Vyper sha3 codegen bug

Summary

There is an error in the stack management when compiling the IR for sha3_64. Concretely, the height variable is miscalculated. The vulnerability can't be triggered without writing the IR by hand. That is, it cannot be triggered from regular vyper code, it can only be triggered by using the fang binary directly (this binary used to be called vyper-ir prior to v0.3.4).

Details

To compile sha3_64, the arg[0] and arg[1] have to be compiled: https://github.com/vyperlang/vyper/blob/c150fc49ee9375a930d177044559b83cb95f7963/vyper/ir/compile_ir.py#L585-L586

As can be seen, after compiling the 0th arg, the height variable isn't increased. If new withargs are defined in the inner scope, they are manipulated correctly, because both their height is off and also the global height is off and thus their placement on the stack is computed correctly.

sha3_64 is used for retrieval in mappings. No flow that would cache the key was found, the issue shouldn't be possible to trigger when compiling the compiler-generated IR.

PoC

Suppose the following hand-written IR:

(with _loc
	(with val 1 
		(with key 2 
			(sha3_64 val key))) 
				(seq 
					(sstore _loc 
					(with x (sload _loc) 
						(with ans (add x 1) (seq (assert (ge ans x)) ans))))))

after compilation:

the generated bytecode: 6001600281806020525f5260405f2090509050805460018101818110610026579050815550005b5f80fd

0000    60  PUSH1 0x01
0002    60  PUSH1 0x02
0004    81  DUP2
0005    80  DUP1       *********** bad code here!!!!!!
0006    60  PUSH1 0x20
0008    52  MSTORE

It can be seen that the second DUP will dup the item on the top of the stack which is incorrect.

Patches

Patched in https://github.com/vyperlang/vyper/pull/4063.

Impact

Versions v0.2.0-v0.3.10 were evaluated, and access of the variable with the invalid height is not reachable from IR generated by the vyper front-end. Because the issue isn't triggered during normal compilation of vyper code, the impact is considered low.