6 <main+0x86> + 9b1: 48 89 df mov %rbx,%rdi + 9b4: ff 10 callq *(%rax) + [...] + 9d6: 0f 0b ud2 + +Virtual Table Layout +~~~~~~~~~~~~~~~~~~~~ + +The compiler lays out classes of disjoint hierarchies in separate regions +of the object file. At worst, bit vectors in disjoint hierarchies only +need to cover their disjoint hierarchy. But the closer that classes in +sub-hierarchies are laid out to each other, the smaller the bit vectors for +those sub-hierarchies need to be (see "Stripping Leading/Trailing Zeros in Bit +Vectors" above). The `GlobalLayoutBuilder`_ class is responsible for laying +out the globals efficiently to minimize the sizes of the underlying bitsets. + +.. _GlobalLayoutBuilder: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Transforms/IPO/LowerBitSets.h?view=markup + +Alignment +~~~~~~~~~ + +If all gaps between address points in a particular bit vector are multiples +of powers of 2, the compiler can compress the bit vector by strengthening +the alignment requirements of the virtual table pointer. For example, given +this class hierarchy: + +.. code-block:: c++ + + struct A { + virtual void f1(); + virtual void f2(); + }; + + struct B : A { + virtual void f1(); + virtual void f2(); + virtual void f3(); + virtual void f4(); + virtual void f5(); + virtual void f6(); + }; + + struct C : A { + virtual void f1(); + virtual void f2(); + }; + +The virtual tables will be laid out like this: + +.. csv-table:: Virtual Table Layout for A, B, C + :header: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 + + A::offset-to-top, &A::rtti, &A::f1, &A::f2, B::offset-to-top, &B::rtti, &B::f1, &B::f2, &B::f3, &B::f4, &B::f5, &B::f6, C::offset-to-top, &C::rtti, &C::f1, &C::f2 + +Notice that each address point for A is separated by 4 words. This lets us +emit a compressed bit vector for A that looks like this: + +.. csv-table:: + :header: 2, 6, 10, 14 + + 1, 1, 0, 1 + +At call sites, the compiler will strengthen the alignment requirements by +using a different rotate count. For example, on a 64-bit machine where the +address points are 4-word aligned (as in A from our example), the ``rol`` +instruction may look like this: + +.. code-block:: none + + dd2: 48 c1 c1 3b rol $0x3b,%rcx + +Padding to Powers of 2 +~~~~~~~~~~~~~~~~~~~~~~ + +Of course, this alignment scheme works best if the address points are +in fact aligned correctly. To make this more likely to happen, we insert +padding between virtual tables that in many cases aligns address points to +a power of 2. Specifically, our padding aligns virtual tables to the next +highest power of 2 bytes; because address points for specific base classes +normally appear at fixed offsets within the virtual table, this normally +has the effect of aligning the address points as well. + +This scheme introduces tradeoffs between decreased space overhead for +instructions and bit vectors and increased overhead in the form of padding. We +therefore limit the amount of padding so that we align to no more than 128 +bytes. This number was found experimentally to provide a good tradeoff. + +Eliminating Bit Vector Checks for All-Ones Bit Vectors +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If the bit vector is all ones, the bit vector check is redundant; we simply +need to check that the address is in range and well aligned. This is more +likely to occur if the virtual tables are padded. |
