fix unsoundness in Vec::extract_if

The implementation of the `Vec::extract_if` iterator invokes UB by
always constructing a mutable slice for the entire length of the vector
even though that span of memory can contain holes from items already
drained. The safety contract of `slice::from_raw_parts` requires that
all elements must be properly initialized.

As an example we can look at the following code:

```rust
let mut v = vec![Box::new(0u64), Box::new(1u64)];
for item in v.extract_if(.., |x| **x == 0) {
    drop(item);
}
```

In the second iteration a `&mut [Box<u64>]` slice of length 2 will be
constructed. The first slot of the slice contains the bitpattern of an
already deallocated box, causing UB.

This fixes the issue by only creating references to valid items and
using pointer manipulation for the rest. I have also taken the liberty
to remove the big `unsafe` blocks in place of targetted ones with a
SAFETY comment. The approach closely mirrors the implementation of
`Vec::retain_mut`.

Signed-off-by: Petros Angelatos <[email protected]>

Author: petrosagg

library/alloc/src/vec/extract_if.rs

@@ -64,27 +64,29 @@ where
     type Item = T;
 
     fn next(&mut self) -> Option<T> {
-        unsafe {
-            while self.idx < self.end {
-                let i = self.idx;
-                let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
-                let drained = (self.pred)(&mut v[i]);
-                // Update the index *after* the predicate is called. If the index
-                // is updated prior and the predicate panics, the element at this
-                // index would be leaked.
-                self.idx += 1;
-                if drained {
-                    self.del += 1;
-                    return Some(ptr::read(&v[i]));
-                } else if self.del > 0 {
-                    let del = self.del;
-                    let src: *const T = &v[i];
-                    let dst: *mut T = &mut v[i - del];
-                    ptr::copy_nonoverlapping(src, dst, 1);
+        while self.idx < self.end {
+            let i = self.idx;
+            // SAFETY: Unchecked element must be valid.
+            let cur = unsafe { &mut *self.vec.as_mut_ptr().add(i) };
+            let drained = (self.pred)(cur);
+            // Update the index *after* the predicate is called. If the index
+            // is updated prior and the predicate panics, the element at this
+            // index would be leaked.
+            self.idx += 1;
+            if drained {
+                self.del += 1;
+                // SAFETY: We never touch this element again after returning it.
+                return Some(unsafe { ptr::read(cur) });
+            } else if self.del > 0 {
+                // SAFETY: `self.del` > 0, so the hole slot must not overlap with current element.
+                // We use copy for move, and never touch this element again.
+                unsafe {
+                    let hole_slot = self.vec.as_mut_ptr().add(i - self.del);
+                    ptr::copy_nonoverlapping(cur, hole_slot, 1);
                 }
             }
-            None
         }
+        None
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
@@ -95,14 +97,18 @@ where
 #[stable(feature = "extract_if", since = "1.87.0")]
 impl<T, F, A: Allocator> Drop for ExtractIf<'_, T, F, A> {
     fn drop(&mut self) {
-        unsafe {
-            if self.idx < self.old_len && self.del > 0 {
-                let ptr = self.vec.as_mut_ptr();
-                let src = ptr.add(self.idx);
-                let dst = src.sub(self.del);
-                let tail_len = self.old_len - self.idx;
-                src.copy_to(dst, tail_len);
+        if self.del > 0 {
+            // SAFETY: Trailing unchecked items must be valid since we never touch them.
+            unsafe {
+                ptr::copy(
+                    self.vec.as_ptr().add(self.idx),
+                    self.vec.as_mut_ptr().add(self.idx - self.del),
+                    self.old_len - self.idx,
+                );
             }
+        }
+        // SAFETY: After filling holes, all items are in contiguous memory.
+        unsafe {
             self.vec.set_len(self.old_len - self.del);
         }
     }