OpenBSD Journal

RETGUARD for clang (amd64) added to -current

Contributed by rueda on from the d(e)ropping-the-gadgets dept.

Todd Mortimer (mortimer@) has committed "RETGUARD" for clang (for amd64).

This is a new anti-ROP security mechanism, which uses random per-function cookies to protect return addresses on the stack.

The full commit message reads:

Module name:	src
Changes by:	2018/06/05 18:14:29

Modified files:
	gnu/llvm/include/llvm: InitializePasses.h 
	gnu/llvm/include/llvm/CodeGen: MachineFrameInfo.h Passes.h 
	gnu/llvm/lib/CodeGen: CMakeLists.txt PrologEpilogInserter.cpp 
	gnu/llvm/lib/Target/X86: X86FrameLowering.cpp X86FrameLowering.h 
	gnu/llvm/tools/clang/include/clang/Frontend: CodeGenOptions.def 
	gnu/llvm/tools/clang/lib/CodeGen: CGCall.cpp 
	gnu/llvm/tools/clang/lib/Driver/ToolChains: Clang.cpp 
	gnu/llvm/tools/clang/lib/Frontend: CompilerInvocation.cpp 
	gnu/usr.bin/clang/libLLVMCodeGen: Makefile 
	share/mk       : 
Added files:
	gnu/llvm/lib/CodeGen: ReturnProtectorPass.cpp 

Log message:
Add RETGUARD to clang for amd64. This security mechanism uses per-function
random cookies to protect access to function return instructions, with the
effect that the integrity of the return address is protected, and function
return instructions are harder to use in ROP gadgets.

On function entry the return address is combined with a per-function random
cookie and stored in the stack frame. The integrity of this value is verified
before function return, and if this check fails, the program aborts. In this way
RETGUARD is an improved stack protector, since the cookies are per-function. The
verification routine is constructed such that the binary space immediately
before each ret instruction is padded with int03 instructions, which makes these
return instructions difficult to use in ROP gadgets. In the kernel, this has the
effect of removing approximately 50% of total ROP gadgets, and 15% of unique
ROP gadgets compared to the 6.3 release kernel. Function epilogues are
essentially gadget free, leaving only the polymorphic gadgets that result from
jumping into the instruction stream partway through other instructions. Work to
remove these gadgets will continue through other mechanisms.

Remaining work includes adding this mechanism to assembly routines, which must
be done by hand. Many thanks to all those who helped test and provide feedback,
especially deaadt, tb, espie and naddy.

ok deraadt@

The FAQ page on following -current has been updated accordingly.

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