4.19-stable review patch. If anyone has any objections, please let me know.
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From: Todd Kjos tkjos@android.com
commit 7bada55ab50697861eee6bb7d60b41e68a961a9c upstream.
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@google.com Acked-by: Arve Hjønnevåg arve@android.com Cc: stable stable@vger.kernel.org # 4.14 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/android/binder.c | 21 ++++++++++++--------- drivers/android/binder_alloc.c | 16 ++++++---------- drivers/android/binder_alloc.h | 3 +-- 3 files changed, 19 insertions(+), 21 deletions(-)
--- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -2971,7 +2971,6 @@ static void binder_transaction(struct bi 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; @@ -3465,14 +3464,18 @@ static int binder_thread_write(struct bi
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, --- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -151,16 +151,12 @@ static struct binder_buffer *binder_allo 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_allo
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", --- 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;