the main executable) is the address of that function’s PLT record in
the main executable. This would break the CFI checks.
+Backward-edge CFI for return statements (RCFI)
+==============================================
+
+This section is a proposal. As of March 2017 it is not implemented.
+
+Backward-edge control flow (`RET` instructions) can be hijacked
+via overwriting the return address (`RA`) on stack.
+Various mitigation techniques (e.g. `SafeStack`_, `RFG`_, `Intel CET`_)
+try to detect or prevent `RA` corruption on stack.
+
+RCFI enforces the expected control flow in several different ways described below.
+RCFI heavily relies on LTO.
+
+Leaf Functions
+--------------
+If `f()` is a leaf function (i.e. it has no calls
+except maybe no-return calls) it can be called using a special calling convention
+that stores `RA` in a dedicated register `R` before the `CALL` instruction.
+`f()` does not spill `R` and does not use the `RET` instruction,
+instead it uses the value in `R` to `JMP` to `RA`.
+
+This flavour of CFI is *precise*, i.e. the function is guaranteed to return
+to the point exactly following the call.
+
+An alternative approach is to
+copy `RA` from stack to `R` in the first instruction of `f()`,
+then `JMP` to `R`.
+This approach is simpler to implement (does not require changing the caller)
+but weaker (there is a small window when `RA` is actually stored on stack).
+
+
+Functions called once
+---------------------
+Suppose `f()` is called in just one place in the program
+(assuming we can verify this in LTO mode).
+In this case we can replace the `RET` instruction with a `JMP` instruction
+with the immediate constant for `RA`.
+This will *precisely* enforce the return control flow no matter what is stored on stack.
+
+Another variant is to compare `RA` on stack with the known constant and abort
+if they don't match; then `JMP` to the known constant address.
+
+Functions called in a small number of call sites
+------------------------------------------------
+We may extend the above approach to cases where `f()`
+is called more than once (but still a small number of times).
+With LTO we know all possible values of `RA` and we check them
+one-by-one (or using binary search) against the value on stack.
+If the match is found, we `JMP` to the known constant address, otherwise abort.
+
+This protection is *near-precise*, i.e. it guarantees that the control flow will
+be transferred to one of the valid return addresses for this function,
+but not necessary to the point of the most recent `CALL`.
+
+General case
+------------
+For functions called multiple times a *return jump table* is constructed
+in the same manner as jump tables for indirect function calls (see above).
+The correct jump table entry (or it's index) is passed by `CALL` to `f()`
+(as an extra argument) and then spilled to stack.
+The `RET` instruction is replaced with a load of the jump table entry,
+jump table range check, and `JMP` to the jump table entry.
+
+This protection is also *near-precise*.
+
+Returns from functions called indirectly
+----------------------------------------
+
+If a function is called indirectly, the return jump table is constructed for the
+equivalence class of functions instead of a single function.
+
+Cross-DSO calls
+---------------
+Consider two instrumented DSOs, `A` and `B`. `A` defines `f()` and `B` calls it.
+
+This case will be handled similarly to the cross-DSO scheme using the slow path callback.
+
+Non-goals
+---------
+
+RCFI does not protect `RET` instructions:
+ * in non-instrumented DSOs,
+ * in instrumented DSOs for functions that are called from non-instrumented DSOs,
+ * embedded into other instructions (e.g. `0f4fc3 cmovg %ebx,%eax`).
+
+.. _SafeStack: https://clang.llvm.org/docs/SafeStack.html
+.. _RFG: http://xlab.tencent.com/en/2016/11/02/return-flow-guard
+.. _Intel CET: https://software.intel.com/en-us/blogs/2016/06/09/intel-release-new-technology-specifications-protect-rop-attacks
Hardware support
================
We believe that the above design can be efficiently implemented in hardware.
-A single new instruction added to an ISA would allow to perform the CFI check
+A single new instruction added to an ISA would allow to perform the forward-edge CFI check
with fewer bytes per check (smaller code size overhead) and potentially more
efficiently. The current software-only instrumentation requires at least
32-bytes per check (on x86_64).
projects / clang / commitdiff