[PATCH v4 3/7] binder: forbid vma splitting

[Carlos Llamas <cmllamas@google.com>]

Binder does not support splitting its mappings. Allowing so, leads to
potential attacks that stem from a partial munmap(), such as closing the
tail range and replacing it with a new mapping.

Close the loophole by explicitly rejecting vma splitting.

Cc: stable@vger.kernel.org
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260901163521.1355535-1-cmllamas@google.com?part=2
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 drivers/android/binder.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 1b492ed48ea6..185128577829 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -6032,10 +6032,16 @@ VISIBLE_IF_KUNIT vm_fault_t binder_vm_fault(struct vm_fault *vmf)
 }
 EXPORT_SYMBOL_IF_KUNIT(binder_vm_fault);
 
+static int binder_vma_may_split(struct vm_area_struct *vma, unsigned long addr)
+{
+	return -EINVAL;
+}
+
 static const struct vm_operations_struct binder_vm_ops = {
 	.open = binder_vma_open,
 	.close = binder_vma_close,
 	.fault = binder_vm_fault,
+	.may_split = binder_vma_may_split,
 };
 
 static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
-- 
2.55.0.966.g6673acef38-goog