// SPDX-License-Identifier: GPL-2.0 //! Slices to user space memory regions.//!//! C header: [`include/linux/uaccess.h`](srctree/include/linux/uaccess.h) use crate::{    alloc::{Allocator, Flags},    bindings,    error::Result,    ffi::c_void,    prelude::*,    transmute::{AsBytes, FromBytes},};use core::mem::{size_of, MaybeUninit}; /// The type used for userspace addresses.pub type UserPtr = usize; /// 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 (time-of-check to/// time-of-use) bugs. Every time a memory location is read, the reader's position is advanced by/// the read length and the next read will start from there. This helps prevent accidentally reading/// the same location twice and causing a TOCTOU bug.////// Creating a [`UserSliceReader`] and/or [`UserSliceWriter`] consumes the `UserSlice`, helping/// ensure that there aren't multiple readers or writers to the same location.////// If double-fetching a memory location is necessary for some reason, then that is done by creating/// multiple readers to the same memory location, e.g. using [`clone_reader`].////// # Examples////// Takes a region of userspace memory from the current process, and modify it by adding one to/// every byte in the region.////// ```no_run/// use kernel::ffi::c_void;/// use kernel::uaccess::{UserPtr, UserSlice};////// fn bytes_add_one(uptr: UserPtr, len: usize) -> Result {///     let (read, mut write) = UserSlice::new(uptr, len).reader_writer();//////     let mut buf = KVec::new();///     read.read_all(&mut buf, GFP_KERNEL)?;//////     for b in &mut buf {///         *b = b.wrapping_add(1);///     }//////     write.write_slice(&buf)?;///     Ok(())/// }/// ```////// Example illustrating a TOCTOU (time-of-check to time-of-use) bug.////// ```no_run/// use kernel::ffi::c_void;/// use kernel::uaccess::{UserPtr, UserSlice};////// /// Returns whether the data in this region is valid./// fn is_valid(uptr: UserPtr, len: usize) -> Result<bool> {///     let read = UserSlice::new(uptr, len).reader();//////     let mut buf = KVec::new();///     read.read_all(&mut buf, GFP_KERNEL)?;//////     todo!()/// }////// /// Returns the bytes behind this user pointer if they are valid./// fn get_bytes_if_valid(uptr: UserPtr, len: usize) -> Result<KVec<u8>> {///     if !is_valid(uptr, len)? {///         return Err(EINVAL);///     }//////     let read = UserSlice::new(uptr, len).reader();//////     let mut buf = KVec::new();///     read.read_all(&mut buf, GFP_KERNEL)?;//////     // THIS IS A BUG! The bytes could have changed since we checked them.///     /////     // To avoid this kind of bug, don't call `UserSlice::new` multiple///     // times with the same address.///     Ok(buf)/// }/// ```////// [`std::io`]: https://doc.rust-lang.org/std/io/index.html/// [`clone_reader`]: UserSliceReader::clone_readerpub struct UserSlice {    ptr: UserPtr,    length: usize,} 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). Validity of the pointer is checked when you    /// attempt to read or write, not in the call to `UserSlice::new`.    ///    /// 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: UserPtr, length: usize) -> Self {        UserSlice { ptr, length }    }     /// Reads the entirety of the user slice, appending it to the end of the provided buffer.    ///    /// Fails with [`EFAULT`] if the read happens on a bad address.    pub fn read_all<A: Allocator>(self, buf: &mut Vec<u8, A>, flags: Flags) -> Result {        self.reader().read_all(buf, flags)    }     /// Constructs a [`UserSliceReader`].    pub fn reader(self) -> UserSliceReader {        UserSliceReader {            ptr: self.ptr,            length: self.length,        }    }     /// Constructs a [`UserSliceWriter`].    pub fn writer(self) -> UserSliceWriter {        UserSliceWriter {            ptr: self.ptr,            length: self.length,        }    }     /// Constructs both a [`UserSliceReader`] and a [`UserSliceWriter`].    ///    /// Usually when this is used, you will first read the data, and then overwrite it afterwards.    pub fn reader_writer(self) -> (UserSliceReader, UserSliceWriter) {        (            UserSliceReader {                ptr: self.ptr,                length: self.length,            },            UserSliceWriter {                ptr: self.ptr,                length: self.length,            },        )    }} /// A reader for [`UserSlice`].////// Used to incrementally read from the user slice.pub struct UserSliceReader {    ptr: UserPtr,    length: usize,} impl UserSliceReader {    /// Skip the provided number of bytes.    ///    /// Returns an error if skipping more than the length of the buffer.    pub fn skip(&mut self, num_skip: usize) -> Result {        // Update `self.length` first since that's the fallible part of this operation.        self.length = self.length.checked_sub(num_skip).ok_or(EFAULT)?;        self.ptr = self.ptr.wrapping_add(num_skip);        Ok(())    }     /// Create a reader that can access the same range of data.    ///    /// Reading from the clone does not advance the current reader.    ///    /// The caller should take care to not introduce TOCTOU issues, as described in the    /// documentation for [`UserSlice`].    pub fn clone_reader(&self) -> UserSliceReader {        UserSliceReader {            ptr: self.ptr,            length: self.length,        }    }     /// Returns the number of bytes left to be read from this reader.    ///    /// Note that even reading less than this number of bytes may fail.    pub fn len(&self) -> usize {        self.length    }     /// Returns `true` if no data is available in the io buffer.    pub fn is_empty(&self) -> bool {        self.length == 0    }     /// Reads raw data from the user slice into a kernel buffer.    ///    /// For a version that uses `&mut [u8]`, please see [`UserSliceReader::read_slice`].    ///    /// Fails with [`EFAULT`] if the read happens on a bad address, or if the read goes out of    /// bounds of this [`UserSliceReader`]. This call may modify `out` even if it returns an error.    ///    /// # Guarantees    ///    /// After a successful call to this method, all bytes in `out` are initialized.    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);        }        // SAFETY: `out_ptr` points into a mutable slice of length `len`, so we may write        // that many bytes to it.        let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr as *const c_void, len) };        if res != 0 {            return Err(EFAULT);        }        self.ptr = self.ptr.wrapping_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, or if the read goes out of    /// bounds of this [`UserSliceReader`]. This call may modify `out` even if it returns an error.    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)    }     /// Reads a value of the specified type.    ///    /// Fails with [`EFAULT`] if the read happens on a bad address, or if the read goes out of    /// bounds of this [`UserSliceReader`].    pub fn read<T: FromBytes>(&mut self) -> Result<T> {        let len = size_of::<T>();        if len > self.length {            return Err(EFAULT);        }        let mut out: MaybeUninit<T> = MaybeUninit::uninit();        // SAFETY: The local variable `out` is valid for writing `size_of::<T>()` bytes.        //        // By using the _copy_from_user variant, we skip the check_object_size check that verifies        // the kernel pointer. This mirrors the logic on the C side that skips the check when the        // length is a compile-time constant.        let res = unsafe {            bindings::_copy_from_user(                out.as_mut_ptr().cast::<c_void>(),                self.ptr as *const c_void,                len,            )        };        if res != 0 {            return Err(EFAULT);        }        self.ptr = self.ptr.wrapping_add(len);        self.length -= len;        // SAFETY: The read above has initialized all bytes in `out`, and since `T` implements        // `FromBytes`, any bit-pattern is a valid value for this type.        Ok(unsafe { out.assume_init() })    }     /// Reads the entirety of the user slice, appending it to the end of the provided buffer.    ///    /// Fails with [`EFAULT`] if the read happens on a bad address.    pub fn read_all<A: Allocator>(mut self, buf: &mut Vec<u8, A>, flags: Flags) -> Result {        let len = self.length;        buf.reserve(len, flags)?;         // The call to `reserve` was successful, so the spare capacity is at least `len` bytes long.        self.read_raw(&mut buf.spare_capacity_mut()[..len])?;         // SAFETY: Since the call to `read_raw` was successful, so the next `len` bytes of the        // vector have been initialized.        unsafe { buf.inc_len(len) };        Ok(())    }} /// A writer for [`UserSlice`].////// Used to incrementally write into the user slice.pub struct UserSliceWriter {    ptr: UserPtr,    length: usize,} impl UserSliceWriter {    /// Returns the amount of space remaining in this buffer.    ///    /// Note that even writing less than this number of bytes may fail.    pub fn len(&self) -> usize {        self.length    }     /// Returns `true` if no more data can be written to this buffer.    pub fn is_empty(&self) -> bool {        self.length == 0    }     /// Writes raw data to this user pointer from a kernel buffer.    ///    /// Fails with [`EFAULT`] if the write happens on a bad address, or if the write goes out of    /// bounds of this [`UserSliceWriter`]. This call may modify the associated userspace slice even    /// if it returns an error.    pub fn write_slice(&mut self, data: &[u8]) -> Result {        let len = data.len();        let data_ptr = data.as_ptr().cast::<c_void>();        if len > self.length {            return Err(EFAULT);        }        // SAFETY: `data_ptr` points into an immutable slice of length `len`, so we may read        // that many bytes from it.        let res = unsafe { bindings::copy_to_user(self.ptr as *mut c_void, data_ptr, len) };        if res != 0 {            return Err(EFAULT);        }        self.ptr = self.ptr.wrapping_add(len);        self.length -= len;        Ok(())    }     /// Writes the provided Rust value to this userspace pointer.    ///    /// Fails with [`EFAULT`] if the write happens on a bad address, or if the write goes out of    /// bounds of this [`UserSliceWriter`]. This call may modify the associated userspace slice even    /// if it returns an error.    pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {        let len = size_of::<T>();        if len > self.length {            return Err(EFAULT);        }        // SAFETY: The reference points to a value of type `T`, so it is valid for reading        // `size_of::<T>()` bytes.        //        // By using the _copy_to_user variant, we skip the check_object_size check that verifies the        // kernel pointer. This mirrors the logic on the C side that skips the check when the length        // is a compile-time constant.        let res = unsafe {            bindings::_copy_to_user(                self.ptr as *mut c_void,                (value as *const T).cast::<c_void>(),                len,            )        };        if res != 0 {            return Err(EFAULT);        }        self.ptr = self.ptr.wrapping_add(len);        self.length -= len;        Ok(())    }}