Lmdb-rs: Undefined Behavior In `from_mdb_value` Function
Introduction
In this comprehensive article, we'll explore a critical issue discovered within the lmdb-rs crate, specifically focusing on undefined behavior arising from the safe function from_mdb_value. This exploration is crucial for developers using lmdb-rs and those interested in the intricacies of Rust's memory safety and unsafe code interactions. The initial discovery and discussion took place in the vhbit/lmdb-rs repository, highlighting the importance of community contributions in identifying and addressing potential vulnerabilities.
Understanding the Core Issue
The heart of the problem lies within the from_mdb_value function, a safe abstraction intended to convert MDB_val (a structure from the underlying LMDB library) into Rust types. The vulnerability stems from the function's reliance on raw pointer manipulation without adequate validation. Specifically, the function doesn't properly track pointer provenance before invoking the unsafe slice::from_raw_parts function. This oversight can lead to undefined behavior if the provided raw pointer is invalid, such as an integer-cast pointer or a null pointer with a non-zero length. Undefined behavior in Rust, as in many other languages, can lead to unpredictable program behavior, including crashes, data corruption, and security vulnerabilities. Therefore, understanding and mitigating such risks is paramount.
Detailed Explanation of the Vulnerability
The from_mdb_value function in lmdb-rs is designed to facilitate the conversion of data stored in an LMDB database into Rust-native types. It achieves this by taking an MDB_val struct, which contains a pointer to the data (mv_data) and the size of the data (mv_size), and creating a Rust slice that references this data. The problematic code snippet, found in src/traits.rs, utilizes slice::from_raw_parts to achieve this:
slice::from_raw_parts(ptr, value.get_size()).to_vec()
The slice::from_raw_parts function is inherently unsafe because it creates a slice from a raw pointer and a length. It's the caller's responsibility to ensure that the pointer is valid, non-null, and points to a memory region that is at least as large as the specified length. If these conditions are not met, the behavior is undefined. The from_mdb_value function, being a safe function, should ideally handle these checks internally to prevent users from accidentally triggering undefined behavior. However, the original implementation lacked these crucial checks, making it vulnerable to invalid MDB_val inputs.
Proof of Concept (PoC) Explained
To demonstrate the vulnerability, a Proof of Concept (PoC) was developed. This PoC highlights how crafting specific MDB_val instances with invalid pointers can trigger undefined behavior. Let's break down the PoC code:
use std::ffi::c_void;
use std::ptr;
use lmdb_rs::{MDB_val, FromMdbValue, ToMdbValue};
fn provoke_vec(m: &MDB_val) {
let mdb_value = m.to_mdb_value();
let _ = <Vec<u8> as FromMdbValue>::from_mdb_value(&mdb_value);
}
fn main() {
let bad_ptr = 1usize as *const c_void;
let m = MDB_val { mv_size: 8, mv_data: bad_ptr };
let _ = std::panic::catch_unwind(|| provoke_vec(&m)); // first to invoke UB
let m_null = MDB_val { mv_size: 4, mv_data: ptr::null() };
let _ = std::panic::catch_unwind(|| provoke_vec(&m_null)); // second to invoke UB
}
The PoC constructs two scenarios that trigger undefined behavior:
- Invalid Pointer (Integer Cast):
bad_ptris created by casting the integer1usizeto a raw pointer (*const c_void). This creates a pointer that is highly unlikely to point to valid memory.- An
MDB_val(m) is then created using thisbad_ptrand a non-zero size (mv_size: 8). - When
from_mdb_valueis called with thisMDB_val,slice::from_raw_partsattempts to create a slice from this invalid pointer, resulting in undefined behavior.
- Null Pointer with Non-Zero Length:
m_nullis created with a null pointer (ptr::null()) and a non-zero size (mv_size: 4).- Similar to the first case, calling
from_mdb_valuewithm_nullcausesslice::from_raw_partsto be invoked with a null pointer and a non-zero length, leading to undefined behavior.
The use of std::panic::catch_unwind is a common technique in Rust for handling potential panics, which can occur as a result of undefined behavior. However, it's important to note that catching a panic doesn't necessarily prevent undefined behavior from occurring; it simply allows the program to continue executing after the panic, albeit in a potentially corrupted state.
The command cargo +nightly miri run is used to run the PoC under Miri, a Rust interpreter that can detect certain kinds of undefined behavior at runtime. Miri is an invaluable tool for ensuring the safety of Rust code, especially when dealing with unsafe code or FFI (Foreign Function Interface) boundaries.
Deeper Dive into the Errors Reported by Miri
When running the PoC with Miri, two distinct error messages are generated, each highlighting a specific instance of undefined behavior:
- Invalid Pointer (Integer Cast) Error:
This error message clearly indicates that the pointererror: Undefined Behavior: pointer not dereferenceable: pointer must be dereferenceable for 8 bytes, but got 0x1[noalloc] which is a dangling pointer (it has no provenance) --> /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lmdb-rs-0.7.6/src/traits.rs:101:13 | 101 | slice::from_raw_parts(ptr, value.get_size()).to_vec() | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here |0x1, created by casting the integer1usize, is not dereferenceable. Miri identifies this pointer as[noalloc], meaning it has no associated memory allocation and is therefore considered a dangling pointer. Attempting to create a slice from this pointer with a length of 8 bytes violates the preconditions ofslice::from_raw_partsand results in undefined behavior. The term "provenance" is also key here. In Rust's memory model, provenance tracks the origin and validity of pointers. A pointer without provenance is essentially a "ghost" pointer, not tied to any allocated memory. - Null Pointer with Non-Zero Length Error:
This error message highlights the second scenario in the PoC, where a null pointer is used with a non-zero length. While a null pointer is a valid pointer value in Rust, it can only be dereferenced (or used to create a slice) when the length is zero. Attempting to create a slice of length 4 from a null pointer is a clear violation oferror: Undefined Behavior: pointer not dereferenceable: pointer must be dereferenceable for 4 bytes, but got null pointer --> /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lmdb-rs-0.7.6/src/traits.rs:101:13 | 101 | slice::from_raw_parts(ptr, value.get_size()).to_vec() | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here | = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information = note: BACKTRACE: = note: inside `<std::vec::Vec<u8> as lmdb_rs::FromMdbValue>::from_mdb_value` at /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lmdb-rs-0.7.6/src/traits.rs:101:13: 101:57slice::from_raw_partspreconditions and leads to undefined behavior. The error message also provides valuable context, indicating that this is a bug in the program and referencing the Rust documentation on undefined behavior.
Implications and Mitigation Strategies
The discovery of this vulnerability in lmdb-rs underscores the challenges of writing safe abstractions over unsafe code. It's crucial for library authors to carefully consider the potential for invalid inputs and implement appropriate validation and error handling mechanisms. In this specific case, the from_mdb_value function should perform checks to ensure that the pointer is non-null and points to a valid memory region before calling slice::from_raw_parts.
Here are some potential mitigation strategies:
- Pointer Validation: The most direct solution is to add checks within
from_mdb_valueto validate the pointer and size. This could involve checking for null pointers and ensuring that the pointer points to a valid memory allocation with sufficient size. - Using Safer Alternatives: Consider using safer alternatives to
slice::from_raw_partsif available. While there isn't a direct safe alternative that provides the same functionality, one could potentially use a combination of pointer arithmetic and bounds checking to achieve a similar result in a safer manner. - Fuzzing: Fuzzing is a powerful technique for automatically discovering vulnerabilities in software. By providing a wide range of randomly generated inputs, fuzzers can often uncover edge cases and unexpected behaviors that manual testing might miss. Integrating fuzzing into the
lmdb-rsbuild process could help identify similar issues in the future. - Miri Testing: As demonstrated by the PoC, Miri is an invaluable tool for detecting undefined behavior in Rust code. Regularly running tests under Miri, especially for code that involves unsafe operations or FFI, can help catch potential vulnerabilities early in the development cycle.
Conclusion
The undefined behavior vulnerability in the from_mdb_value function of lmdb-rs serves as a valuable lesson in the importance of careful design and implementation when working with unsafe code in Rust. By understanding the root cause of the issue, the errors reported by Miri, and potential mitigation strategies, developers can write more robust and secure Rust libraries. The community's prompt response and willingness to contribute a fix highlight the collaborative nature of the Rust ecosystem and its commitment to memory safety. Remember, when working with raw pointers, thorough validation is key to preventing undefined behavior and ensuring the reliability of your code. Always strive to create safe abstractions that protect users from the complexities and dangers of unsafe operations. For more information on undefined behavior in Rust, please refer to the official Rust documentation.
