[PATCH] binder: fix race that allows malicious free of live buffer

[Todd Kjos]

Malicious code can attempt to free buffers using the
BC_FREE_BUFFER ioctl to binder. There are protections
against a user freeing a buffer while in use by the
kernel, however there was a window where BC_FREE_BUFFER
could be used to free a recently allocated buffer that
was not completely initialized. This resulted in a
use-after-free detected by KASAN with a malicious
test program.

This window is closed by setting the buffer's
allow_user_free attribute to 0 when the buffer
is allocated or when the user has previously
freed it instead of waiting for the caller
to set it. The problem was that when the struct
buffer was recycled, allow_user_free was stale
and set to 1 allowing a free to go through.

Signed-off-by: Todd Kjos <tkjos@xxxxxxxxxx>
Acked-by: Arve HjÃnnevÃg <arve@xxxxxxxxxxx>
---
 drivers/android/binder.c       | 21 ++++++++++++---------
 drivers/android/binder_alloc.c | 16 ++++++----------
 drivers/android/binder_alloc.h |  3 +--
 3 files changed, 19 insertions(+), 21 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index cb30a524d16d8..9f1000d2a40c7 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2974,7 +2974,6 @@ static void binder_transaction(struct binder_proc *proc,
 		t->buffer = NULL;
 		goto err_binder_alloc_buf_failed;
 	}
-	t->buffer->allow_user_free = 0;
 	t->buffer->debug_id = t->debug_id;
 	t->buffer->transaction = t;
 	t->buffer->target_node = target_node;
@@ -3510,14 +3509,18 @@ static int binder_thread_write(struct binder_proc *proc,
 
 			buffer = binder_alloc_prepare_to_free(&proc->alloc,
 							      data_ptr);
-			if (buffer == NULL) {
-				binder_user_error("%d:%d BC_FREE_BUFFER u%016llx no match\n",
-					proc->pid, thread->pid, (u64)data_ptr);
-				break;
-			}
-			if (!buffer->allow_user_free) {
-				binder_user_error("%d:%d BC_FREE_BUFFER u%016llx matched unreturned buffer\n",
-					proc->pid, thread->pid, (u64)data_ptr);
+			if (IS_ERR_OR_NULL(buffer)) {
+				if (PTR_ERR(buffer) == -EPERM) {
+					binder_user_error(
+						"%d:%d BC_FREE_BUFFER u%016llx matched unreturned or currently freeing buffer\n",
+						proc->pid, thread->pid,
+						(u64)data_ptr);
+				} else {
+					binder_user_error(
+						"%d:%d BC_FREE_BUFFER u%016llx no match\n",
+						proc->pid, thread->pid,
+						(u64)data_ptr);
+				}
 				break;
 			}
 			binder_debug(BINDER_DEBUG_FREE_BUFFER,
diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 64fd96eada31f..030c98f35cca7 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -151,16 +151,12 @@ static struct binder_buffer *binder_alloc_prepare_to_free_locked(
 		else {
 			/*
 			 * Guard against user threads attempting to
-			 * free the buffer twice
+			 * free the buffer when in use by kernel or
+			 * after it's already been freed.
 			 */
-			if (buffer->free_in_progress) {
-				binder_alloc_debug(BINDER_DEBUG_USER_ERROR,
-						   "%d:%d FREE_BUFFER u%016llx user freed buffer twice\n",
-						   alloc->pid, current->pid,
-						   (u64)user_ptr);
-				return NULL;
-			}
-			buffer->free_in_progress = 1;
+			if (!buffer->allow_user_free)
+				return ERR_PTR(-EPERM);
+			buffer->allow_user_free = 0;
 			return buffer;
 		}
 	}
@@ -500,7 +496,7 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
 
 	rb_erase(best_fit, &alloc->free_buffers);
 	buffer->free = 0;
-	buffer->free_in_progress = 0;
+	buffer->allow_user_free = 0;
 	binder_insert_allocated_buffer_locked(alloc, buffer);
 	binder_alloc_debug(BINDER_DEBUG_BUFFER_ALLOC,
 		     "%d: binder_alloc_buf size %zd got %pK\n",
diff --git a/drivers/android/binder_alloc.h b/drivers/android/binder_alloc.h
index 9ef64e5638566..fb3238c74c8a8 100644
--- a/drivers/android/binder_alloc.h
+++ b/drivers/android/binder_alloc.h
@@ -50,8 +50,7 @@ struct binder_buffer {
 	unsigned free:1;
 	unsigned allow_user_free:1;
 	unsigned async_transaction:1;
-	unsigned free_in_progress:1;
-	unsigned debug_id:28;
+	unsigned debug_id:29;
 
 	struct binder_transaction *transaction;
 
-- 
2.19.1.930.g4563a0d9d0-goog