[PATCH 5.10 559/717] binder: add flag to clear buffer on txn complete

Greg Kroah-Hartman
2020-12-28T14:26:27Z

From: Todd Kjos <tkjos@google.com>

commit 0f966cba95c78029f491b433ea95ff38f414a761 upstream.

Add a per-transaction flag to indicate that the buffer
must be cleared when the transaction is complete to
prevent copies of sensitive data from being preserved
in memory.

Signed-off-by: Todd Kjos <tkjos@google.com>
Link: https://lore.kernel.org/r/20201120233743.3617529-1-tkjos@google.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/android/binder.c | 1
drivers/android/binder_alloc.c | 48 ++++++++++++++++++++++++++++++++++++
drivers/android/binder_alloc.h | 4 ++-
include/uapi/linux/android/binder.h | 1
4 files changed, 53 insertions(+), 1 deletion(-)

--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -3146,6 +3146,7 @@ static void binder_transaction(struct bi
t->buffer->debug_id = t->debug_id;
t->buffer->transaction = t;
t->buffer->target_node = target_node;
+ t->buffer->clear_on_free = !!(t->flags & TF_CLEAR_BUF);
trace_binder_transaction_alloc_buf(t->buffer);

if (binder_alloc_copy_user_to_buffer(
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -696,6 +696,8 @@ static void binder_free_buf_locked(struc
binder_insert_free_buffer(alloc, buffer);
}

+static void binder_alloc_clear_buf(struct binder_alloc *alloc,
+ struct binder_buffer *buffer);
/**
* binder_alloc_free_buf() - free a binder buffer
* @alloc: binder_alloc for this proc
@@ -706,6 +708,18 @@ static void binder_free_buf_locked(struc
void binder_alloc_free_buf(struct binder_alloc *alloc,
struct binder_buffer *buffer)
{
+ /*
+ * We could eliminate the call to binder_alloc_clear_buf()
+ * from binder_alloc_deferred_release() by moving this to
+ * binder_alloc_free_buf_locked(). However, that could
+ * increase contention for the alloc mutex if clear_on_free
+ * is used frequently for large buffers. The mutex is not
+ * needed for correctness here.
+ */
+ if (buffer->clear_on_free) {
+ binder_alloc_clear_buf(alloc, buffer);
+ buffer->clear_on_free = false;
+ }
mutex_lock(&alloc->mutex);
binder_free_buf_locked(alloc, buffer);
mutex_unlock(&alloc->mutex);
@@ -802,6 +816,10 @@ void binder_alloc_deferred_release(struc
/* Transaction should already have been freed */
BUG_ON(buffer->transaction);

+ if (buffer->clear_on_free) {
+ binder_alloc_clear_buf(alloc, buffer);
+ buffer->clear_on_free = false;
+ }
binder_free_buf_locked(alloc, buffer);
buffers++;
}
@@ -1136,6 +1154,36 @@ static struct page *binder_alloc_get_pag
}

/**
+ * binder_alloc_clear_buf() - zero out buffer
+ * @alloc: binder_alloc for this proc
+ * @buffer: binder buffer to be cleared
+ *
+ * memset the given buffer to 0
+ */
+static void binder_alloc_clear_buf(struct binder_alloc *alloc,
+ struct binder_buffer *buffer)
+{
+ size_t bytes = binder_alloc_buffer_size(alloc, buffer);
+ binder_size_t buffer_offset = 0;
+
+ while (bytes) {
+ unsigned long size;
+ struct page *page;
+ pgoff_t pgoff;
+ void *kptr;
+
+ page = binder_alloc_get_page(alloc, buffer,
+ buffer_offset, &pgoff);
+ size = min_t(size_t, bytes, PAGE_SIZE - pgoff);
+ kptr = kmap(page) + pgoff;
+ memset(kptr, 0, size);
+ kunmap(page);
+ bytes -= size;
+ buffer_offset += size;
+ }
+}
+
+/**
* binder_alloc_copy_user_to_buffer() - copy src user to tgt user
* @alloc: binder_alloc for this proc
* @buffer: binder buffer to be accessed
--- a/drivers/android/binder_alloc.h
+++ b/drivers/android/binder_alloc.h
@@ -23,6 +23,7 @@ struct binder_transaction;
* @entry: entry alloc->buffers
* @rb_node: node for allocated_buffers/free_buffers rb trees
* @free: %true if buffer is free
+ * @clear_on_free: %true if buffer must be zeroed after use
* @allow_user_free: %true if user is allowed to free buffer
* @async_transaction: %true if buffer is in use for an async txn
* @debug_id: unique ID for debugging
@@ -41,9 +42,10 @@ struct binder_buffer {
struct rb_node rb_node; /* free entry by size or allocated entry */
/* by address */
unsigned free:1;
+ unsigned clear_on_free:1;
unsigned allow_user_free:1;
unsigned async_transaction:1;
- unsigned debug_id:29;
+ unsigned debug_id:28;

struct binder_transaction *transaction;

--- a/include/uapi/linux/android/binder.h
+++ b/include/uapi/linux/android/binder.h
@@ -248,6 +248,7 @@ enum transaction_flags {
TF_ROOT_OBJECT = 0x04, /* contents are the component's root object */
TF_STATUS_CODE = 0x08, /* contents are a 32-bit status code */
TF_ACCEPT_FDS = 0x10, /* allow replies with file descriptors */
+ TF_CLEAR_BUF = 0x20, /* clear buffer on txn complete */
};

struct binder_transaction_data {