Ubuntu Security Notice 4367-2 - USN-4367-1 fixed vulnerabilities in the 5.4 Linux kernel. Unfortunately, that update introduced a regression in overlayfs. This update corrects the problem. It was discovered that the btrfs implementation in the Linux kernel did not properly detect that a block was marked dirty in some situations. An attacker could use this to specially craft a file system image that, when unmounted, could cause a denial of service. Various other issues were also addressed.
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