CVE ID:	CVE-2021-47505
Description:	In the Linux kernel, the following vulnerability has been resolved: aio: fix use-after-free due to missing POLLFREE handling signalfd_poll() and binder_poll() are special in that they use a waitqueue whose lifetime is the current task, rather than the struct file as is normally the case. This is okay for blocking polls, since a blocking poll occurs within one task; however, non-blocking polls require another solution. This solution is for the queue to be cleared before it is freed, by sending a POLLFREE notification to all waiters. Unfortunately, only eventpoll handles POLLFREE. A second type of non- blocking poll, aio poll, was added in kernel v4.18, and it doesn't handle POLLFREE. This allows a use-after-free to occur if a signalfd or binder fd is polled with aio poll, and the waitqueue gets freed. Fix this by making aio poll handle POLLFREE. A patch by Ramji Jiyani (https://lore.kernel.org/r/20211027011834.24974 84-1-ramjiyani@google.com) tried to do this by making aio_poll_wake() always complete the request inline if POLLFREE is seen. However, that solution had two bugs. First, it introduced a deadlock, as it unconditionally locked the aio context while holding the waitqueue lock, which inverts the normal locking order. Second, it didn't consider that POLLFREE notifications are missed while the request has been temporarily de-queued. The second problem was solved by my previous patch. This patch then properly fixes the use-after-free by handling POLLFREE in a deadlock-free way. It does this by taking advantage of the fact that freeing of the waitqueue is RCU-delayed, similar to what eventpoll does.
Test IDs:	None available
Cross References:	Common Vulnerability Exposure (CVE) ID: CVE-2021-47505 https://git.kernel.org/stable/c/321fba81ec034f88aea4898993c1bf15605c023f https://git.kernel.org/stable/c/321fba81ec034f88aea4898993c1bf15605c023f https://git.kernel.org/stable/c/4105e6a128e8a98455dfc9e6dbb2ab0c33c4497f https://git.kernel.org/stable/c/4105e6a128e8a98455dfc9e6dbb2ab0c33c4497f https://git.kernel.org/stable/c/47ffefd88abfffe8a040bcc1dd0554d4ea6f7689 https://git.kernel.org/stable/c/47ffefd88abfffe8a040bcc1dd0554d4ea6f7689 https://git.kernel.org/stable/c/50252e4b5e989ce64555c7aef7516bdefc2fea72 https://git.kernel.org/stable/c/50252e4b5e989ce64555c7aef7516bdefc2fea72 https://git.kernel.org/stable/c/60d311f9e6381d779d7d53371f87285698ecee24 https://git.kernel.org/stable/c/60d311f9e6381d779d7d53371f87285698ecee24