CVE-2019-19039

__btrfs_free_extent in fs/btrfs/extent-tree.c in the Linux kernel through 5.3.12 calls btrfs_print_leaf in a certain ENOENT case, which allows local users to obtain potentially sensitive information about register values via the dmesg program. NOTE: The BTRFS development team disputes this issues as not being a vulnerability because “1) The kernel provide facilities to restrict access to dmesg - dmesg_restrict=1 sysctl option. So it’s really up to the system administrator to judge whether dmesg access shall be disallowed or not. 2) WARN/WARN_ON are widely used macros in the linux kernel. If this CVE is considered valid this would mean there are literally thousands CVE lurking in the kernel - something which clearly is not the case.

Mitigation

Its likely that this flaw will not end up being fixed, due to this being expected behavior.

If this behavior is not derirable, it can be mitigated by explicitly setting the kernel parameter to restrict non privileged users with the command:

sudo sysctl -w kernel.dmesg_restrict=1

And to make persistent between systme reboots.

echo 'kernel.dmesg_restrict=1' | sudo tee -a /etc/sysctl.conf