[PATCH v5 2/5] rust: bitmap: restrict bitmap length to at most i32::MAX

[Eliot Courtney]

It is currently possible to construct a non-`BitmapVec` backed
`Bitmap` using `Bitmap::from_raw` that is larger than `i32::MAX`, and
it is not part of the unsafe requirements. Restricting all bitmaps
(even non-`BitmapVec` backed ones) to a maximum size of `i32::MAX`
simplifies a few things and matches `BitmapVec::MAX_LEN`.

Add that requirement to the unsafe requirements on `Bitmap::from_raw`
and `Bitmap::from_raw_mut`, and to the invariants on `Bitmap`.

This also fixes u32 casts truncating in `copy_and_extend`, which could
otherwise lead to OOB writes.

Fixes: 11eca92a2cae ("rust: add bitmap API.")
Link: https://lore.kernel.org/DKG0U8RLO7LZ.2I1AIH0S38PAP@xxxxxxxxxx
Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
---
 rust/kernel/bitmap.rs | 68 +++++++++++++++++++++++++++++++++++----------------
 1 file changed, 47 insertions(+), 21 deletions(-)

diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index a43bfe0ec3dc..fdcfc0409773 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -17,24 +17,57 @@
 /// # Invariants
 ///
 /// Must reference a `[c_ulong]` long enough to fit `data.len()` bits.
+/// Must not be longer than `i32::MAX` bits.
 #[cfg_attr(CONFIG_64BIT, repr(align(8)))]
 #[cfg_attr(not(CONFIG_64BIT), repr(align(4)))]
 pub struct Bitmap {
     data: [()],
 }
 
+macro_rules! bitmap_assert {
+    ($cond:expr, $($arg:tt)+) => {
+        #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
+        assert!($cond, $($arg)*);
+    }
+}
+
+macro_rules! bitmap_assert_return {
+    ($cond:expr, $($arg:tt)+) => {
+        #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
+        assert!($cond, $($arg)*);
+
+        #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
+        if !($cond) {
+            pr_err!($($arg)*);
+            return
+        }
+    }
+}
+
 impl Bitmap {
     /// Borrows a C bitmap.
     ///
+    /// # Panics
+    ///
+    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `nbits` exceeds `i32::MAX`.
+    ///
     /// # Safety
     ///
     /// * `ptr` holds a non-null address of an initialized array of `unsigned long`
     ///   that is large enough to hold `nbits` bits.
+    /// * `nbits` must not exceed `i32::MAX`.
     /// * the array must not be freed for the lifetime of this [`Bitmap`]
     /// * concurrent access only happens through atomic operations
     pub unsafe fn from_raw<'a>(ptr: *const usize, nbits: usize) -> &'a Bitmap {
+        bitmap_assert!(
+            nbits <= i32::MAX as usize,
+            "`nbits` must be <= {}, was {}",
+            i32::MAX,
+            nbits
+        );
         let data: *const [()] = core::ptr::slice_from_raw_parts(ptr.cast(), nbits);
         // INVARIANT: `data` references an initialized array that can hold `nbits` bits.
+        // INVARIANT: the caller guarantees that `nbits` does not exceed `i32::MAX`.
         // SAFETY:
         // The caller guarantees that `data` (derived from `ptr` and `nbits`)
         // points to a valid, initialized, and appropriately sized memory region
@@ -51,15 +84,27 @@ pub unsafe fn from_raw<'a>(ptr: *const usize, nbits: usize) -> &'a Bitmap {
 
     /// Borrows a C bitmap exclusively.
     ///
+    /// # Panics
+    ///
+    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `nbits` exceeds `i32::MAX`.
+    ///
     /// # Safety
     ///
     /// * `ptr` holds a non-null address of an initialized array of `unsigned long`
     ///   that is large enough to hold `nbits` bits.
+    /// * `nbits` must not exceed `i32::MAX`.
     /// * the array must not be freed for the lifetime of this [`Bitmap`]
     /// * no concurrent access may happen.
     pub unsafe fn from_raw_mut<'a>(ptr: *mut usize, nbits: usize) -> &'a mut Bitmap {
+        bitmap_assert!(
+            nbits <= i32::MAX as usize,
+            "`nbits` must be <= {}, was {}",
+            i32::MAX,
+            nbits
+        );
         let data: *mut [()] = core::ptr::slice_from_raw_parts_mut(ptr.cast(), nbits);
         // INVARIANT: `data` references an initialized array that can hold `nbits` bits.
+        // INVARIANT: the caller guarantees that `nbits` does not exceed `i32::MAX`.
         // SAFETY:
         // The caller guarantees that `data` (derived from `ptr` and `nbits`)
         // points to a valid, initialized, and appropriately sized memory region
@@ -96,26 +141,6 @@ union BitmapRepr {
     ptr: NonNull<usize>,
 }
 
-macro_rules! bitmap_assert {
-    ($cond:expr, $($arg:tt)+) => {
-        #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
-        assert!($cond, $($arg)*);
-    }
-}
-
-macro_rules! bitmap_assert_return {
-    ($cond:expr, $($arg:tt)+) => {
-        #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
-        assert!($cond, $($arg)*);
-
-        #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
-        if !($cond) {
-            pr_err!($($arg)*);
-            return
-        }
-    }
-}
-
 /// Represents an owned bitmap.
 ///
 /// Wraps underlying C bitmap API. See [`Bitmap`] for available
@@ -415,7 +440,8 @@ pub fn clear_bit_atomic(&self, index: usize) {
     #[inline]
     pub fn copy_and_extend(&mut self, src: &Bitmap) {
         let len = core::cmp::min(src.len(), self.len());
-        // SAFETY: access to `self` and `src` is within bounds.
+        // SAFETY: access to `self` and `src` is within bounds. Both lengths fit in `u32`
+        // because a `Bitmap` is at most `i32::MAX` bits, so the casts are lossless.
         unsafe {
             bindings::bitmap_copy_and_extend(
                 self.as_mut_ptr(),

-- 
2.55.0