Control Flow Integrity Design Documentation¶
This page documents the design of the Control Flow Integrity 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:
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:
| 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:
| 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:
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:
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.
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:
| 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:
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:
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:
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.
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:
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:
| 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:
| 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:
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.
Forward-Edge CFI for Indirect Function Calls¶
Under forward-edge CFI for indirect function calls, each unique function type has its own bit vector, and at each call site we need to check that the function pointer is a member of the function type’s bit vector. This scheme works in a similar way to forward-edge CFI for virtual calls, the distinction being that we need to build bit vectors of function entry points rather than of virtual tables.
Unlike when re-arranging global variables, we cannot re-arrange functions in a particular order and base our calculations on the layout of the functions’ entry points, as we have no idea how large a particular function will end up being (the function sizes could even depend on how we arrange the functions). Instead, we build a jump table, which is a block of code consisting of one branch instruction for each of the functions in the bit set that branches to the target function, and redirect any taken function addresses to the corresponding jump table entry. In this way, the distance between function entry points is predictable and controllable. In the object file’s symbol table, the symbols for the target functions also refer to the jump table entries, so that addresses taken outside the module will pass any verification done inside the module.
In more concrete terms, suppose we have three functions f, g, h which are members of a single bitset, and a function foo that returns their addresses:
f:
mov 0, %eax
ret
g:
mov 1, %eax
ret
h:
mov 2, %eax
ret
foo:
mov f, %eax
mov g, %edx
mov h, %ecx
ret
Our jump table will (conceptually) look like this:
f:
jmp .Ltmp0 ; 5 bytes
int3 ; 1 byte
int3 ; 1 byte
int3 ; 1 byte
g:
jmp .Ltmp1 ; 5 bytes
int3 ; 1 byte
int3 ; 1 byte
int3 ; 1 byte
h:
jmp .Ltmp2 ; 5 bytes
int3 ; 1 byte
int3 ; 1 byte
int3 ; 1 byte
.Ltmp0:
mov 0, %eax
ret
.Ltmp1:
mov 1, %eax
ret
.Ltmp2:
mov 2, %eax
ret
foo:
mov f, %eax
mov g, %edx
mov h, %ecx
ret
Because the addresses of f, g, h are evenly spaced at a power of 2, and function types do not overlap (unlike class types with base classes), we can normally apply the Alignment and Eliminating Bit Vector Checks for All-Ones Bit Vectors optimizations thus simplifying the check at each call site to a range and alignment check.