CVE-2021-47505 Affecting kernel-default-base package, versions <5.14.21-150400.24.122.2.150400.24.58.2


Severity

Recommended
0.0
high
0
10

Based on SUSE Linux Enterprise Server security rating.

Threat Intelligence

EPSS
0.05% (18th percentile)

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  • Snyk IDSNYK-SLES154-KERNELDEFAULTBASE-7388457
  • published26 Jun 2024
  • disclosed25 Jun 2024

Introduced: 25 Jun 2024

CVE-2021-47505  (opens in a new tab)

How to fix?

Upgrade SLES:15.4 kernel-default-base to version 5.14.21-150400.24.122.2.150400.24.58.2 or higher.

NVD Description

Note: Versions mentioned in the description apply only to the upstream kernel-default-base package and not the kernel-default-base package as distributed by SLES. See How to fix? for SLES:15.4 relevant fixed versions and status.

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.

CVSS Scores

version 3.1