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Fix UB in NtUnicodeStrMut::try_from_u16() #2

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Merged
ColinFinck merged 2 commits into from
May 16, 2026
Merged

Fix UB in NtUnicodeStrMut::try_from_u16() #2

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8d4d3b4
Fix UB in NtUnicodeStrMut::try_from_u16()
smichaku Apr 27, 2026
df111ec
Move newly added helper functions to `NtUnicodeStr`
ColinFinck May 16, 2026
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87 changes: 51 additions & 36 deletions src/unicode_string/str.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
// Copyright 2023 Colin Finck <colin@reactos.org>
// Copyright 2023-2026 Colin Finck <colin@reactos.org>
// SPDX-License-Identifier: MIT OR Apache-2.0

use core::cmp::Ordering;
Expand Down Expand Up @@ -158,12 +158,7 @@ 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 = Self::try_length_from_u16(buffer)?;

Ok(Self {
raw: RawNtString {
Expand Down Expand Up @@ -191,9 +186,48 @@ 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;
let (length, maximum_length) = Self::try_length_from_u16_until_nul(buffer)?;

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

pub(crate) fn try_length_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 try_length_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))
}

pub(crate) fn try_length_from_u16_until_nul(buffer: &[u16]) -> Result<(u16, u16)> {
match buffer.iter().position(|x| *x == 0) {
Some(nul_pos) => {
// Include the terminating NUL character in `maximum_length` ...
Expand All @@ -203,23 +237,16 @@ impl<'a> NtUnicodeStr<'a> {
let maximum_length_usize = maximum_elements
.checked_mul(mem::size_of::<u16>())
.ok_or(NtStringError::BufferSizeExceedsU16)?;
maximum_length = u16::try_from(maximum_length_usize)
let 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 - mem::size_of::<u16>() as u16;

Ok(Self {
raw: RawNtString {
length,
maximum_length,
buffer: buffer.as_ptr(),
},
_lifetime: PhantomData,
})
Ok((length, maximum_length))
}
None => Err(NtStringError::NulNotFound),
}
}

pub(crate) fn u16_iter(&'a self) -> Copied<Iter<'a, u16>> {
Expand Down Expand Up @@ -314,25 +341,13 @@ 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) = Self::try_length_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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Open in desktop
Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
// Copyright 2023 Colin Finck <colin@reactos.org>
// Copyright 2023-2026 Colin Finck <colin@reactos.org>
// SPDX-License-Identifier: MIT OR Apache-2.0

use core::cmp::Ordering;
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 = NtUnicodeStr::try_length_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) = NtUnicodeStr::try_length_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) = NtUnicodeStr::try_length_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
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