SIMD-0179: SBPF stricter verification constraints

proposals/0179-stricter-verification.md

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+---
+simd: '0179'
+title: SBPF Stricter verification constraints
+authors:
+  - Alexander Meißner
+  - Lucas Steuernagel
+category: Standard
+type: Core
+status: Draft
+created: 2024-10-03
+---
+
+## Summary
+
+This SIMD proposes improvements in the verification rules of SBPF programs. 
+The introduction of the syscall instruction in SIMD-0178 allows for the 
+differentiation of internal and external calls, opening up for stricter 
+verification constraints and increased security of deployed programs.
+
+## Motivation
+
+The `call` instruction (opcode `0x85`) in SBF, especially when it requires a 
+relocation, is ambiguous. It does not carry information about whether it is 
+a system call or an internal call. This ambiguity prevents more rigid 
+verification constraints that would reject programs jumping to inconvenient 
+locations, like invalid destinations or the middle of an `lddw` instruction. 
+The new syscall instruction introduced in SIMD-0178 permits the 
+differentiation of internal and external calls, so we can introduce new 
+verification rules to prevent any unwarranted behavior.
+
+## New Terminology
+
+None.
+
+## Detailed Design
+
+The following must go into effect if and only if a program indicates the SBF 
+version XX or higher in its ELF header e_flags field, according to the 
+specification of SIMD-0161.
+
+### Restrict functions’ last instruction
+
+Functions must only end with the `ja` (opcode `0x05`) or the exit (opcode 
+`0x9D` since SIMD-0178) instruction. Allowing calls to be the last instruction 
+of functions was inconvenient, because when the call returns, and there is no 
+other instruction to redirect the control flow, we will execute the very next 
+program counter, resulting in a fallthrough into another function’s code. 
+Offending this new validation condition must throw an 
+`VerifierError::InvalidFunction` error.
+
+### Restrict jump instruction destination
+
+All jump instructions, except for `call` (opcode `0x85`) and `callx` (opcode 
+`0x8D`), must now jump to a code location inside their own function. Jumping 
+to arbitrary locations hinders a precise program verification. 
+`VerifierError::JumpOutOfCode` must be thrown for offending this rule.
+
+`call imm` (opcode `0x85`) must only be allowed to jump to a program counter 
+previously registered as the start of a function. Otherwise 
+`VerifierError::InvalidFunction` must be thrown. Functions must be registered 
+if they are present in the symbol table. The entrypoint to the program must 
+also define a valid function.
+
+### Runtime check for callx
+
+The jump destination of `callx` (opcode `0x8D`) must be checked during 
+execution time to match the initial address of a registered function. If this 
+is not the case, a `EbpfError::UnsupportedInstruction` must be thrown. This 
+measure is supposed to improve security of programs, disallowing the malicious 
+use of callx.
+
+A function is registered according to the rules mentioned in the previous 
+section: be present in the symbol table or be the entrypoint.
+
+### Limit where a function can start
+
+Presently, functions may start in any part of the ELF text section, however 
+this is an encumbrance when it comes to `lddw` (opcode `0x18`) instructions. 
+As they occupy two instruction slots in an ELF, it is possible for functions 
+to start between these lddw slots, very likely resulting in undesired 
+behavior. The verifier must throw an `VerifierError::JumpToMiddleOfLddw` error 
+when that is the case.
+
+### Removal of ExecutionOverrun error
+
+As the jump instructions destinations are now limited to the range of the 
+function they are located in, and the call instructions can only move the 
+program counter to a known location, the `EbpfError::ExecutionOverrun` must not 
+be reachable, so it must be removed.
+
+## Alternatives Considered
+
+None.
+
+## Impact
+
+The changes proposed in this SIMD are transparent to dApp developers. The 
+compiler toolchain will emit correct code for the specified SBF version. The 
+enhanced verification restrictions will improve the security of programs 
+deployed to the blockchain, filtering out unwarranted behavior that could be 
+used with malicious intents.
+
+## Security Considerations
+
+None.