CVE-2021-47505

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 <ramjiyani@google.com> (https://lore.kernel.org/r/20211027011834.2497484-1-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.

• CWE-416

• https://git.kernel.org/stable/c/321fba81ec034f88aea4898993c1bf15605c023fPatch
• https://git.kernel.org/stable/c/4105e6a128e8a98455dfc9e6dbb2ab0c33c4497fPatch
• https://git.kernel.org/stable/c/47ffefd88abfffe8a040bcc1dd0554d4ea6f7689Patch
• https://git.kernel.org/stable/c/50252e4b5e989ce64555c7aef7516bdefc2fea72Patch
• https://git.kernel.org/stable/c/60d311f9e6381d779d7d53371f87285698ecee24Patch
• https://git.kernel.org/stable/c/321fba81ec034f88aea4898993c1bf15605c023fPatch
• https://git.kernel.org/stable/c/4105e6a128e8a98455dfc9e6dbb2ab0c33c4497fPatch
• https://git.kernel.org/stable/c/47ffefd88abfffe8a040bcc1dd0554d4ea6f7689Patch
• https://git.kernel.org/stable/c/50252e4b5e989ce64555c7aef7516bdefc2fea72Patch
• https://git.kernel.org/stable/c/60d311f9e6381d779d7d53371f87285698ecee24Patch

Published

May 24, 2024

Last Modified

Aug 4, 2026

Status

Modified

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
• Autonomous PentestingAI agents that find and validate exploitable vulnerabilities like this one across your applications.
• PR ReviewsPentest every pull request so vulnerable code is caught before it ships to production.
• AI Penetration TestingHow AI-driven penetration testing continuously covers your attack surface.

• CVE-2021-47499In the Linux kernel, the following vulnerability has been re…5.5
• CVE-2021-47500In the Linux kernel, the following vulnerability has been re…7.8
• CVE-2021-47501In the Linux kernel, the following vulnerability has been re…5.5
• CVE-2021-47502In the Linux kernel, the following vulnerability has been re…5.5
• CVE-2021-47503In the Linux kernel, the following vulnerability has been re…6.2
• CVE-2021-47504Rejected reason: This CVE ID has been rejected or withdrawn …
• CVE-2021-47506In the Linux kernel, the following vulnerability has been re…7.8
• CVE-2021-47507In the Linux kernel, the following vulnerability has been re…4.7
• CVE-2021-47508In the Linux kernel, the following vulnerability has been re…5.5
• CVE-2021-47509In the Linux kernel, the following vulnerability has been re…5.5
• CVE-2021-47510In the Linux kernel, the following vulnerability has been re…5.5
• CVE-2021-47511In the Linux kernel, the following vulnerability has been re…5.5