Re: [PATCH v3] binder: add mutex_lock for mmap and NULL when free

Kassey Li
2023-10-09T12:22:32Z

On 2023/10/8 7:16, Carlos Llamas wrote:
> On Sat, Oct 07, 2023 at 08:28:13PM +0800, Kassey Li wrote:
>> - Enforce alloc->mutex in binder_alloc_mmap_handler when add the entry to
>> list.
>>
>> - Assign the freed pages/page_ptr to NULL to catch possible use after free
>> with NULL pointer access.
>>
>> Fixes: 19c987241ca1 ("binder: separate out binder_alloc functions")
>> Fixes: f2517eb76f1f ("android: binder: Add global lru shrinker to binder")
>> CC: Todd Kjos <tkjos@android.com>
>> CC: Sherry Yang <sherryy@android.com>
>> Link: https://lore.kernel.org/all/20231007034046.2352124-1-quic_yingangl@quicinc.com/
>> Signed-off-by: Kassey Li <quic_yingangl@quicinc.com>
>> ---
>> V1 -> V2: Add Fixes info.
>> V2 -> V3: Add this history.
>> ---
>> drivers/android/binder_alloc.c | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
>> index e3db8297095a..c7d126e04343 100644
>> --- a/drivers/android/binder_alloc.c
>> +++ b/drivers/android/binder_alloc.c
>> @@ -775,6 +775,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
>> }
>>
>> buffer->user_data = alloc->buffer;
>> + mutex_lock(&alloc->mutex);
>
> At this stage we are already holding the mm->mmap_lock. If you take the
> alloc->mutex here you will deadlock. You should see this warning with
> lockdep enabled. Also, you don't need the lock here...

lockdep did not warn, but i agree with you, i did not aware the
mm->mmap_lock. thanks for the remind.
>
>> list_add(&buffer->entry, &alloc->buffers);
>> buffer->free = 1;
>> binder_insert_free_buffer(alloc, buffer);
>> @@ -782,7 +783,7 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
>>
>> /* Signal binder_alloc is fully initialized */
>> binder_alloc_set_vma(alloc, vma);
>
> This barrier will set the alloc->vma pointer. Once set, it signals that
> the alloc-> space has been initialized and it is safe to access.
>
>> -
>> + mutex_unlock(&alloc->mutex);
>> return 0;
>>
>> err_alloc_buf_struct_failed:
>> @@ -856,9 +857,11 @@ void binder_alloc_deferred_release(struct binder_alloc *alloc)
>> __func__, alloc->pid, i, page_addr,
>> on_lru ? "on lru" : "active");
>> __free_page(alloc->pages[i].page_ptr);
>> + alloc->pages[i].page_ptr = NULL;
>> page_count++;
>> }
>> kfree(alloc->pages);
>> + alloc->pages = NULL;
>
> The process is dying and there aren't any more references to it. It is
> pointless to mark the pages NULL. No one is or will use them after.

looks correct.
>
>> }
>> mutex_unlock(&alloc->mutex);
>> if (alloc->mm)
>> --
>> 2.25.1
>>
>
> I see that you reported a crash on the previous thread here:
> https://lore.kernel.org/all/26988068-8c9f-8591-db6e-44c8105af638@quicinc.com/
> ...unfortunately, it doesn't seem to me like setting alloc->pages = NULL
> would help fix your issue.
>
> I do agree this sounds like a use-after-free though. Are you able to
> reproduce the issue with KASAN enabled? This should tell us where the
> actual problem is.

kasan is trying but not reproduced yet, we will chasing with this
test enable kasan.
>
> --
> Carlos Llamas