[PATCH 17/37] binder: protect against two threads freeing buffer

[Todd Kjos]

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 <tk...@google.com>
---
 drivers/android/binder.c       |  4 ++--
 drivers/android/binder_alloc.c | 22 +++++++++++++++++-----
 drivers/android/binder_alloc.h |  7 ++++---
 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,
-- 
2.13.2.725.g09c95d1e9-goog