Over the past few weeks, those of you who frequent the DAY[0] streams over on our Twitch may have seen me working on trying to understand the recent Android Binder Use-After-Free (UAF) published by Google’s Project Zero (p0). This bug is actually not new, the issue was discovered and fixed in the mainline kernel in February 2018, however, p0 discovered many popular devices did not receive the patch downstream. Some of these devices include the Pixel 2, the Huawei P20, and Samsung Galaxy S7, S8, and S9 phones. I believe many of these devices received security patches within the last couple weeks that finally killed the bug.

After a few streams of poking around with a kernel debugger on a virtual machine (running Android-x86), and testing with a vulnerable Pixel 2, I’ve came to understand the exploit written by Jann Horn and Maddie Stone pretty well. Without an understanding of Binder (the binder_thread object specifically), as well as how Vectored I/O works, the exploit can be pretty confusing. It’s also quite clever how they exploited this issue, so I thought it would be cool to write up how the exploit works.

source: grugq