Re: [PATCH v5 1/4] rust: uaccess: add userspace pointers

[Benno Lossin <benno.lossin@proton.me>]

On 15.04.24 11:44, Alice Ryhl wrote:
> On Mon, Apr 15, 2024 at 11:37 AM Benno Lossin <benno.lossin@proton.me> wrote:
>>
>> On 15.04.24 09:13, Alice Ryhl wrote:
>>> +impl UserSlice {
>>> +    /// Constructs a user slice from a raw pointer and a length in bytes.
>>> +    ///
>>> +    /// Constructing a [`UserSlice`] performs no checks on the provided address and length, it can
>>> +    /// safely be constructed inside a kernel thread with no current userspace process. Reads and
>>> +    /// writes wrap the kernel APIs `copy_from_user` and `copy_to_user`, which check the memory map
>>> +    /// of the current process and enforce that the address range is within the user range (no
>>> +    /// additional calls to `access_ok` are needed).
>>> +    ///
>>> +    /// Callers must be careful to avoid time-of-check-time-of-use (TOCTOU) issues. The simplest way
>>> +    /// is to create a single instance of [`UserSlice`] per user memory block as it reads each byte
>>> +    /// at most once.
>>> +    pub fn new(ptr: *mut c_void, length: usize) -> Self {
>>
>> What would happen if I call this with a kernel pointer and then
>> read/write to it? For example
>>
>>       let mut arr = [MaybeUninit::uninit(); 64];
>>       let ptr: *mut [MaybeUninit<u8>] = &mut arr;
>>       let ptr = ptr.cast::<c_void>();
>>
>>       let slice = UserSlice::new(ptr, 64);
>>       let (mut r, mut w) = slice.reader_writer();
>>
>>       r.read_raw(&mut arr)?;
>>       // SAFETY: `arr` was initialized above.
>>       w.write_slice(unsafe { MaybeUninit::slice_assume_init_ref(&arr) })?;
>>
>> I think this would violate the exclusivity of `&mut` without any
>> `unsafe` code. (the `unsafe` block at the end cannot possibly be wrong)
> 
> This will fail with an EFAULT error. There is a check on the C side
> that verifies that the address is in userspace. (The access_ok call.)

I see, that makes a lot of sense.

Regardless of whether you fix the nit about the guarantees section:

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

-- 
Cheers,
Benno