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Diffstat (limited to 'rust/kernel/user_ptr.rs')
| -rw-r--r-- | rust/kernel/user_ptr.rs | 175 |
1 files changed, 175 insertions, 0 deletions
diff --git a/rust/kernel/user_ptr.rs b/rust/kernel/user_ptr.rs new file mode 100644 index 000000000000..084535675c4a --- /dev/null +++ b/rust/kernel/user_ptr.rs @@ -0,0 +1,175 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! User pointers. +//! +//! C header: [`include/linux/uaccess.h`](../../../../include/linux/uaccess.h) + +use crate::{ + bindings, + error::code::*, + error::Result, + io_buffer::{IoBufferReader, IoBufferWriter}, +}; +use alloc::vec::Vec; + +/// A reference to an area in userspace memory, which can be either +/// read-only or read-write. +/// +/// All methods on this struct are safe: invalid pointers 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. +/// +/// All APIs enforce the invariant that a given byte of memory from userspace +/// may only be read once. By preventing double-fetches we avoid TOCTOU +/// vulnerabilities. This is accomplished by taking `self` by value to prevent +/// obtaining multiple readers on a given [`UserSlicePtr`], and the readers +/// only permitting forward reads. +/// +/// 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). +/// +/// [`std::io`]: https://doc.rust-lang.org/std/io/index.html +pub struct UserSlicePtr(*mut core::ffi::c_void, usize); + +impl UserSlicePtr { + /// Constructs a user slice from a raw pointer and a length in bytes. + /// + /// # Safety + /// + /// Callers must be careful to avoid time-of-check-time-of-use + /// (TOCTOU) issues. The simplest way is to create a single instance of + /// [`UserSlicePtr`] per user memory block as it reads each byte at + /// most once. + pub unsafe fn new(ptr: *mut core::ffi::c_void, length: usize) -> Self { + UserSlicePtr(ptr, length) + } + + /// Reads the entirety of the user slice. + /// + /// Returns `EFAULT` if the address does not currently point to + /// mapped, readable memory. + pub fn read_all(self) -> Result<Vec<u8>> { + self.reader().read_all() + } + + /// Constructs a [`UserSlicePtrReader`]. + pub fn reader(self) -> UserSlicePtrReader { + UserSlicePtrReader(self.0, self.1) + } + + /// Writes the provided slice into the user slice. + /// + /// Returns `EFAULT` if the address does not currently point to + /// mapped, writable memory (in which case some data from before the + /// fault may be written), or `data` is larger than the user slice + /// (in which case no data is written). + pub fn write_all(self, data: &[u8]) -> Result { + self.writer().write_slice(data) + } + + /// Constructs a [`UserSlicePtrWriter`]. + pub fn writer(self) -> UserSlicePtrWriter { + UserSlicePtrWriter(self.0, self.1) + } + + /// Constructs both a [`UserSlicePtrReader`] and a [`UserSlicePtrWriter`]. + pub fn reader_writer(self) -> (UserSlicePtrReader, UserSlicePtrWriter) { + ( + UserSlicePtrReader(self.0, self.1), + UserSlicePtrWriter(self.0, self.1), + ) + } +} + +/// A reader for [`UserSlicePtr`]. +/// +/// Used to incrementally read from the user slice. +pub struct UserSlicePtrReader(*mut core::ffi::c_void, usize); + +impl IoBufferReader for UserSlicePtrReader { + /// Returns the number of bytes left to be read from this. + /// + /// Note that even reading less than this number of bytes may fail. + fn len(&self) -> usize { + self.1 + } + + /// Reads raw data from the user slice into a raw kernel buffer. + /// + /// # Safety + /// + /// The output buffer must be valid. + unsafe fn read_raw(&mut self, out: *mut u8, len: usize) -> Result { + if len > self.1 || len > u32::MAX as usize { + return Err(EFAULT); + } + let res = unsafe { bindings::copy_from_user(out as _, self.0, len as _) }; + if res != 0 { + return Err(EFAULT); + } + // Since this is not a pointer to a valid object in our program, + // we cannot use `add`, which has C-style rules for defined + // behavior. + self.0 = self.0.wrapping_add(len); + self.1 -= len; + Ok(()) + } +} + +/// A writer for [`UserSlicePtr`]. +/// +/// Used to incrementally write into the user slice. +pub struct UserSlicePtrWriter(*mut core::ffi::c_void, usize); + +impl IoBufferWriter for UserSlicePtrWriter { + fn len(&self) -> usize { + self.1 + } + + fn clear(&mut self, mut len: usize) -> Result { + let mut ret = Ok(()); + if len > self.1 { + ret = Err(EFAULT); + len = self.1; + } + + // SAFETY: The buffer will be validated by `clear_user`. We ensure that `len` is within + // bounds in the check above. + let left = unsafe { bindings::clear_user(self.0, len as _) } as usize; + if left != 0 { + ret = Err(EFAULT); + len -= left; + } + + self.0 = self.0.wrapping_add(len); + self.1 -= len; + ret + } + + unsafe fn write_raw(&mut self, data: *const u8, len: usize) -> Result { + if len > self.1 || len > u32::MAX as usize { + return Err(EFAULT); + } + let res = unsafe { bindings::copy_to_user(self.0, data as _, len as _) }; + if res != 0 { + return Err(EFAULT); + } + // Since this is not a pointer to a valid object in our program, + // we cannot use `add`, which has C-style rules for defined + // behavior. + self.0 = self.0.wrapping_add(len); + self.1 -= len; + Ok(()) + } +} |