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Fix UB in NtUnicodeStrMut::try_from_u16() #2

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57 changes: 57 additions & 0 deletions src/helpers.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,11 @@
// SPDX-License-Identifier: MIT OR Apache-2.0

use core::cmp::Ordering;
use core::mem;

use widestring::U16CStr;

use crate::error::{NtStringError, Result};

/// Generic memory layout unified for `ANSI_STRING`, `OEM_STRING`, `UNICODE_STRING`,
/// in their mutable and immutable versions.
Expand All @@ -16,6 +21,58 @@ pub(crate) struct RawNtString<T> {
pub(crate) buffer: T,
}

pub(crate) fn check_from_u16(buffer: &[u16]) -> Result<u16> {
let elements = buffer.len();
let length_usize = elements
.checked_mul(mem::size_of::<u16>())
.ok_or(NtStringError::BufferSizeExceedsU16)?;
let length = u16::try_from(length_usize).map_err(|_| NtStringError::BufferSizeExceedsU16)?;
Ok(length)
}

pub(crate) fn check_from_u16_until_nul(buffer: &[u16]) -> Result<(u16, u16)> {
let length;
let maximum_length;

match buffer.iter().position(|x| *x == 0) {
Some(nul_pos) => {
// Include the terminating NUL character in `maximum_length` ...
let maximum_elements = nul_pos
.checked_add(1)
.ok_or(NtStringError::BufferSizeExceedsU16)?;
let maximum_length_usize = maximum_elements
.checked_mul(mem::size_of::<u16>())
.ok_or(NtStringError::BufferSizeExceedsU16)?;
maximum_length = u16::try_from(maximum_length_usize)
.map_err(|_| NtStringError::BufferSizeExceedsU16)?;

// ... but not in `length`
length = maximum_length - mem::size_of::<u16>() as u16;
}
None => return Err(NtStringError::NulNotFound),
};

Ok((length, maximum_length))
}

pub(crate) fn check_from_u16_cstr(u16cstr: &U16CStr) -> Result<(u16, u16)> {
let buffer = u16cstr.as_slice_with_nul();

// Include the terminating NUL character in `maximum_length` ...
let maximum_length_in_elements = buffer.len();
let maximum_length_in_bytes = maximum_length_in_elements
.checked_mul(mem::size_of::<u16>())
.ok_or(NtStringError::BufferSizeExceedsU16)?;
let maximum_length =
u16::try_from(maximum_length_in_bytes).map_err(|_| NtStringError::BufferSizeExceedsU16)?;

// ... but not in `length`
debug_assert!(maximum_length >= mem::size_of::<u16>() as u16);
let length = maximum_length - mem::size_of::<u16>() as u16;

Ok((length, maximum_length))
}

/// Compare any two `u16` iterators and return an [`Ordering`] value.
///
/// Can be used to implement `cmp`/`partial_cmp` and `eq`/`partial_eq`.
Expand Down
48 changes: 7 additions & 41 deletions src/unicode_string/str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,9 @@ use core::{fmt, mem, slice};
use widestring::{U16CStr, U16Str};

use crate::error::{NtStringError, Result};
use crate::helpers::{cmp_iter, RawNtString};
use crate::helpers::{
check_from_u16, check_from_u16_cstr, check_from_u16_until_nul, cmp_iter, RawNtString,
};

use super::iter::{Chars, CharsLossy};

Expand Down Expand Up @@ -158,12 +160,7 @@ impl<'a> NtUnicodeStr<'a> {
///
/// [`try_from_u16_until_nul`]: Self::try_from_u16_until_nul
pub fn try_from_u16(buffer: &'a [u16]) -> Result<Self> {
let elements = buffer.len();
let length_usize = elements
.checked_mul(mem::size_of::<u16>())
.ok_or(NtStringError::BufferSizeExceedsU16)?;
let length =
u16::try_from(length_usize).map_err(|_| NtStringError::BufferSizeExceedsU16)?;
let length = check_from_u16(buffer)?;

Ok(Self {
raw: RawNtString {
Expand Down Expand Up @@ -191,26 +188,7 @@ impl<'a> NtUnicodeStr<'a> {
///
/// [`try_from_u16`]: Self::try_from_u16
pub fn try_from_u16_until_nul(buffer: &'a [u16]) -> Result<Self> {
let length;
let maximum_length;

match buffer.iter().position(|x| *x == 0) {
Some(nul_pos) => {
// Include the terminating NUL character in `maximum_length` ...
let maximum_elements = nul_pos
.checked_add(1)
.ok_or(NtStringError::BufferSizeExceedsU16)?;
let maximum_length_usize = maximum_elements
.checked_mul(mem::size_of::<u16>())
.ok_or(NtStringError::BufferSizeExceedsU16)?;
maximum_length = u16::try_from(maximum_length_usize)
.map_err(|_| NtStringError::BufferSizeExceedsU16)?;

// ... but not in `length`
length = maximum_length - mem::size_of::<u16>() as u16;
}
None => return Err(NtStringError::NulNotFound),
};
let (length, maximum_length) = check_from_u16_until_nul(buffer)?;

Ok(Self {
raw: RawNtString {
Expand Down Expand Up @@ -314,25 +292,13 @@ impl<'a> TryFrom<&'a U16CStr> for NtUnicodeStr<'a> {
/// The internal buffer will be NUL-terminated.
/// See the [module-level documentation](super) for the implications of that.
fn try_from(value: &'a U16CStr) -> Result<Self> {
let buffer = value.as_slice_with_nul();

// Include the terminating NUL character in `maximum_length` ...
let maximum_length_in_elements = buffer.len();
let maximum_length_in_bytes = maximum_length_in_elements
.checked_mul(mem::size_of::<u16>())
.ok_or(NtStringError::BufferSizeExceedsU16)?;
let maximum_length = u16::try_from(maximum_length_in_bytes)
.map_err(|_| NtStringError::BufferSizeExceedsU16)?;

// ... but not in `length`
debug_assert!(maximum_length >= mem::size_of::<u16>() as u16);
let length = maximum_length - mem::size_of::<u16>() as u16;
let (length, maximum_length) = check_from_u16_cstr(value)?;

Ok(Self {
raw: RawNtString {
length,
maximum_length,
buffer: buffer.as_ptr(),
buffer: value.as_ptr(),
},
_lifetime: PhantomData,
})
Expand Down
50 changes: 28 additions & 22 deletions src/unicode_string/strmut.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@ use core::{fmt, mem, slice};
use widestring::{U16CStr, U16Str};

use crate::error::Result;
use crate::helpers::RawNtString;
use crate::helpers::{check_from_u16, check_from_u16_cstr, check_from_u16_until_nul, RawNtString};
use crate::NtStringError;

use super::{impl_eq, impl_partial_cmp, NtUnicodeStr};
Expand Down Expand Up @@ -122,14 +122,16 @@ impl<'a> NtUnicodeStrMut<'a> {
///
/// [`try_from_u16_until_nul`]: Self::try_from_u16_until_nul
pub fn try_from_u16(buffer: &mut [u16]) -> Result<Self> {
let unicode_str = NtUnicodeStr::try_from_u16(buffer)?;
let length = check_from_u16(buffer)?;

// SAFETY: `unicode_str` was created from a mutable `buffer` and
// `NtUnicodeStr` and `NtUnicodeStrMut` have the same memory layout,
// so we can safely transmute `NtUnicodeStr` to `NtUnicodeStrMut`.
let unicode_str_mut = unsafe { mem::transmute(unicode_str) };

Ok(unicode_str_mut)
Ok(Self {
raw: RawNtString {
length,
maximum_length: length,
buffer: buffer.as_mut_ptr(),
},
_lifetime: PhantomData,
})
}

/// Creates an [`NtUnicodeStrMut`] from an existing [`u16`] string buffer that contains at least one NUL character.
Expand All @@ -148,14 +150,16 @@ impl<'a> NtUnicodeStrMut<'a> {
///
/// [`try_from_u16`]: Self::try_from_u16
pub fn try_from_u16_until_nul(buffer: &mut [u16]) -> Result<Self> {
let unicode_str = NtUnicodeStr::try_from_u16_until_nul(buffer)?;
let (length, maximum_length) = check_from_u16_until_nul(buffer)?;

// SAFETY: `unicode_str` was created from a mutable `buffer` and
// `NtUnicodeStr` and `NtUnicodeStrMut` have the same memory layout,
// so we can safely transmute `NtUnicodeStr` to `NtUnicodeStrMut`.
let unicode_str_mut = unsafe { mem::transmute(unicode_str) };

Ok(unicode_str_mut)
Ok(Self {
raw: RawNtString {
length,
maximum_length,
buffer: buffer.as_mut_ptr(),
},
_lifetime: PhantomData,
})
}
}

Expand Down Expand Up @@ -192,14 +196,16 @@ impl<'a> TryFrom<&'a mut U16CStr> for NtUnicodeStrMut<'a> {
/// The internal buffer will be NUL-terminated.
/// See the [module-level documentation](super) for the implications of that.
fn try_from(value: &'a mut U16CStr) -> Result<Self> {
let unicode_str = NtUnicodeStr::try_from(&*value)?;
let (length, maximum_length) = check_from_u16_cstr(value)?;

// SAFETY: `unicode_str` was created from a mutable `value` and
// `NtUnicodeStr` and `NtUnicodeStrMut` have the same memory layout,
// so we can safely transmute `NtUnicodeStr` to `NtUnicodeStrMut`.
let unicode_str_mut = unsafe { mem::transmute(unicode_str) };

Ok(unicode_str_mut)
Ok(Self {
raw: RawNtString {
length,
maximum_length,
buffer: value.as_mut_ptr(),
},
_lifetime: PhantomData,
})
}
}

Expand Down