On 15.04.24 11:44, Alice Ryhl wrote:
> On Mon, Apr 15, 2024 at 11:37 AM Benno Lossin <benno.lossin@xxxxxxxxx> 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@xxxxxxxxx>
--
Cheers,
Benno
