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

[Alice Ryhl]

Trevor Gross <tmgross@xxxxxxxxx> writes:
> On Mon, Apr 15, 2024 at 3:14 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>>
>> From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
>>
>> A pointer to an area in userspace memory, which can be either read-only
>> or read-write.
>>
>> All methods on this struct are safe: attempting to read or write on bad
>> addresses (either out of the bound of the slice or unmapped addresses)
>> will return `EFAULT`. Concurrent access, *including data races to/from
>> userspace memory*, is permitted, because fundamentally another userspace
>> thread/process could always be modifying memory at the same time (in the
>> same way that userspace Rust's `std::io` permits data races with the
>> contents of files on disk). In the presence of a race, the exact byte
>> values read/written are unspecified but the operation is well-defined.
>> Kernelspace code should validate its copy of data after completing a
>> read, and not expect that multiple reads of the same address will return
>> the same value.
>>
>> These APIs are designed to make it difficult to accidentally write
>> TOCTOU bugs. Every time you read from a memory location, the pointer is
>> advanced by the length so that you cannot use that reader to read the
>> same memory location twice. Preventing double-fetches avoids TOCTOU
>> bugs. This is accomplished by taking `self` by value to prevent
>> obtaining multiple readers on a given `UserSlicePtr`, and the readers
>> only permitting forward reads. If double-fetching a memory location is
>> necessary for some reason, then that is done by creating multiple
>> readers to the same memory location.
>>
>> Constructing a `UserSlicePtr` 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).
>>
>> This code is based on something that was originally written by Wedson on
>> the old rust branch. It was modified by Alice by removing the
>> `IoBufferReader` and `IoBufferWriter` traits, and various other changes.
>>
>> Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
>> Co-developed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
>> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> 
> Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>
> 
> I left some suggestions for documentation improvements and one
> question, but mostly LGTM.

Thanks for taking a look!

>> +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).
> 
> I would just add a note that pointer should be a valid userspace
> pointer, but that gets checked at read/write time

Will do.

>> +    /// 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 {
>> +        UserSlice { ptr, length }
>> +    }
> 
>> +impl UserSliceReader {
>> [...]
>> +    /// Reads raw data from the user slice into a kernel buffer.
>> +    ///
>> +    /// After a successful call to this method, all bytes in `out` are initialized.
> 
> If this is guaranteed, could it return `Result<&mut [u8]>`? So the
> caller doesn't need to unsafely `assume_init` anything.

It could, but I don't think it's that useful. All existing callers will
want to record it somewhere with something like `Vec::set_len`, which
this doesn't help with. There are ways to do something like that, but it
complicates the API further which I am not interested in.

>> +    /// Fails with `EFAULT` if the read happens on a bad address.
> 
> This should also mention that the slice cannot be bigger than the
> reader's length.

I can add a note.
 
>> +    pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
>> +        let len = out.len();
>> +        let out_ptr = out.as_mut_ptr().cast::<c_void>();
>> +        if len > self.length {
>> +            return Err(EFAULT);
>> +        }
>> +        let Ok(len_ulong) = c_ulong::try_from(len) else {
>> +            return Err(EFAULT);
>> +        };
>> +        // SAFETY: `out_ptr` points into a mutable slice of length `len_ulong`, so we may write
>> +        // that many bytes to it.
>> +        let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr, len_ulong) };
>> +        if res != 0 {
>> +            return Err(EFAULT);
>> +        }
>> +        // Userspace pointers are not directly dereferencable by the kernel, so we cannot use `add`,
>> +        // which has C-style rules for defined behavior.
>> +        self.ptr = self.ptr.wrapping_byte_add(len);
>> +        self.length -= len;
>> +        Ok(())
>> +    }
>> +
>> +    /// Reads raw data from the user slice into a kernel buffer.
>> +    ///
>> +    /// Fails with `EFAULT` if the read happens on a bad address.
>> +    pub fn read_slice(&mut self, out: &mut [u8]) -> Result {
>> +        // SAFETY: The types are compatible and `read_raw` doesn't write uninitialized bytes to
>> +        // `out`.
>> +        let out = unsafe { &mut *(out as *mut [u8] as *mut [MaybeUninit<u8>]) };
>> +        self.read_raw(out)
>> +    }
> 
> If this is just a safe version of read_raw, could you crosslink the docs?

Okay.

>> +impl UserSliceWriter {
>> +
>> +    /// Writes raw data to this user pointer from a kernel buffer.
>> +    ///
>> +    /// Fails with `EFAULT` if the write happens on a bad address.
>> +    pub fn write_slice(&mut self, data: &[u8]) -> Result {
>>   [...]
>> +    }
> 
> Could use a note about length like `read_raw`.

Okay.

Alice