binder: protect against two threads freeing buffer

Adds protection against malicious user code freeing
the same buffer at the same time which could cause
a crash. Cannot happen under normal use.

Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 23 insertions, 10 deletions

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 3bbfb2455b70..a1912a22c89c 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2024,8 +2024,8 @@ static int binder_thread_write(struct binder_proc *proc,
                                 return -EFAULT;
                         ptr += sizeof(binder_uintptr_t);
 
-                        buffer = binder_alloc_buffer_lookup(&proc->alloc,
-                                                            data_ptr);
+                        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);
diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index a0af1419cc79..2a2e41b13de5 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -116,7 +116,7 @@ static void binder_insert_allocated_buffer_locked(
         rb_insert_color(&new_buffer->rb_node, &alloc->allocated_buffers);
 }
 
-static struct binder_buffer *binder_alloc_buffer_lookup_locked(
+static struct binder_buffer *binder_alloc_prepare_to_free_locked(
                 struct binder_alloc *alloc,
                 uintptr_t user_ptr)
 {
@@ -135,8 +135,19 @@ static struct binder_buffer *binder_alloc_buffer_lookup_locked(
                         n = n->rb_left;
                 else if (kern_ptr > buffer)
                         n = n->rb_right;
-                else
+                else {
+                        /*
+                         * Guard against user threads attempting to
+                         * free the buffer twice
+                         */
+                        if (buffer->free_in_progress) {
+                                pr_err("%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;
                         return buffer;
+                }
         }
         return NULL;
 }
@@ -152,13 +163,13 @@ static struct binder_buffer *binder_alloc_buffer_lookup_locked(
  *
  * Return:      Pointer to buffer or NULL
  */
-struct binder_buffer *binder_alloc_buffer_lookup(struct binder_alloc *alloc,
-                                                 uintptr_t user_ptr)
+struct binder_buffer *binder_alloc_prepare_to_free(struct binder_alloc *alloc,
+                                                   uintptr_t user_ptr)
 {
         struct binder_buffer *buffer;
 
         mutex_lock(&alloc->mutex);
-        buffer = binder_alloc_buffer_lookup_locked(alloc, user_ptr);
+        buffer = binder_alloc_prepare_to_free_locked(alloc, user_ptr);
         mutex_unlock(&alloc->mutex);
         return buffer;
 }
@@ -358,6 +369,7 @@ struct binder_buffer *binder_alloc_new_buf_locked(struct binder_alloc *alloc,
 
         rb_erase(best_fit, &alloc->free_buffers);
         buffer->free = 0;
+        buffer->free_in_progress = 0;
         binder_insert_allocated_buffer_locked(alloc, buffer);
         if (buffer_size != size) {
                 struct binder_buffer *new_buffer = (void *)buffer->data + size;
diff --git a/drivers/android/binder_alloc.h b/drivers/android/binder_alloc.h
index 721c511431f9..088e4ffc6230 100644
--- a/drivers/android/binder_alloc.h
+++ b/drivers/android/binder_alloc.h
@@ -48,7 +48,8 @@ struct binder_buffer {
         unsigned free:1;
         unsigned allow_user_free:1;
         unsigned async_transaction:1;
-        unsigned debug_id:29;
+        unsigned free_in_progress:1;
+        unsigned debug_id:28;
 
         struct binder_transaction *transaction;
 
@@ -109,8 +110,8 @@ extern struct binder_buffer *binder_alloc_new_buf(struct binder_alloc *alloc,
 extern void binder_alloc_init(struct binder_alloc *alloc);
 extern void binder_alloc_vma_close(struct binder_alloc *alloc);
 extern struct binder_buffer *
-binder_alloc_buffer_lookup(struct binder_alloc *alloc,
-                           uintptr_t user_ptr);
+binder_alloc_prepare_to_free(struct binder_alloc *alloc,
+                             uintptr_t user_ptr);
 extern void binder_alloc_free_buf(struct binder_alloc *alloc,
                                   struct binder_buffer *buffer);
 extern int binder_alloc_mmap_handler(struct binder_alloc *alloc,