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 doesnt 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 didnt 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.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/321fba81ec034f88aea4898993c1bf15605c023f
• https://git.kernel.org/stable/c/4105e6a128e8a98455dfc9e6dbb2ab0c33c4497f
• https://git.kernel.org/stable/c/47ffefd88abfffe8a040bcc1dd0554d4ea6f7689
• https://git.kernel.org/stable/c/50252e4b5e989ce64555c7aef7516bdefc2fea72
• https://git.kernel.org/stable/c/60d311f9e6381d779d7d53371f87285698ecee24
• https://git.kernel.org/stable/c/321fba81ec034f88aea4898993c1bf15605c023f
• https://git.kernel.org/stable/c/4105e6a128e8a98455dfc9e6dbb2ab0c33c4497f
• https://git.kernel.org/stable/c/47ffefd88abfffe8a040bcc1dd0554d4ea6f7689
• https://git.kernel.org/stable/c/50252e4b5e989ce64555c7aef7516bdefc2fea72
• https://git.kernel.org/stable/c/60d311f9e6381d779d7d53371f87285698ecee24