On Fri, May 5, 2023 at 1:30 PM Carlos Llamas cmllamas@google.com wrote:

In binder_transaction_buffer_release() the 'failed_at' offset indicates the number of objects to clean up. However, this function was changed by commit 44d8047f1d87 ("binder: use standard functions to allocate fds"), to release all the objects in the buffer when 'failed_at' is zero.

This introduced an issue when a transaction buffer is released without any objects having been processed so far. In this case, 'failed_at' is indeed zero yet it is misinterpreted as releasing the entire buffer.

This leads to use-after-free errors where nodes are incorrectly freed and subsequently accessed. Such is the case in the following KASAN report:

================================================================== BUG: KASAN: slab-use-after-free in binder_thread_read+0xc40/0x1f30 Read of size 8 at addr ffff4faf037cfc58 by task poc/474

CPU: 6 PID: 474 Comm: poc Not tainted 6.3.0-12570-g7df047b3f0aa #5 Hardware name: linux,dummy-virt (DT) Call trace: dump_backtrace+0x94/0xec show_stack+0x18/0x24 dump_stack_lvl+0x48/0x60 print_report+0xf8/0x5b8 kasan_report+0xb8/0xfc __asan_load8+0x9c/0xb8 binder_thread_read+0xc40/0x1f30 binder_ioctl+0xd9c/0x1768 __arm64_sys_ioctl+0xd4/0x118 invoke_syscall+0x60/0x188 [...]

Allocated by task 474: kasan_save_stack+0x3c/0x64 kasan_set_track+0x2c/0x40 kasan_save_alloc_info+0x24/0x34 __kasan_kmalloc+0xb8/0xbc kmalloc_trace+0x48/0x5c binder_new_node+0x3c/0x3a4 binder_transaction+0x2b58/0x36f0 binder_thread_write+0x8e0/0x1b78 binder_ioctl+0x14a0/0x1768 __arm64_sys_ioctl+0xd4/0x118 invoke_syscall+0x60/0x188 [...]

Freed by task 475: kasan_save_stack+0x3c/0x64 kasan_set_track+0x2c/0x40 kasan_save_free_info+0x38/0x5c __kasan_slab_free+0xe8/0x154 __kmem_cache_free+0x128/0x2bc kfree+0x58/0x70 binder_dec_node_tmpref+0x178/0x1fc binder_transaction_buffer_release+0x430/0x628 binder_transaction+0x1954/0x36f0 binder_thread_write+0x8e0/0x1b78 binder_ioctl+0x14a0/0x1768 __arm64_sys_ioctl+0xd4/0x118 invoke_syscall+0x60/0x188 [...] ==================================================================

In order to avoid these issues, let's always calculate the intended 'failed_at' offset beforehand. This is renamed and wrapped in a helper function to make it clear and convenient.

Fixes: 32e9f56a96d8 ("binder: don't detect sender/target during buffer cleanup") Reported-by: Zi Fan Tan zifantan@google.com Cc: stable@vger.kernel.org Signed-off-by: Carlos Llamas cmllamas@google.com

Acked-by: Todd Kjos tkjos@google.com

---

v2: rename 'failed_at' to 'off_end_offsets' and drop the now unecessary comments after the rename per Todd's feedback.

drivers/android/binder.c | 28 +++++++++++++++++++++------- 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c index fb56bfc45096..8fb7672021ee 100644 --- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -1934,24 +1934,23 @@ static void binder_deferred_fd_close(int fd) static void binder_transaction_buffer_release(struct binder_proc *proc, struct binder_thread *thread, struct binder_buffer *buffer,

•                                         binder_size_t failed_at,
  

•                                         binder_size_t off_end_offset,
                                          bool is_failure)
  

{ int debug_id = buffer->debug_id;

•   binder_size_t off_start_offset, buffer_offset, off_end_offset;
  

•   binder_size_t off_start_offset, buffer_offset;
  
    binder_debug(BINDER_DEBUG_TRANSACTION,
                 "%d buffer release %d, size %zd-%zd, failed at %llx\n",
                 proc->pid, buffer->debug_id,
                 buffer->data_size, buffer->offsets_size,
  

•                (unsigned long long)failed_at);
  

•                (unsigned long long)off_end_offset);
  
    if (buffer->target_node)
            binder_dec_node(buffer->target_node, 1, 0);
  
    off_start_offset = ALIGN(buffer->data_size, sizeof(void *));
  

•   off_end_offset = is_failure && failed_at ? failed_at :
  

•                           off_start_offset + buffer->offsets_size;
  

•   for (buffer_offset = off_start_offset; buffer_offset < off_end_offset;
         buffer_offset += sizeof(binder_size_t)) {
            struct binder_object_header *hdr;
  

@@ -2111,6 +2110,21 @@ static void binder_transaction_buffer_release(struct binder_proc *proc, } }

+/* Clean up all the objects in the buffer */ +static inline void binder_release_entire_buffer(struct binder_proc *proc,

•                                           struct binder_thread *thread,
  

•                                           struct binder_buffer *buffer,
  

•                                           bool is_failure)
  

+{

•   binder_size_t off_end_offset;
  

•   off_end_offset = ALIGN(buffer->data_size, sizeof(void *));
  

•   off_end_offset += buffer->offsets_size;
  

•   binder_transaction_buffer_release(proc, thread, buffer,
  

•                                     off_end_offset, is_failure);
  

+}

static int binder_translate_binder(struct flat_binder_object *fp, struct binder_transaction *t, struct binder_thread *thread) @@ -2806,7 +2820,7 @@ static int binder_proc_transaction(struct binder_transaction *t, t_outdated->buffer = NULL; buffer->transaction = NULL; trace_binder_transaction_update_buffer_release(buffer);

•           binder_transaction_buffer_release(proc, NULL, buffer, 0, 0);
  

•           binder_release_entire_buffer(proc, NULL, buffer, false);
            binder_alloc_free_buf(&proc->alloc, buffer);
            kfree(t_outdated);
            binder_stats_deleted(BINDER_STAT_TRANSACTION);
  

@@ -3775,7 +3789,7 @@ binder_free_buf(struct binder_proc *proc, binder_node_inner_unlock(buf_node); } trace_binder_transaction_buffer_release(buffer);

•   binder_transaction_buffer_release(proc, thread, buffer, 0, is_failure);
  

•   binder_release_entire_buffer(proc, thread, buffer, is_failure);
    binder_alloc_free_buf(&proc->alloc, buffer);
  

}

-- 2.40.1.521.gf1e218fcd8-goog