BACKPORT: FROMLIST: rust_binder: avoid reading the written value in offsets array

When sending a transaction, its offsets array is first copied into the
target proc's vma, and then the values are read back from there. This is
normally fine because the vma is a read-only mapping, so the target
process cannot change the value under us.

However, if the target process somehow gains the ability to write to its
own vma, it could change the offset before it's read back, causing the
kernel to misinterpret what the sender meant. If the sender happens to
send a payload with a specific shape, this could in the worst case lead
to the receiver being able to privilege escalate into the sender.

The intent is that gaining the ability to change the read-only vma of
your own process should not be exploitable, so remove this TOCTOU read
even though it's unexploitable without another Binder bug.

Cc: stable@vger.kernel.org
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Reported-by: Jann Horn <jannh@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20260218-binder-vma-check-v2-2-60f9d695a990@google.com
Bug: 485023800
[aliceryhl: adjust UserSlice creation due to UserPtr missing]
Change-Id: I5da1e2dab72951663ad9803c61e0a2be206245fd
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/android/binder/thread.rs

diff --git a/drivers/android/binder/thread.rs b/drivers/android/binder/thread.rs
index 62e2d0c..3ecd4ae 100644
--- a/drivers/android/binder/thread.rs
+++ b/drivers/android/binder/thread.rs
@@ -1137,10 +1137,8 @@ pub(crate) fn copy_transaction_data(
 
         // Copy offsets if there are any.
         if offsets_size > 0 {
-            {
-                let mut reader = UserSlice::new(trd_data_ptr.offsets as _, offsets_size).reader();
-                alloc.copy_into(&mut reader, aligned_data_size, offsets_size)?;
-            }
+            let mut offsets_reader =
+                UserSlice::new(trd_data_ptr.offsets as _, offsets_size).reader();
 
             let offsets_start = aligned_data_size;
             let offsets_end = aligned_data_size + offsets_size;
@@ -1161,11 +1159,9 @@ pub(crate) fn copy_transaction_data(
                 .step_by(size_of::<u64>())
                 .enumerate()
             {
-                let offset: usize = view
-                    .alloc
-                    .read::<u64>(index_offset)?
-                    .try_into()
-                    .map_err(|_| EINVAL)?;
+                let offset = offsets_reader.read::<u64>()?;
+                view.alloc.write(index_offset, &offset)?;
+                let offset: usize = offset.try_into().map_err(|_| EINVAL)?;
 
                 if offset < end_of_previous_object || !is_aligned(offset, size_of::<u32>()) {
                     pr_warn!("Got transaction with invalid offset.");