Tags, or metadata, enrich software with domain-specific information that is consumed by hardware to enforce security and testing policies during runtime. However, given a target architecture, developing custom compilers that encode tags can be tedious and time-consuming. We present COGENT, a highly flexible and feature-rich compiler toolchain for instruction tag generation on the RISC-V architecture. Central to this effort is a LLVM-based compiler that is supplemented with a tag-aware disassembler and a tag integrity checker. COGENT is capable of: (a) generating tags at one or more of varying granularity (per function, per basic block, or per instruction), and (b) associating variable-width tags (1-32 bits) to instructions, and arbitrary-width tags to each function or basic block. Additionally, COGENT is capable of emitting control-flow labels, which are crucial in asserting control-flow integrity (CFI), a runtime property that aids in detecting bugs and exploits that violate control flow. We evaluate the correctness of tags generated by COGENT's compiler and the associated performance penalties, along with how well COGENT preserves IR-level tags at the lower level. We provide three exemplar applications-Control Flow Integrity, Adaptive Tracing, and Hardware-Level Function Tracing that can leverage COGENT. The tagged code incurs an average cycle count overhead from 5.24% to 0.94% in the worst and best cases, respectively, making it ideal for debugging and testing applications, including fuzzing.

Control flow attacks exploit software vulnerabilities to divert the flow of control into unintended paths to ultimately execute attack code. This paper explores the use of instruction and data tagging as a general means of thwarting such control flow attacks, including attacks that rely on violating pointer integrity. Using specific types of narrow-width data tags along with narrow-width instruction tags embedded within the binary facilitates the security policies required to protect against such attacks, leading to a practically viable solution. Co-locating instruction tags close to their corresponding instructions within cache lines eliminates the need for separate mechanisms for instruction tag accesses. Information gleaned from the analysis phase of a compiler is augmented and used to generate the instruction and data tags. A full-stack implementation that consists of a modified LLVM compiler, modified Linux OS support for tags and a FPGA-implemented CPU hardware prototype for enforcing CFI, data pointer and code pointer integrity is demonstrated. With a modest hardware enhancement, the execution time of benchmark applications on the prototype system is shown to be limited to low, single-digit percentages of a baseline system without tagging.