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Subject[PATCH 17/37] binder: protect against two threads freeing buffer
Date
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>
---
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
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